An Alkali-Free Surfactant System Based on Petroleum Sulfonate and Extended Surfactant for Enhanced Oil Recovery

An Alkali-Free Surfactant System Based on Petroleum Sulfonate and Extended Surfactant for Enhanced Oil Recovery | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article An Alkali-Free Surfactant System Based on Petroleum Sulfonate and Extended Surfactant for Enhanced Oil Recovery Jingchun wu, Yangyang hou, yang zhao, junzhen Wu, xin yu, LiYuan Cai, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9477351/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract To address the scaling and corrosion problems associated with long-term weak-alkali ASP flooding in Daqing Oilfield, an alkali-free oil-displacement system based on petroleum sulfonate (WPS) and an extended surfactant (C12P9E3S) was developed. The formulation was designed according to the equivalent alkane carbon number (EACN) of the target crude oil and optimized through experimental evaluation of interfacial tension and emulsification performance. The optimal system, with a WPS/C12P9E3S mass ratio of 2:1, total surfactant concentration of 0.3 wt%, and NaCl concentration of 1.6 wt%, achieved ultralow interfacial tension at the 10 − 3 mN/m level and exhibited improved emulsion stability compared with the conventional weak-alkali ASP system. Core flooding experiments showed that the alkali-free system increased oil recovery by 36.72% after water flooding, which is 5.80 percentage points higher than that of the weak-alkali ASP system. In addition, the system demonstrated more stable displacement behavior and a longer effective flooding stage. The improved oil recovery performance is mainly associated with enhanced emulsification and better transport characteristics of the dispersed oil phase in porous media. The results indicate that the developed alkali-free system can effectively reduce alkali-related operational risks while maintaining favorable oil-displacement efficiency, providing a practical alternative for enhanced oil recovery in mature oilfields. Physical sciences/Chemistry Physical sciences/Energy science and technology Physical sciences/Engineering Physical sciences/Materials science extended surfactant petroleum sulfonate interfacial behavior emulsification stability enhanced oil recovery Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 1 Introduction Ternary chemical flooding has been widely recognized as an effective enhanced oil recovery (EOR) technique for mature oilfields [ 1 ] and has been extensively applied in highly water-flooded reservoirs such as those in the Daqing Oilfield [ 2 ] . However, conventional weak-alkali ternary systems may cause scaling and corrosion during long-term injection and production, which complicates field operation and reduces engineering adaptability [ 3 ] . Therefore, the development of alkali-free oil-displacement systems that maintain satisfactory displacement efficiency while improving field applicability has become an important research direction in chemical flooding [ 4 ] . In recent years, various strategies have been proposed to reduce or eliminate alkali in chemical flooding, including alkali-free binary systems [ 5 , 6 ] and alkali-free ternary systems [ 7 ] . Among the surfactants used in such systems, petroleum sulfonates have attracted considerable attention because of their broad raw-material availability, low cost, and relatively good compatibility with crude oil. Previous studies have shown that petroleum sulfonates can still exhibit strong interfacial activity under alkali-free conditions, achieve ultralow oil-water interfacial tension, and provide promising oil-displacement performance [ 9 ] . In particular, alkali-free systems containing petroleum sulfonates blended with amphoteric or nonionic surfactants have been reported to maintain relatively low interfacial tension over a wide range of salinity and temperature, indicating favorable reservoir adaptability [ 10 – 12 ] . Existing studies have also demonstrated the feasibility of alkali-free petroleum sulfonate systems in terms of interfacial behavior, emulsification, salt and calcium tolerance, and structural characterization, while also revealing their compositional complexity and the difficulty of establishing accurate structure-property relationships [ 24 ] . Early studies on alkali-free systems mainly focused on achieving ultralow interfacial tension, because low interfacial tension has long been regarded as a key requirement for chemical EOR [ 8 ] . Indeed, through appropriate surfactant selection, formulation design, and salinity matching, alkali-free systems can still achieve ultralow interfacial tension at the 10^-3 mN/m level or even lower within a certain salinity range, and can deliver promising EOR performance in both laboratory and field tests. However, recent studies have shown that the effectiveness of chemical flooding systems depends not only on ultralow interfacial tension, but also on adsorption control, formulation stability, and the synergistic operation of multiple recovery mechanisms under reservoir conditions [ 13 ] . It is now widely recognized that ultralow interfacial tension alone cannot fully determine oil-displacement performance [ 14 ] . During the displacement process, crude-oil emulsification and dispersion [ 15 ] , interfacial-film properties [ 16 , 17 ] , and the deformation and transport of emulsion droplets in porous media can all significantly affect residual-oil mobilization and final recovery efficiency [ 18 ] . Accordingly, emulsification performance has increasingly been incorporated, together with interfacial tension, into the screening and evaluation of alkali-free systems [ 19 ] . Some newly developed alkali-free binary systems have also shown improved displacement performance beyond simple interfacial-tension reduction [ 20 ] . Similarly, alkali-free SP systems formulated with commercial surfactants such as AES and ABS have been reported to achieve ultralow interfacial tension and markedly improve oil-displacement efficiency under appropriate salinity conditions [ 21 ] . In the Daqing Oilfield, an alkali-free ternary system in which sodium chloride replaced sodium carbonate was reported to maintain relatively high displacement efficiency while alleviating scaling-related problems in the injection-production system, indicating good potential for field application [ 22 ] . These studies suggest that the evaluation of alkali-free systems should move beyond a sole emphasis on interfacial-tension reduction toward a more balanced consideration of both interfacial behavior and emulsification stability [ 23 ] . Nevertheless, although single petroleum sulfonate systems can achieve ultralow interfacial tension under specific conditions, they still show limitations in maintaining ultralow interfacial tension over a broad range, in formulation adaptability, and in the fine regulation of emulsion or microemulsion structures under alkali-free conditions. As a result, such systems may have difficulty simultaneously satisfying the multiple requirements of residual-oil mobilization, stripping, and transport in porous media. Therefore, the development of blended systems with stronger interfacial-regulation capability is an important strategy for improving the overall displacement performance of alkali-free petroleum sulfonate systems. In this context, extended surfactants [ 25 ] have attracted increasing interest. These surfactants, in which propylene oxide (PO) and/or ethylene oxide (EO) units are introduced between the hydrophobic chain and the hydrophilic headgroup, exhibit more flexible interfacial adsorption configurations and stronger interfacial-regulation capability [ 26 ] . Previous studies have shown that extended surfactants favor the formation of microemulsion systems with high solubilization capacity and ultralow interfacial tension, often at relatively low surfactant concentrations [ 27 ] . In addition, their molecular structure, particularly the alkyl-chain length and the composition of the PO/EO segments, plays an important role in interfacial adsorption, interfacial-tension reduction, and microemulsion formation [ 28 ] . Therefore, introducing extended surfactants into alkali-free petroleum sulfonate systems may help overcome the limitations of single petroleum sulfonate systems in fine interfacial regulation and synergistic oil displacement across multiple scales [ 29 ] , providing a potentially effective route to balancing cost effectiveness and displacement performance. Despite these advances, most current studies still remain at the level of formulation optimization and phenomenological performance comparison. The intrinsic links among interfacial regulation, emulsification enhancement, and pore-scale transport have not yet been systematically clarified. In particular, the coupled mechanisms by which interfacial-film regulation, microemulsion formation, and droplet transport through pore throats synergistically contribute to enhanced oil recovery remain insufficiently understood [ 30 , 31 ] . In this study, a series of extended surfactants was designed and synthesized based on the equivalent alkane carbon number (EACN) characteristics of the target crude oil and blended with petroleum sulfonate to construct alkali-free flooding systems. Ultralow interfacial tension was treated as a prerequisite rather than the sole screening criterion, and the formulation was further optimized by combining interfacial activity with emulsion-stability-related indicators. The optimized alkali-free system was then compared with a weak-alkali ASP system in terms of interfacial behavior, emulsion stability, flooding performance, and produced-fluid characteristics. On this basis, the possible origins of its displacement advantage were analyzed from the perspectives of interfacial regulation, emulsification behavior, droplet transport, and component-migration differences. 2 Experimental Section 2.1 Experimental Materials Chemicals The experimental chemicals included a series of extended surfactants synthesized in-house; refining petrochemical petroleum sulfonate (WPS, active content 40%, Daqing Refining & Chemical Company); sodium chloride (NaCl) and sodium carbonate (Na 2 CO 3 , used only in the weak-alkali ASP control system), both of analytical reagent grade (AR, Tianjin Damao Chemical Reagent Factory); partially hydrolyzed polyacrylamide (HPAM, relative molecular weight approximately 2.5×10 7 , active content 90%, industrial grade, Daqing Refining & Chemical Company); n-Alkanes C8–C15 were supplied by Shandong Keyuan Biochemical. The TRS anionic surfactant, with an active content of 40%, was obtained from Daqing Refining & Chemical Company. Oil. The oil used in the experiments was crude oil collected from the target reservoir interval of a block in the Daqing Oilfield, with a dead-oil viscosity of 33 mPa·s at 45°C. Simulated oil was prepared by diluting the crude oil with aviation kerosene, yielding a viscosity of 10 mPa·s at 45°C. Water. The experimental water consisted of the actual reinjection water and formation water from the target reservoir interval of a block in Daqing. Their ionic compositions are listed in Table 1 and Table 2 . Table 1 Ionic composition of reinjection wastewater. Na + K + Ca 2+ Mg 2+ HCO 3- SO 4 2- Cl - Total salinity Ion concentration (mg/L) 3350 5.489 20.2 42.1 2049.3 96.6 1237.7 6798.87 Table 2 Ionic composition of the formation water. Na + K + Ca 2+ Mg 2+ HCO 3- SO 4 2- Cl - Total salinity Ion concentration (mg/L) 2183.821 10.489 23.109 66.6798 2053.857 77.086 2363.213 6778 Oil sand. The experimental oil sand was obtained from the SII reservoir group in the target reservoir interval of a block in the Daqing Oilfield, with a particle size distribution of 100–200 mesh. Core samples. Epoxy-resin-cast Berea cores with dimensions of 4.5 cm × 4.5 cm × 30 cm were used in the experiments. The detailed core properties are summarized in Table 6 . Table 6 Core flooding results of different flooding systems. Flooding system Core No. Water permeability (mD) Oil saturation (%) Waterflood recovery (%) Chemical flooding recovery (%) Average chemical flooding recovery (%) Polymer-only system 230204-1 214.7 64.52 39.63 12.07 11.9 230204-2 206.2 63.88 40.12 11.8 230204-3 199.2 64.2 40.0 11.86 Single C12P9E3S system 230501-1 214.2 63.89 40.37 20.12 20 230501-2 224.3 64.03 39.69 20.3 230501-3 179.4 63.54 39.13 19.6 Single WPS system 230601-1 225.3 63.88 38.9 20.41 21.15 ± 0.69 230601-2 216.4 64.12 39.4 21.23 230601-3 220.0 63.65 40.22 21.8 Alkali-free WPS/C12P9E3S formulated system 230723-1 216.5 64.25 39.13 37.27 36.72 230723-2 186.5 63.75 41.88 35.88 230723-3 203.4 64.12 40.22 37.03 Weak-alkali ASP system 230308-1 210.5 65.06 39.42 30.99 30.92 230308-2 196.5 64.54 40.02 30.2 230308-3 208.4 64.86 39.33 31.58 2.2 Design of the target surfactant 2.2.1 Crude oil characterization and EACN analysis To achieve a better match between the molecular structure of the surfactant and the interfacial behavior of the target crude oil, the average molecular weight, compositional characteristics, and equivalent alkane carbon number (EACN) of the target crude oil were first determined and used as the basis for the design of the extended surfactant. The average molecular weight and compositional characteristics of the crude oil from the target block were measured in accordance with GB/T 17282 − 2012. The EACN of the crude oil was determined with reference to the method proposed by Huang Yanhua et al [ 32 ] . An anionic surfactant solution with a mass fraction of 0.3% was prepared using produced water, and the NaCl concentration in the system was adjusted to 1.5%. Under a temperature of 45°C, the dynamic interfacial tension between the target crude oil and different n-alkane systems was measured using the spinning-drop method. Each experiment was conducted for 1 h at a rotational speed of 5000 r/min with an oil-drop volume of 1 µL, and the interfacial tension was recorded every 10 min. A variation of no more than 10% within 60 min was taken as the criterion for quasi-equilibrium. Meanwhile, the minimum interfacial tension observed during the measurement was also recorded to provide a supplementary evaluation of the dynamic interfacial tension reduction performance of the system. $$\:{EACN}_{O}=[{\left(EACN\right)}_{mo}{x}_{mo}-{\left(EACN\right)}_{l}{x}_{l}]/{x}_{o}$$ 1 Where: \(\:{EACN}_{mo}\) —is the equivalent alkane carbon number of the mixed oil sample; \(\:{EACN}_{l}\) —is the carbon number of the n-alkane; \(\:{EACN}_{O}\) —is the equivalent alkane carbon number of the crude oil; \(\:{X}_{l}\) —is the mole fraction of the n-alkane; \(\:{X}_{mO}\) —is the mole fraction of the mixed oil sample; \(\:{X}_{O}\) —is the mole fraction of the crude oil. 2.3 Synthesis and characterization of the target surfactant 2.3.1 Synthesis of the extended surfactant The synthesis of C12P9E3S is taken as a representative example. The preparation procedure involved alkoxylation, sulfation, and subsequent neutralization/purification. In the alkoxylation step, 93.0 g of dodecanol and 5.0 g of catalyst were charged into a 500 mL high-pressure reactor. The reactor was heated to 130°C, and propylene oxide and ethylene oxide were introduced sequentially at a molar ratio of dodecanol:propylene oxide:ethylene oxide = 1:9:3. After completion of the reaction, the mixture was cooled for 5 h to obtain the intermediate alkoxylated product. The intermediate was then subjected to sulfation. For this purpose, 18 g of the ether intermediate and an equal mass of dry 1,2-dichloroethane were added to a four-neck flask equipped with an off-gas absorption device. Under ice-water-bath cooling and mechanical stirring, chlorosulfonic acid was slowly added dropwise to the reaction system. After the addition was completed, the reaction mixture was neutralized with sodium hydroxide solution to approximately pH 9 to suppress hydrolysis. The resulting product was subsequently purified. Most inorganic salts were removed by centrifugation, and the solvent was removed by rotary evaporation. Residual inorganic salts were further removed with anhydrous ethanol, followed by rotary evaporation to remove ethanol and obtain the final product. The active matter content of the product was determined by the two-phase titration method. The reaction scheme is shown in Fig. 1 . (1) Reaction scheme for the synthesis of C12P9E3: 2.3.2 Structural characterization of the series of extended surfactants The samples were characterized by Fourier transform infrared spectroscopy (FTIR) using a Nicolet iS20 spectrometer (Thermo Fisher, USA). The measurements were performed in ATR mode over a scanning range of 4000–450 cm⁻¹, with 32 scans. 1H NMR spectra were recorded on a Bruker AVANCE 600 MHz nuclear magnetic resonance spectrometer (Bruker, Germany), using CDCl 3 as the solvent and tetramethylsilane (TMS) as the internal standard at 298 K. 2.3.3 Performance evaluation of single-surfactant systems The interfacial tension of the single-surfactant systems was measured by the spinning-drop method. The test temperature was 45°C, the measurement duration was 1 h, the rotational speed was 5000 r/min, and the oil-drop volume was 1 µL. Interfacial tension was recorded every 10 min, and both the quasi-equilibrium interfacial tension and the minimum interfacial tension observed during the measurement were reported. Phase behavior and emulsion stability were preliminarily evaluated by salt-concentration scanning. Specifically, 2 mL of surfactant solution with a concentration of 0.3 wt% was prepared, with the NaCl concentration set at 0.4%, 0.8%, 1.2%, 1.6%, 2.0%, 2.5%, and 3.0%, respectively. Then, 2 mL of simulated oil was added to each sample to obtain an oil/water volume ratio of 1:1. The samples were mixed on a rotating mixer at 2 r/min for 24 h and then allowed to stand. The phase behavior and oil-water separation were recorded after 0, 1, 3, 7, 15, 30, and 60 d. 2.4 Screening of the alkali-free formulated flooding system The formulation was screened by taking ultralow interfacial tension as a necessary prerequisite, while further considering emulsion stability, water-separation behavior, TSI value, and surfactant dosage. In this way, the selected system was not determined by the minimum interfacial tension alone, but by a combined evaluation of interfacial activity and emulsion-related performance. First, the interfacial and phase behaviors of the individual n-C12P9EnS (n = 3, 6, 9) were evaluated to identify the target extended surfactant. The selected extended surfactant was then blended with WPS at different ratios, followed by a multifactorial screening of salinity and total surfactant concentration. During the screening process, under the premise of maintaining low interfacial tension, priority was given to systems with a relatively wide emulsification window, higher emulsion stability, and lower total surfactant dosage, and the optimal formulation was determined accordingly. 2.5 Comparison of the flooding system performance To ensure comparability among different flooding systems, five types of flooding formulations were prepared in this study: a polymer-only system, a single C12P9E3S system, a single WPS system, an alkali-free WPS/C12P9E3S formulated system, and a weak-alkali ASP system. In all systems except the polymer-only system, the HPAM concentration was fixed at 1600 mg/L. In the polymer-only system, the polymer concentration was adjusted to 1400 mg/L to maintain a comparable viscosity. At 45°C, the viscosity of all systems was controlled at approximately 38 mPa·s. The polymer protection slug was prepared at a polymer concentration of 1400 mg/L, with a viscosity of approximately 38 mPa·s. These five systems were mainly used for staged contribution analysis in core flooding experiments to distinguish the respective contributions of mobility control, the effect of individual surfactants, and the synergistic effect of surfactant formulation to oil recovery. Among them, the alkali-free WPS/C12P9E3S formulated system and the weak-alkali ASP system were further selected for representative comparison in terms of interfacial performance, emulsion stability, and anti-adsorption behavior. The detailed formulations are listed in Table 3 . Table 3 Chemical compositions of the different flooding systems. system Surfactant composition Polymer (HPAM) Salinity / alkali polymer-only system / 1400 mg/L 1.6 wt% NaCl Single C12P9E3S system 0.3 wt% C12P9E3S 1600 mg/L HPAM 1.6 wt% NaCl Single WPS system 0.3 wt% WPS 1600 mg/L HPAM 1.6 wt% NaCl Alkali-free WPS/C12P9E3S formulated system 0.2 wt% WPS + 0.1 wt% C12P9E3S 1600 mg/L HPAM 1.6 wt% NaCl Weak-alkali ASP system 0.3 wt% WPS 1600 mg/L HPAM 1.2 wt% Na 2 CO 3 Note: HPAM = partially hydrolyzed polyacrylamide. 2.5.1 Formulations of the flooding systems 2.5.2 Emulsion stability Each of the two system solutions was mixed with the simulated oil at a volume ratio of 1:1. The mixtures were rotated at 2 r/min for 24 h using a rotary mixer and then transferred into a 30 cm-high test bottle. The transmittance and backscattering profiles of the emulsions were analyzed using a Turbiscan Lab stability analyzer. The scanning time was 2 h, and the test temperature was maintained at 45°C. The Turbiscan Stability Index (TSI) was automatically calculated to evaluate emulsion stability. Each experiment was performed in triplicate. 2.5.3 Interfacial tension measurement Interfacial tension was measured by the spinning-drop method at 45°C for 1 h. The quasi-equilibrium interfacial tension and the minimum instantaneous interfacial tension were selected as the effective evaluation parameters. 2.5.4 Emulsification performance of the systems Each of the two system solutions was mixed with the simulated oil at a volume ratio of 1:1 and rotated at 2 r/min for 24 h using a rotary mixer. The mixtures were then placed in a constant-temperature oven at 45°C and allowed to stand. At fixed time intervals of 30 min, the volume of separated water was observed and recorded. The water separation ratio was then calculated based on the cumulative volume of separated water over time. Each experiment was conducted in triplicate. 2.5.5 Zeta potential of the systems An appropriate amount of the test system was added to the sample cell of the instrument, and the zeta potential of the emulsion was measured under the preset temperature and instrumental parameters. Each sample was measured three times, and the average value was taken as the final result. 2.5.6 Anti-adsorption performance of the systems The anti-adsorption performance was evaluated by a multiple adsorption test using oil sand. Specifically, 20 g of the target oil sand was weighed, and the test flooding system solution was added at a solid-to-liquid mass ratio of 1:9. After shaking at 45°C for 24 h, the supernatant was collected, and its interfacial tension against crude oil was measured. The entire supernatant was then transferred to an equal amount of fresh oil sand, and the above procedure was repeated. The variation in interfacial tension of the system after multiple adsorption cycles was used to characterize its anti-adsorption capacity. 2.6 Laboratory oil displacement experiments and produced-fluid analysis 2.6.1 Laboratory oil displacement experiments Epoxy-resin-cast Berea cores from the same batch, with water-phase permeability in the range of 150–200 mD, were used in the core flooding experiments. The cores were vacuumed for 8 h using a vacuum pump and then saturated with formation water from the target reservoir. The ionic composition of the formation water is listed in Tables 1 and 2 . Simulated oil was injected at a constant rate of 0.1 mL/min until no water was produced from the outlet end of the core. The core was then aged in a constant-temperature oven at 45 ± 0.5°C for 24 h. Water flooding was subsequently conducted at a flow rate of 0.3 mL/min until the water cut at the outlet reached 100%. Thereafter, 0.3 PV of the main flooding slug and 0.2 PV of the polymer protection slug were injected, followed by subsequent water flooding until the water cut exceeded 98%. Each experiment was repeated three times, and the results are reported as the average of the three parallel runs. 2.6.2 Produced-fluid analysis The produced-fluid analysis included measurements of salinity, polymer concentration, and surfactant concentration. For salinity determination, 1 mL of the lower aqueous phase of the produced fluid was diluted tenfold with 9 mL of distilled water and mixed thoroughly. The conductivity was then measured, and the measured conductivity value was multiplied by 10 and substituted into the NaCl conductivity calibration curve to determine the salt concentration. For polymer concentration determination, a 721 UV-Vis spectrophotometer was used to measure the absorbance at 470 nm. The measured absorbance value was multiplied by 10 and substituted into the polymer concentration–absorbance calibration curve to obtain the polymer concentration. For surfactant concentration determination, the two-phase titration method was employed. At the titration endpoint, the lower phase appeared light gray. After the addition of two drops of Hyamine solution, the lower phase turned light blue. The volume of Hyamine solution consumed was recorded and used to calculate the active surfactant content. 3 Results 3.1 Determination of the EACN of the target crude oil and basis for the design of the target surfactant To achieve structural matching between the surfactant and the interfacial behavior of the target crude oil, the equivalent alkane carbon number (EACN) of the target crude oil was determined by an indirect method. Under the conditions of 45°C, 0.3 wt% anionic surfactant, and 1.5 wt% NaCl, the interfacial tension (IFT)-time curves of the target crude oil against n-alkanes from C8 to C15 showed that the system reached the lowest quasi-equilibrium IFT when the alkane carbon number was C12. Accordingly, the minimum IFT was obtained at C12, as shown in Fig. 2 a,b. According to measurements performed three times in accordance with GB/T 17282 − 2012, the average molecular weight of the crude oil was 453.5 g/mol. The EACN was further back-calculated using crude oil/n-alkane mixed oil samples. When the volume ratio of crude oil to n-pentadecane was 9:1, the IFT between the mixed oil sample and the aqueous phase reached a minimum value of 0.0033 mN/m, as shown in Fig. 2 (c). Under this condition, (EACN) l = 15, (EACN) mo = 12, x l = 0.1, x o = 0.9, x mo = 1. Substitution into Eq. ( 1 ) gave an EACN value of approximately 11.67 (about 12). This result provided the basis for the subsequent design of extended surfactants using a C12-based hydrophobe. 3.2 Structural characterization of the extended surfactants 3.2.1 FTIR characterization of n-C12P9EnS (n = 3, 6, 9) As shown in Fig. 3 (a), taking C12P9E3S as an example, the absorption peaks at approximately 2931.7cm − 1 and 2846.8cm − 1 are assigned to the stretching vibrations of − C−H, while the peak near 3493cm − 1 corresponds to the stretching vibration of − O−H. The peak at around 1092.4 cm − 1 is attributed to the stretching vibration of − C−O − S−, and the peak near 1321.9cm − 1 is assigned to the stretching vibration of S = O, indicating that sulfation was successfully achieved. In addition, the peak near 721.1cm − 1 , corresponding to the stretching vibration of S − O, further suggests that the target product, C12P9E3S was successfully synthesized. As shown in Fig. 3 (b) and Table 4 , the actual numbers of added PO and EO units in the n-C12PmEnS series were generally consistent with the theoretical design values. Among the synthesized surfactants, n-C12P9E3S exhibited the closest agreement with the target structure, indicating relatively good structural controllability and supporting its use in the subsequent evaluation of interfacial and oil-displacement properties. Table 4 Average numbers of added PO and EO units in n-C12PmEnS molecules. n-C12PmEnS Number of PO units (m) Number of EO units (n) n-C12P9E3S 9 3 n-C12P9E6S 9 6 n-C12P9E9S 9 9 3.3 Screening of the alkali-free formulated system Using ultralow interfacial tension on the order of 10 − 3 mN/m as a prerequisite, the formulation was screened comprehensively based on the width of the emulsification window, water separation ratio, TSI value, and total surfactant dosage. On this basis, the optimal formulation was determined to have a WPS/C12P9E3S mass ratio of 2:1 and an NaCl concentration of 1.6 wt%. 3.3.1 Performance evaluation of the individual n-C12PmEnS(m = 9, n = 3, 6, 9) The interfacial tension and phase behavior of the individual n-C12PmEnS (m = 9, n = 3, 6 ,9) were first evaluated. As shown in Fig. 4 , none of the three single-surfactant systems achieved ultralow interfacial tension on the order of 10 − 3 mN/m, among them, the C12P9E3S system exhibited relatively lower interfacial tension over a relatively wide salinity range and showed the widest salinity range for emulsification. Therefore, C12P9E3S was selected as the target extended surfactant for subsequent formulation with WPS. 3.3.2 Emulsification performance of the WPS/C12P9E3S formulated systems As shown in Fig. 5 , to examine whether formulation with C12P9E3S could compensate for the limited emulsification performance of WPS under alkali-free conditions, the salinity-scan phase behavior results of WPS and its mixtures with C12P9E3S at different ratios were compared. The results showed that, at an oil/water ratio of 1:1, the formulated systems with a WPS/C12P9E3S ratio below 2:1 exhibited a wider salinity range for emulsification and a higher emulsion volume fraction. In addition, the emulsification performance was further improved at lower WPS proportions. Therefore, the WPS/C12P9E3S formulated system was selected for subsequent investigation. 3.3.3 Effect of the WPS/C12P9E3S mixing ratio on the interfacial activity of the formulated systems As shown in Fig. 6 (a), the alkali-free WPS/C12P9E3S system at a mixing ratio of 2:1 exhibited a relatively wide ultralow-interfacial-tension region. Interfacial tension on the order of 10 − 3 mN/m was achieved over a total surfactant concentration range of 0.1–0.3 wt% and an NaCl concentration range of 0.8–2.0 wt%. By comparison, as shown in Fig. 6 (b), the alkali-free system at a WPS/C12P9E3S ratio of 1:1 showed a narrower ultralow-interfacial-tension region, reaching the 10 − 3 mN/m level only when the total surfactant concentration was 0.1–0.3 wt% and the NaCl concentration was 1.8–2.0 wt%. Considering both emulsification performance and interfacial tension, the optimal alkali-free formulated system was finally determined to be WPS/C12P9E3S = 2:1 with an NaCl of 1.6 wt%. 3.3.4 Micromorphology of the alkali-free WPS/C12P9E3S formulated system Under the optimal conditions, the microstructure of the alkali-free WPS/C12P9E3S formulated system was further examined. As shown in Fig. 7 , the upper phase was dominated by a water-in-oil structure, whereas the lower phase was dominated by an oil-in-water structure. The middle phase exhibited localized water-in-oil and oil-in-water morphologies simultaneously, together with bicontinuous characteristics. These observations suggest that the optimal formulated system has the potential to form a complex microemulsion structure at the corresponding salinity, which may provide a phase-behavior basis for the attainment of relatively low interfacial tension and relatively high emulsion stability. 3.4 Comparison of the interfacial and emulsification properties between the alkali-free formulated system and the weak-alkali ASP system To compare the alkali-free formulated system with the engineering baseline system, the interfacial performance, emulsion stability, and anti-adsorption behavior of the alkali-free WPS/C12P9E3S formulated system and the weak-alkali ASP system were comparatively analyzed under the same temperature, the same oil/water ratio, the same test methods, and comparable viscosity-matching conditions. 3.4.1 Comparison of the interfacial performance of the flooding systems As shown in Fig. 8 , comparison of the interfacial-tension windows of the two systems under the same temperature, oil/water ratio, and viscosity-matching conditions indicates that the weak-alkali ASP system could achieve ultralow interfacial tension over a wider alkali-concentration range, and under some conditions the minimum interfacial tension reached the 10 − 4 mN/m level. In contrast, the alkali-free WPS/C12P9E3S formulated system mainly achieved ultralow interfacial tension on the order of 10 − 3 mN/m. These results suggest that, in terms of interfacial tension reduction alone, the weak-alkali ASP system still has a certain advantage. Accordingly, if the alkali-free formulated system shows superior oil-displacement performance overall, its advantage is more likely to arise from other factors, such as emulsion stability and transport behavior in porous media, rather than being determined solely by the minimum interfacial tension. 3.4.2 Comparison of the emulsion stability of the flooding systems As shown in Fig. 9 (a-c), the results of water separation ratio, TSI, and zeta potential indicate that the emulsion stability of the alkali-free WPS/C12P9E3S formulated system was clearly superior to that of the weak-alkali ASP system. Within 300 min, the water separation ratio of the formulated system increased from 0% to 49%, which was markedly lower than the 74% observed for the weak-alkali ASP system. Meanwhile, the formulated system exhibited consistently lower TSI values throughout the test period, indicating stronger macroscopic stability. In addition, the formulated system showed a larger absolute zeta potential, suggesting stronger electrostatic repulsion between emulsion droplets and a greater resistance to coalescence. These results indicate that although the alkali-free formulated system did not show an advantage in terms of minimum interfacial tension, it exhibited a clear advantage in emulsion stability, which may provide an important basis for sustained emulsification and oil transport during subsequent displacement. 3.4.3 Comparison of the anti-adsorption performance of the flooding systems As shown in Table 5 , after five adsorption cycles on oil sand, the interfacial tension (IFT) of both systems increased to the 10 − 2 mN/m level, and the difference between the two systems became relatively small after the fifth adsorption cycle. The IFT of the alkali-free WPS/C12P9E3S formulated system was 1.904×10 − 2 mN/m, compared with 1.688×10 − 2 mN/m for the weak-alkali ASP system. These results indicate that the alkali-free WPS/C12P9E3S formulated system and the weak-alkali ASP system showed only a limited difference in resistance to adsorption on oil sand. This suggests that the improvement in oil-displacement efficiency of the alkali-free WPS/C12P9E3S formulated system was not mainly attributable to enhanced anti-adsorption performance, but was more likely associated with its superior emulsion stability, weaker chromatographic separation, and more favorable matching between emulsion droplets and pore throats. Table 5 Interfacial tension after repeated adsorption on oil sand. Flooding system Interfacial tension (mN/m) Before adsorption After 1 adsorption cycle After 2 adsorption cycles After 3 adsorption cycles After 4 adsorption cycles After 5 adsorption cycles Alkali-free WPS/C12P9E3S formulated system 1.38×10 − 3 2.8×10 − 3 1.87×10 − 3 5.7×10 − 3 8.5×10 − 3 1.9×10 − 2 Weak-alkali ASP system 6.7×10 − 4 9×10 − 4 2×10 − 3 6.8×10 − 3 7.6×10 − 3 1.62×10 − 2 3.5 Core flooding and produced-fluid characterization 3.5.1 Results of the core flooding experiments To distinguish the respective contributions of mobility control, individual surfactant action, and formulation synergy to oil recovery, the core flooding results of the polymer-only system, the single C12P9E3S system, the alkali-free formulated system, and the weak-alkali ASP system were further compared. As shown in Table 6 , all four systems were able to further improve oil recovery after water flooding, although the incremental recovery beyond water flooding differed markedly among the systems. As summarized in Table 7 , the polymer-only system provided an average incremental recovery of 11.9% over water flooding, indicating that mobility control plays a fundamental role in enlarging the swept volume. The single C12P9E3S system yielded an average incremental recovery of 20.0% over water flooding, representing a further increase of 8.1 percentage points compared with the polymer-only system. The single WPS system gave an average incremental recovery of 21.15% over water flooding, which was 9.24 percentage points higher than that of the polymer-only system. These results indicate that the action of an individual surfactant can further mobilize part of the residual oil on the basis of mobility control, although the magnitude of the improvement remains limited. By contrast, the alkali-free WPS/C12P9E3S formulated system increased oil recovery by an additional 16.72 percentage points relative to the single C12P9E3S system and by 15.57 percentage points relative to the single WPS system. This suggests a pronounced synergistic effect in the formulated system, and its oil-displacement performance cannot be explained by any single mechanism alone. Table 7 Incremental oil recovery and staged contribution analysis for different flooding systems. Flooding system Chemical-flooding incremental recovery (%) Average chemical flooding recovery (%) Relative to polymer flooding (%) Relative to the single C12P9E3S system (%) Relative to the single WPS system (%) Polymer-only system 12.07 11.9 ± 0.14 / / / 11.8 11.86 Single C12P9E3S system 20.12 20 ± 0.36 8.1 / / 20.3 19.6 Single WPS system 20.41 21.15 ± 0.69 9.24 / / 21.23 21.8 Alkali-free WPS/C12P9E3S formulated system 37.27 36.72 ± 0.74 24.82 16.72 15.57 35.88 37.03 Weak-alkali ASP system 30.99 30.92 ± 0.69 19.02 10.92 9.77 30.2 31.58 In addition, the alkali-free WPS/C12P9E3S formulated system achieved a further 5.80 percentage-point increase in oil recovery compared with the weak-alkali ASP system. This result indicates that, even under alkali-free conditions, the formulation of petroleum sulfonate with an extended surfactant can still deliver substantial incremental oil recovery and shows the potential to compete with the conventional weak-alkali ASP system. 3.5.2 Injection pressure and flow-resistance characteristics As shown in Fig. 10 (a), the injection-pressure curves indicate that the polymer-only system exhibited the highest peak pressure, followed by the weak-alkali ASP system, whereas the alkali-free WPS/C12P9E3S formulated system showed the lowest and most stable pressure throughout the flooding process. During the entire displacement stage, the injection pressure of the WPS/C12P9E3S = 2:1 system remained consistently lower than that of the weak-alkali ASP system. The maximum pressure of the former during flooding was 0.4 MPa, which was 0.5 MPa lower than that of the latter. As shown in Fig. 10 (b) and Fig. 11 , the water-cut curves and observations of the produced fluids further indicate that the alkali-free WPS/C12P9E3S formulated system maintained a longer low-water-cut stage and a more sustained emulsification-and-transport process. Its minimum water cut was 50.91%, approximately 15% lower than that of the weak-alkali ASP system. In addition, during the subsequent waterflooding stage, emulsification in the formulated system persisted for about 0.32 PV, compared with about 0.24 PV for the weak-alkali ASP system, corresponding to an extension of approximately 0.08 PV in emulsification-assisted transport. No obvious emulsification was observed in the polymer-only system, the single C12P9E3S system, or the single WPS system. These results suggest that the alkali-free WPS/C12P9E3S formulated system was more favorable for maintaining oil droplets in a dispersed state and for stable transport by the aqueous phase over a wider pore-volume range, thereby prolonging the effective window for mobilizing residual oil. Combined with the subsequent droplet-size analysis, it may be inferred that the lower injection pressure of this system is related to a more favorable matching relationship between emulsion droplets and pore throats. 3.5.3 Relative concentration distribution of produced fluids The relative concentration profiles of the chemical components in the produced fluids indicate that, in the alkali-free WPS/C12P9E3S = 2:1 system, the peak positions of the different components were closer to one another, suggesting a smaller difference in component retardation during transport through porous media. A smaller difference in the PV corresponding to the concentration peaks generally indicates better component synchrony and a greater likelihood of maintaining an effective formulation ratio. As shown in Fig. 12 (a), for the WPS/C12P9E3S formulated system, the concentration peaks of polymer, surfactant, and salt appeared at 1.04, 1.12, and 0.97 PV, respectively, with a maximum difference of 0.15 PV. As shown in Fig. 12 (b), in contrast, the corresponding peak positions in the weak-alkali ASP system differed by approximately 0.24 PV. This suggests that the alkali-free WPS/C12P9E3S formulated system may exhibit a smaller retardation difference among components. In addition, the interfacial tension of the produced fluid from the formulated system decreased to the 10 − 3 mN/m level over the range of 0.9–1.2 PV, whereas the lowest value observed for the weak-alkali ASP system reached only the 10 − 2 mN/m level. This result suggests that the formulated system may be more favorable for maintaining an effective component ratio over a wider pore-volume range. 4 Discussion Based on the above comparative results, the displacement advantage of the alkali-free system is discussed here from a coupled interfacial-transport perspective. In particular, four mutually related aspects are considered: interfacial adsorption behavior, emulsification enhancement, droplet-size-related transport behavior, and differences in component migration. 3.6.1 Proposed interfacial adsorption scenario One important reason why the alkali-free WPS/C12P9E3S formulated system may achieve enhanced oil-displacement efficiency is that it appears to exhibit interfacial behavior at the oil-water interface that differs from that of the weak-alkali ASP system [ 33 ] . As illustrated in Fig. 13 , WPS possesses strong interfacial activity owing to the pronounced hydrophilicity of its sulfonate groups and the lipophilicity of its hydrocarbon chains, allowing it to adsorb rapidly at the oil-water interface and thereby produce an initial reduction in interfacial tension. However, because petroleum sulfonate is compositionally complex, the adsorption film formed at the interface is often relatively loose and of limited stability [ 34 ] . C12P9E3S, as a typical extended surfactant, contains both PO and EO segments in its molecular structure. The intermediate PO/EO chains are conducive to regulating the molecular packing at the interface, thereby improving the structure of the interfacial layer [ 35 , 36 ] . Previous studies have shown that extended surfactants can exhibit adsorption behavior at the oil-water interface that differs from that of conventional anionic surfactants because of their distinctive molecular architecture, and can achieve relatively low interfacial tension under suitable salinity and oil-phase conditions [ 37 ] . However, the comparative results of this study show that although the weak-alkali ASP system has an advantage in achieving lower interfacial tension, the alkali-free WPS/C12P9E3S composite system exhibits higher oil recovery and better emulsion stability. This indicates that, for the systems investigated in this work, the oil-displacement performance is not determined solely by the minimum interfacial tension. Unlike many previous studies that have treated ultralow interfacial tension as the primary evaluation criterion for alkali-free systems, the present results further suggest that differences in interfacial behavior and their subsequent effects on emulsification and displacement processes deserve particular attention. 3.6.2 Emulsification Enhancement and Crude Oil Entrapment–Transport In addition to interfacial tension reduction, emulsification also plays a key role in the oil-displacement process of composite flooding systems [ 38 ] . The formation and stability of emulsions are influenced by multiple factors, including surfactant structure, polymer presence, and flow conditions [ 39 ] . Recent pore-scale visualization studies have further shown that in situ emulsification is not merely a static bottle-test phenomenon, but directly participates in flow redistribution and residual oil mobilization in porous media [ 40 ] . As shown in Fig. 9 , the water separation ratio and TSI value of the alkali-free WPS/C12P9E3S composite system are significantly lower than those of the weak-alkali ASP system, indicating stronger emulsion stability. Meanwhile, its higher absolute zeta potential suggests stronger electrostatic repulsion between emulsion droplets, thereby effectively inhibiting droplet coalescence. In this system, crude oil can be dispersed into fine droplets and remain stably suspended in the aqueous phase, enabling a continuous process of “emulsification–entrainment–transport” [ 16 ] . In addition, as shown in Fig. 11 , the emulsification duration of the alkali-free WPS/C12P9E3S composite system is approximately 0.32 PV, which is clearly longer than the 0.24 PV observed for the weak-alkali ASP system. This suggests that, compared with studies limited to static comparisons of emulsification performance, the present work further relates emulsion stability to dynamic displacement responses. In other words, improved emulsion stability is reflected not only in slower water separation or lower TSI values in bottle tests, but may also translate into a more sustained emulsification-assisted transport process during flooding, thereby helping maintain crude oil dispersion and migration over a longer displacement interval. 3.6.3 Relationship Between Produced-Fluid Emulsion Droplet Size and Pore-Throat Matching As shown in Fig. 14 , the emulsion droplet-size statistics were obtained from microscopic image analysis of the produced fluids. One microscope image was selected at each PV point, and 50 droplets were counted in each image. Previous studies have shown that particle transport behavior in porous media depends on the matching relationship between particle size and pore-throat size, giving rise to different modes such as complete blockage, deformation-assisted passage, and ineffective passage [ 41 ] . More recent studies on emulsion transport in porous media have further demonstrated that the relationship between droplet size and pore-throat scale can significantly affect droplet passage, retention, and local flow resistance [ 42 – 44 ] .When the droplet size exceeds the pore-throat scale, flow resistance increases markedly; in contrast, droplets whose size is better matched to the pore throats are more likely to pass through stably. During the subsequent waterflooding stage of the alkali-free WPS/C12P9E3S composite system, the average size of emulsified oil droplets was mainly concentrated around 30 µm, with a standard deviation of 7 µm. By comparison, the emulsified oil droplets in the weak-alkali ASP system were mostly distributed around 73 µm, with a standard deviation of 8 µm. According to the representative pore-throat radius estimated by the Winland r35 equation [ 45 ] , Winland r35 ≈ 15.1 µm, corresponding to a representative pore-throat diameter of approximately 30 µm. Based on the matching relationship between droplet size and pore-throat diameter, the ratio of droplet diameter to pore-throat diameter in the WPS/C12P9E3S system (D/d ≈ 1) is close to unity, suggesting that the droplets can undergo elastic deformation and pass through pore throats under a relatively low pressure difference, thereby enabling continuous transport. In contrast, for the weak-alkali ASP system, D/d ≈ 2.1, indicating that the droplet size is substantially larger than the pore-throat diameter. Such droplets are more likely to induce pore-throat blockage, leading to additional injection-pressure drop, as shown in Figure . It can therefore be inferred that the former system is more likely to undergo deformation-assisted passage and continuous transport at lower flow resistance, whereas the latter is more prone to generating additional flow resistance. It should be emphasized that this study did not further compare displacement performance under conditions where droplet size is substantially smaller than the pore-throat scale. Therefore, the contribution of this work does not lie in establishing an optimal droplet-to-throat size ratio, but rather in extending droplet-size analysis from a comparison of absolute droplet size to an interpretation based on its relationship with the representative pore-throat scale, and in indicating that droplet sizes significantly larger than the pore-throat scale may be unfavorable for displacement efficiency. 3.6.4 Synergistic Transport of Components and the Mechanism of Sustained Interfacial Activity In multicomponent chemical flooding systems, the transport behavior of individual functional components in porous media is jointly affected by differences in molecular size, adsorption characteristics, and flow pathways. As a result, the migration velocities of different components may vary substantially, leading to spatial separation of components at the displacement front, i.e., the chromatographic separation effect [ 46 ] . In the present study, this effect played an important role in the flooding performance of the systems investigated. As shown in Fig. 11 , the relative concentration profiles of the produced fluids indicate that, in the alkali-free WPS/C12P9E3S composite system, the peak positions of the polymer, surfactant, and salt concentrations are closer to one another, and the maximum difference among these peak positions is smaller than that in the weak-alkali ASP system. This suggests a smaller difference in component retardation in porous media, corresponding to a weaker chromatographic separation effect. Greater synchronization among the components is conducive to maintaining the synergistic action of surfactant-induced interfacial tension reduction and polymer-based mobility control over a wider pore-volume range, thereby enhancing the persistence of interfacial activity within the effective displacement interval. Combined with the interfacial tension evolution of the produced fluids, the results further show that the alkali-free WPS/C12P9E3S composite system maintained relatively low interfacial tension over a wider range of injected pore volumes. This may be one of the important reasons why it achieved higher oil recovery than the weak-alkali ASP system. The contribution of this study lies more specifically in proposing, through the correspondence between component concentration peak positions in the produced fluids and the variation in interfacial tension, a process-response-based interpretation: namely, that smaller differences in component retardation may be one of the key factors contributing to the higher oil recovery of the alkali-free composite system. 3.6.5 Summary In summary, the enhanced oil recovery achieved by the alkali-free WPS/C12P9E3S composite system cannot be attributed to a single factor. Rather, based on the experimental results obtained in this study, the advantage of the alkali-free composite system can be interpreted not merely in terms of a “better formulation,” but more specifically in terms of stronger emulsion stability, a more suitable relationship between emulsion droplet size and pore-throat scale, and smaller differences in component retardation. It should be noted that the mechanistic analysis presented here is based mainly on macroscopic displacement behavior and indirect characterization results. The quantitative mechanisms governing the interfacial film microstructure and component transport behavior still require further verification through interfacial rheological measurements, microscopic visualization experiments, and numerical simulation. 5 Conclusion (1) Based on the characterization of crude oil from the target block, the equivalent alkane carbon number (EACN) was determined to be approximately 12. Accordingly, a series of extended surfactants, n-C12P9EnS (n = 3, 6, and 9), was synthesized, among which C12P9E3S showed the most favorable interfacial and emulsification performance. (2) An alkali-free oil displacement system based on petroleum sulfonate and C12P9E3S was developed. Under optimal conditions (WPS/C12P9E3S = 2:1, total surfactant concentration 0.3 wt%, NaCl concentration 1.6 wt%), the system achieved ultralow interfacial tension at the 10⁻³ mN/m level and exhibited good emulsification performance. (3) Compared with the weak-alkali ASP system, the alkali-free system showed similar anti-adsorption performance but significantly improved emulsion stability, as reflected by lower water separation ratios, lower TSI values, higher absolute zeta potential, and smaller emulsion droplet size. (4) Core flooding results showed that the alkali-free system achieved higher oil recovery. This improvement is mainly associated with enhanced emulsification and more favorable transport behavior in porous media, rather than the minimum interfacial tension alone. These results demonstrate that alkali-free systems can achieve effective oil displacement while reducing operational risks, showing good potential for field application. Declarations Funding This work was supported by the Natural Science Foundation of Heilongjiang Province (Grant No. LH2023D013). Author Contribution Author ContributionsJ.W. and Y.H. designed and performed the experiments; Y.Z. conceived the study, supervised the research, and wrote the manuscript; J.W. analyzed the data and prepared the figures; X.Y. contributed to data analysis and interpretation; L.C. and M.Z. provided materials and technical support; F.S. and Z.S. reviewed and edited the manuscript. All authors have read and approved the final manuscript. Data Availability The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request. References Guo, H. et al. ASP flooding: Theory and practice progress in China. J. Chem. 2017 , 2017(1), 8509563. 10.1155/2017/8509563 Guo, H. et al. Recent advances of alkali-surfactant-polymer flooding in China. In SPE Improved Oil Recovery Conference, ; SPE: D031S016R004. (2022). 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9477351","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":641533637,"identity":"5bf6622e-5caf-40c0-9654-ff77fef12ecd","order_by":0,"name":"Jingchun wu","email":"","orcid":"","institution":"Northeast Petroleum University","correspondingAuthor":false,"prefix":"","firstName":"Jingchun","middleName":"","lastName":"wu","suffix":""},{"id":641533638,"identity":"c4cff552-3601-4d61-9df0-d3681cb2e096","order_by":1,"name":"Yangyang hou","email":"","orcid":"","institution":"Northeast Petroleum University","correspondingAuthor":false,"prefix":"","firstName":"Yangyang","middleName":"","lastName":"hou","suffix":""},{"id":641533639,"identity":"1546d707-d5d3-48d3-aa11-45d113ef4cdb","order_by":2,"name":"yang zhao","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAx0lEQVRIiWNgGAWjYDCCA0CcUGFjx8/MfPgB8Vo+nElLlmxnSzMgWgvjzJbDjBvO8yhIEKWD73j6M2nehjRm48M8DAYMNTbRBLVInnmQJs27w4bP7DDvgQcMx9JyGwhpMbiRcEya90was9lhvgQDxobDxGhJbJPmbTvMuLmZx0CCSC3JbJIzgVo2MBOrRfLMM2YLUCBLHAYGcgIxfgGG2MMb4KjsP3z4wYcaG8JagFHPgoiOBMLKwcqYPxCncBSMglEwCkYsAABEiUPbLusvqgAAAABJRU5ErkJggg==","orcid":"","institution":"Northeast Petroleum University","correspondingAuthor":true,"prefix":"","firstName":"yang","middleName":"","lastName":"zhao","suffix":""},{"id":641533640,"identity":"03784d60-80a5-4826-87e7-55dab636e0c9","order_by":3,"name":"junzhen Wu","email":"","orcid":"","institution":"Northeast Petroleum University","correspondingAuthor":false,"prefix":"","firstName":"junzhen","middleName":"","lastName":"Wu","suffix":""},{"id":641533641,"identity":"847c6680-eb87-4c2d-8839-65a9eb3d5912","order_by":4,"name":"xin yu","email":"","orcid":"","institution":"Northeast Petroleum University","correspondingAuthor":false,"prefix":"","firstName":"xin","middleName":"","lastName":"yu","suffix":""},{"id":641533642,"identity":"7bbfe585-8c6c-4acb-91d0-a84af4d2219c","order_by":5,"name":"LiYuan Cai","email":"","orcid":"","institution":"Northeast Petroleum University","correspondingAuthor":false,"prefix":"","firstName":"LiYuan","middleName":"","lastName":"Cai","suffix":""},{"id":641533643,"identity":"ca9f64e7-b61d-44f5-908b-a1eec1d56d16","order_by":6,"name":"Miaoxin zhang","email":"","orcid":"","institution":"Northeast Petroleum University","correspondingAuthor":false,"prefix":"","firstName":"Miaoxin","middleName":"","lastName":"zhang","suffix":""},{"id":641533644,"identity":"59559fe2-064d-453d-bb5d-21f656f78966","order_by":7,"name":"fang shi","email":"","orcid":"","institution":"Northeast Petroleum University","correspondingAuthor":false,"prefix":"","firstName":"fang","middleName":"","lastName":"shi","suffix":""},{"id":641533645,"identity":"28e328fb-270e-4381-96ec-9261dc74730f","order_by":8,"name":"Zailai Su","email":"","orcid":"","institution":"PetroChinaTarim Oilfield Company","correspondingAuthor":false,"prefix":"","firstName":"Zailai","middleName":"","lastName":"Su","suffix":""}],"badges":[],"createdAt":"2026-04-21 01:53:28","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9477351/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9477351/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":109436298,"identity":"02adc231-2b31-486d-9d43-204ae4673e43","added_by":"auto","created_at":"2026-05-18 06:14:37","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":309780,"visible":true,"origin":"","legend":"\u003cp\u003eReaction scheme for the synthesis of C12P9E3S.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/398604bdd039a35be8e8bdfd.png"},{"id":109760189,"identity":"fa482268-7b24-4bc0-991a-91ef4f4f4b8a","added_by":"auto","created_at":"2026-05-22 07:28:17","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3148745,"visible":true,"origin":"","legend":"\u003cp\u003eDetermination of the EACN of the target crude oil: (a) dynamic interfacial tension curves of TRS against different n-alkanes, (b) variation of minimum interfacial tension with alkane carbon number, (c) interfacial tension curves of different mixed oil samples.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/fd45448cb1faf02db054012b.png"},{"id":109759617,"identity":"c3431b8e-913e-4e9e-a1ac-b74a2bcb6997","added_by":"auto","created_at":"2026-05-22 07:27:26","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":4974144,"visible":true,"origin":"","legend":"\u003cp\u003eStructural characterization of the extended surfactants: (a) FTIR spectra of n-C12PmEnS (m = 9, n= 3, 6, 9), (b) 1H NMR spectra of n-C12PmEnS (m = 9, n = 3, 6, 9)\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/b84704e4e03829fa34321c0f.png"},{"id":109759890,"identity":"c8cd9b24-7f35-44e2-98a7-621d7ef50d61","added_by":"auto","created_at":"2026-05-22 07:27:53","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":436235,"visible":true,"origin":"","legend":"\u003cp\u003eInterfacial tension and salinity-scan phase behavior of n-C12P9EnS single-surfactant systems: (a) interfacial tension versus salinity, (b–d) phase behavior of C12P9E3S, C12P9E6S and\u003c/p\u003e\n\u003cp\u003eC12P9E9S, respectively.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/2c6a7da03b18f984ea065b2e.png"},{"id":109759240,"identity":"8d7b767d-be16-4d83-8773-e52b47930c06","added_by":"auto","created_at":"2026-05-22 07:26:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":193386,"visible":true,"origin":"","legend":"\u003cp\u003eSalinity-scan phase behavior of WPS/C12P9E3S systems with different mixing ratios: (a)–(e).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/a4d09bcdc70f12d040fac1c7.png"},{"id":109760230,"identity":"5dec198e-7f26-40aa-8d26-d8e4b79b00de","added_by":"auto","created_at":"2026-05-22 07:28:19","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1045909,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Interfacial activity map of the alkali-free WPS/C12P9E3S system at a mixing ratio of 2:1, (b) Interfacial activity map of the alkali-free WPS/C12P9E3S system at a mixing ratio of 1:1.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/9d1074a2387bb5ef7571b389.png"},{"id":109759909,"identity":"31c391ae-8423-4fc2-bb8b-5fdea61bea86","added_by":"auto","created_at":"2026-05-22 07:27:54","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1478010,"visible":true,"origin":"","legend":"\u003cp\u003eMicromorphology of the alkali-free WPS/C12P9E3S system (2:1) at 1.6 wt% NaCl.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/153a110016716e647ac90841.png"},{"id":109436301,"identity":"7a38b1ec-d27c-45d5-9677-c761d6614e49","added_by":"auto","created_at":"2026-05-18 06:14:37","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":3137479,"visible":true,"origin":"","legend":"\u003cp\u003eInterfacial activity of the alkali-free WPS/C12P9E3S\u003cstrong\u003e \u003c/strong\u003eand weak-alkali ASP flooding systems\u003cstrong\u003e:\u003c/strong\u003e (a) alkali-free WPS/C12P9E3S system, (b) weak-alkali ASP system\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/bbb186bb58c42d4c30e4ee57.png"},{"id":109759418,"identity":"5aa8c960-cbfb-45d1-8972-b33563a4820b","added_by":"auto","created_at":"2026-05-22 07:26:59","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":3202503,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of emulsion stability between the two flooding systems: (a) water separation ratio as a function of time, (b) TSI as a function of time, and (c) comparison of zeta potential.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/05c138f73280204921bab9d7.png"},{"id":109799697,"identity":"583ef286-2d19-404f-90f3-93c1a0f7b696","added_by":"auto","created_at":"2026-05-22 15:33:17","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":3927865,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Injection pressure of different flooding systems, (b) water cut of different flooding systems.\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/18fee173a97667fd08f16a56.png"},{"id":109436309,"identity":"20818530-99ac-450d-8d35-29cfc5b928db","added_by":"auto","created_at":"2026-05-18 06:14:37","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":871816,"visible":true,"origin":"","legend":"\u003cp\u003eMacroscopic appearance of the produced fluids from different flooding systems:\u003cstrong\u003e \u003c/strong\u003e(a) polymer-only system, (b) single C12P9E3S system, (c) single WPS system, (d) alkali-free WPS/C12P9E3S system (2:1), (e) weak-alkali ASP system.\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/dfda24f82fba4d7c6384f8d3.png"},{"id":109436305,"identity":"fb53fc33-8fd4-42f4-bfb3-a4788a00cc75","added_by":"auto","created_at":"2026-05-18 06:14:37","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":2450171,"visible":true,"origin":"","legend":"\u003cp\u003eRelative concentration profiles and interfacial tension variation of produced fluids for two flooding systems: (a) alkali-free WPS/C12P9E3S system (2:1), and (b) weak-alkali ASP system.\u003c/p\u003e","description":"","filename":"floatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/97de946092ce8213b178ff51.png"},{"id":109759408,"identity":"e9ddf525-6fdd-4d16-a013-a2a9c7876b11","added_by":"auto","created_at":"2026-05-22 07:26:57","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":170146,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of interfacial adsorption at the oil-water interface for the single WPS system and the WPS/C12P9E3S formulated system.\u003c/p\u003e","description":"","filename":"floatimage13.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/3ea82c190cc1324cfaaf26e6.png"},{"id":109799276,"identity":"5fc0ede2-6ff6-4bcb-88f7-fb2064ea7225","added_by":"auto","created_at":"2026-05-22 15:27:05","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":2133704,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of emulsion droplet-size distributions in the produced fluids of the two oil-displacement systems (a) alkali-free WPS/C12P9E3S composite system, (b) weak-alkali ASP system.\u003c/p\u003e","description":"","filename":"floatimage14.png","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/8915d1c23c4b319fb80c93e1.png"},{"id":109799709,"identity":"9b03b6c7-4921-4ee3-82a2-10c9064ab4ac","added_by":"auto","created_at":"2026-05-22 15:33:23","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":27874292,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9477351/v1/c857307a-83ef-4119-b3cf-185713d6f1ae.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"An Alkali-Free Surfactant System Based on Petroleum Sulfonate and Extended Surfactant for Enhanced Oil Recovery","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eTernary chemical flooding has been widely recognized as an effective enhanced oil recovery (EOR) technique for mature oilfields\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e and has been extensively applied in highly water-flooded reservoirs such as those in the Daqing Oilfield\u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. However, conventional weak-alkali ternary systems may cause scaling and corrosion during long-term injection and production, which complicates field operation and reduces engineering adaptability\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e. Therefore, the development of alkali-free oil-displacement systems that maintain satisfactory displacement efficiency while improving field applicability has become an important research direction in chemical flooding\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn recent years, various strategies have been proposed to reduce or eliminate alkali in chemical flooding, including alkali-free binary systems\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e and alkali-free ternary systems\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. Among the surfactants used in such systems, petroleum sulfonates have attracted considerable attention because of their broad raw-material availability, low cost, and relatively good compatibility with crude oil. Previous studies have shown that petroleum sulfonates can still exhibit strong interfacial activity under alkali-free conditions, achieve ultralow oil-water interfacial tension, and provide promising oil-displacement performance\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. In particular, alkali-free systems containing petroleum sulfonates blended with amphoteric or nonionic surfactants have been reported to maintain relatively low interfacial tension over a wide range of salinity and temperature, indicating favorable reservoir adaptability\u003csup\u003e[\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e. Existing studies have also demonstrated the feasibility of alkali-free petroleum sulfonate systems in terms of interfacial behavior, emulsification, salt and calcium tolerance, and structural characterization, while also revealing their compositional complexity and the difficulty of establishing accurate structure-property relationships\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e. Early studies on alkali-free systems mainly focused on achieving ultralow interfacial tension, because low interfacial tension has long been regarded as a key requirement for chemical EOR\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. Indeed, through appropriate surfactant selection, formulation design, and salinity matching, alkali-free systems can still achieve ultralow interfacial tension at the 10^-3 mN/m level or even lower within a certain salinity range, and can deliver promising EOR performance in both laboratory and field tests. However, recent studies have shown that the effectiveness of chemical flooding systems depends not only on ultralow interfacial tension, but also on adsorption control, formulation stability, and the synergistic operation of multiple recovery mechanisms under reservoir conditions\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. It is now widely recognized that ultralow interfacial tension alone cannot fully determine oil-displacement performance\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e. During the displacement process, crude-oil emulsification and dispersion\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e, interfacial-film properties\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e, and the deformation and transport of emulsion droplets in porous media can all significantly affect residual-oil mobilization and final recovery efficiency\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. Accordingly, emulsification performance has increasingly been incorporated, together with interfacial tension, into the screening and evaluation of alkali-free systems\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e. Some newly developed alkali-free binary systems have also shown improved displacement performance beyond simple interfacial-tension reduction\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. Similarly, alkali-free SP systems formulated with commercial surfactants such as AES and ABS have been reported to achieve ultralow interfacial tension and markedly improve oil-displacement efficiency under appropriate salinity conditions\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. In the Daqing Oilfield, an alkali-free ternary system in which sodium chloride replaced sodium carbonate was reported to maintain relatively high displacement efficiency while alleviating scaling-related problems in the injection-production system, indicating good potential for field application\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. These studies suggest that the evaluation of alkali-free systems should move beyond a sole emphasis on interfacial-tension reduction toward a more balanced consideration of both interfacial behavior and emulsification stability\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eNevertheless, although single petroleum sulfonate systems can achieve ultralow interfacial tension under specific conditions, they still show limitations in maintaining ultralow interfacial tension over a broad range, in formulation adaptability, and in the fine regulation of emulsion or microemulsion structures under alkali-free conditions. As a result, such systems may have difficulty simultaneously satisfying the multiple requirements of residual-oil mobilization, stripping, and transport in porous media. Therefore, the development of blended systems with stronger interfacial-regulation capability is an important strategy for improving the overall displacement performance of alkali-free petroleum sulfonate systems. In this context, extended surfactants\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e have attracted increasing interest. These surfactants, in which propylene oxide (PO) and/or ethylene oxide (EO) units are introduced between the hydrophobic chain and the hydrophilic headgroup, exhibit more flexible interfacial adsorption configurations and stronger interfacial-regulation capability\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e. Previous studies have shown that extended surfactants favor the formation of microemulsion systems with high solubilization capacity and ultralow interfacial tension, often at relatively low surfactant concentrations\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e. In addition, their molecular structure, particularly the alkyl-chain length and the composition of the PO/EO segments, plays an important role in interfacial adsorption, interfacial-tension reduction, and microemulsion formation\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. Therefore, introducing extended surfactants into alkali-free petroleum sulfonate systems may help overcome the limitations of single petroleum sulfonate systems in fine interfacial regulation and synergistic oil displacement across multiple scales\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e, providing a potentially effective route to balancing cost effectiveness and displacement performance.\u003c/p\u003e \u003cp\u003eDespite these advances, most current studies still remain at the level of formulation optimization and phenomenological performance comparison. The intrinsic links among interfacial regulation, emulsification enhancement, and pore-scale transport have not yet been systematically clarified. In particular, the coupled mechanisms by which interfacial-film regulation, microemulsion formation, and droplet transport through pore throats synergistically contribute to enhanced oil recovery remain insufficiently understood\u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn this study, a series of extended surfactants was designed and synthesized based on the equivalent alkane carbon number (EACN) characteristics of the target crude oil and blended with petroleum sulfonate to construct alkali-free flooding systems. Ultralow interfacial tension was treated as a prerequisite rather than the sole screening criterion, and the formulation was further optimized by combining interfacial activity with emulsion-stability-related indicators. The optimized alkali-free system was then compared with a weak-alkali ASP system in terms of interfacial behavior, emulsion stability, flooding performance, and produced-fluid characteristics. On this basis, the possible origins of its displacement advantage were analyzed from the perspectives of interfacial regulation, emulsification behavior, droplet transport, and component-migration differences.\u003c/p\u003e"},{"header":"2 Experimental Section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Experimental Materials\u003c/h2\u003e \u003cp\u003e \u003cstrong\u003eChemicals\u003c/strong\u003e \u003cp\u003eThe experimental chemicals included a series of extended surfactants synthesized in-house; refining petrochemical petroleum sulfonate (WPS, active content 40%, Daqing Refining \u0026amp; Chemical Company); sodium chloride (NaCl) and sodium carbonate (Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, used only in the weak-alkali ASP control system), both of analytical reagent grade (AR, Tianjin Damao Chemical Reagent Factory); partially hydrolyzed polyacrylamide (HPAM, relative molecular weight approximately 2.5\u0026times;10\u003csup\u003e7\u003c/sup\u003e, active content 90%, industrial grade, Daqing Refining \u0026amp; Chemical Company); n-Alkanes C8\u0026ndash;C15 were supplied by Shandong Keyuan Biochemical. The TRS anionic surfactant, with an active content of 40%, was obtained from Daqing Refining \u0026amp; Chemical Company.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eOil.\u003c/b\u003e The oil used in the experiments was crude oil collected from the target reservoir interval of a block in the Daqing Oilfield, with a dead-oil viscosity of 33 mPa\u0026middot;s at 45\u0026deg;C. Simulated oil was prepared by diluting the crude oil with aviation kerosene, yielding a viscosity of 10 mPa\u0026middot;s at 45\u0026deg;C.\u003c/p\u003e \u003cp\u003e \u003cb\u003eWater.\u003c/b\u003e The experimental water consisted of the actual reinjection water and formation water from the target reservoir interval of a block in Daqing. Their ionic compositions are listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eIonic composition of reinjection wastewater.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNa\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eK\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCa\u003csup\u003e2+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMg\u003csup\u003e2+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eHCO\u003csup\u003e3-\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eCl\u003csup\u003e-\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003eTotal salinity\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIon concentration\u003c/p\u003e \u003cp\u003e(mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3350\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.489\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e20.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e42.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2049.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e96.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1237.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e6798.87\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eIonic composition of the formation water.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNa\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eK\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCa\u003csup\u003e2+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMg\u003csup\u003e2+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eHCO\u003csup\u003e3-\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eCl\u003csup\u003e-\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003eTotal salinity\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIon concentration\u003c/p\u003e \u003cp\u003e(mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2183.821\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.489\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e23.109\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e66.6798\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2053.857\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e77.086\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e2363.213\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e6778\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eOil sand.\u003c/b\u003e The experimental oil sand was obtained from the SII reservoir group in the target reservoir interval of a block in the Daqing Oilfield, with a particle size distribution of 100\u0026ndash;200 mesh.\u003c/p\u003e \u003cp\u003e \u003cb\u003eCore samples.\u003c/b\u003e Epoxy-resin-cast Berea cores with dimensions of 4.5 cm \u0026times; 4.5 cm \u0026times; 30 cm were used in the experiments. The detailed core properties are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCore flooding results of different flooding systems.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026minus;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFlooding system\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCore No.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eWater permeability (mD)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eOil saturation (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eWaterflood recovery (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eChemical flooding recovery (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eAverage chemical flooding recovery (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003ePolymer-only system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230204-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e214.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e39.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e12.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e11.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230204-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e206.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e63.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e40.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e11.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230204-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e199.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e40.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e11.86\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eSingle C12P9E3S system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230501-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e214.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e63.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e40.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e20.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230501-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e224.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e39.69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e20.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230501-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e179.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e63.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e39.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e19.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eSingle WPS system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230601-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e225.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e63.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e38.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e20.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e21.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.69\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230601-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e216.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e39.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e21.23\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230601-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e220.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e63.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e40.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e21.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eAlkali-free WPS/C12P9E3S formulated system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230723-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e216.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e39.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e37.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e36.72\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230723-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e186.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e63.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e41.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e35.88\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230723-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e203.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e40.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e37.03\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eWeak-alkali ASP system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230308-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e210.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e65.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e39.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e30.92\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230308-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e196.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e40.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c2\"\u003e \u003cp\u003e230308-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e208.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e39.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e31.58\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Design of the target surfactant\u003c/h2\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1 Crude oil characterization and EACN analysis\u003c/h2\u003e \u003cp\u003eTo achieve a better match between the molecular structure of the surfactant and the interfacial behavior of the target crude oil, the average molecular weight, compositional characteristics, and equivalent alkane carbon number (EACN) of the target crude oil were first determined and used as the basis for the design of the extended surfactant. The average molecular weight and compositional characteristics of the crude oil from the target block were measured in accordance with GB/T 17282\u0026thinsp;\u0026minus;\u0026thinsp;2012.\u003c/p\u003e \u003cp\u003eThe EACN of the crude oil was determined with reference to the method proposed by Huang Yanhua et al\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. An anionic surfactant solution with a mass fraction of 0.3% was prepared using produced water, and the NaCl concentration in the system was adjusted to 1.5%. Under a temperature of 45\u0026deg;C, the dynamic interfacial tension between the target crude oil and different n-alkane systems was measured using the spinning-drop method. Each experiment was conducted for 1 h at a rotational speed of 5000 r/min with an oil-drop volume of 1 \u0026micro;L, and the interfacial tension was recorded every 10 min. A variation of no more than 10% within 60 min was taken as the criterion for quasi-equilibrium. Meanwhile, the minimum interfacial tension observed during the measurement was also recorded to provide a supplementary evaluation of the dynamic interfacial tension reduction performance of the system.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{EACN}_{O}=[{\\left(EACN\\right)}_{mo}{x}_{mo}-{\\left(EACN\\right)}_{l}{x}_{l}]/{x}_{o}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{EACN}_{mo}\\)\u003c/span\u003e \u003c/span\u003e\u0026mdash;is the equivalent alkane carbon number of the mixed oil sample;\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{EACN}_{l}\\)\u003c/span\u003e \u003c/span\u003e\u0026mdash;is the carbon number of the n-alkane;\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{EACN}_{O}\\)\u003c/span\u003e \u003c/span\u003e\u0026mdash;is the equivalent alkane carbon number of the crude oil;\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{X}_{l}\\)\u003c/span\u003e \u003c/span\u003e\u0026mdash;is the mole fraction of the n-alkane;\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{X}_{mO}\\)\u003c/span\u003e \u003c/span\u003e\u0026mdash;is the mole fraction of the mixed oil sample;\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{X}_{O}\\)\u003c/span\u003e \u003c/span\u003e\u0026mdash;is the mole fraction of the crude oil.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Synthesis and characterization of the target surfactant\u003c/h2\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.3.1 Synthesis of the extended surfactant\u003c/h2\u003e \u003cp\u003eThe synthesis of C12P9E3S is taken as a representative example. The preparation procedure involved alkoxylation, sulfation, and subsequent neutralization/purification. In the alkoxylation step, 93.0 g of dodecanol and 5.0 g of catalyst were charged into a 500 mL high-pressure reactor. The reactor was heated to 130\u0026deg;C, and propylene oxide and ethylene oxide were introduced sequentially at a molar ratio of dodecanol:propylene oxide:ethylene oxide\u0026thinsp;=\u0026thinsp;1:9:3. After completion of the reaction, the mixture was cooled for 5 h to obtain the intermediate alkoxylated product. The intermediate was then subjected to sulfation. For this purpose, 18 g of the ether intermediate and an equal mass of dry 1,2-dichloroethane were added to a four-neck flask equipped with an off-gas absorption device. Under ice-water-bath cooling and mechanical stirring, chlorosulfonic acid was slowly added dropwise to the reaction system. After the addition was completed, the reaction mixture was neutralized with sodium hydroxide solution to approximately pH 9 to suppress hydrolysis. The resulting product was subsequently purified. Most inorganic salts were removed by centrifugation, and the solvent was removed by rotary evaporation. Residual inorganic salts were further removed with anhydrous ethanol, followed by rotary evaporation to remove ethanol and obtain the final product. The active matter content of the product was determined by the two-phase titration method. The reaction scheme is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e(1) Reaction scheme for the synthesis of C12P9E3:\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e2.3.2 Structural characterization of the series of extended surfactants\u003c/h2\u003e \u003cp\u003eThe samples were characterized by Fourier transform infrared spectroscopy (FTIR) using a Nicolet iS20 spectrometer (Thermo Fisher, USA). The measurements were performed in ATR mode over a scanning range of 4000\u0026ndash;450 cm⁻\u0026sup1;, with 32 scans.\u003c/p\u003e \u003cp\u003e1H NMR spectra were recorded on a Bruker AVANCE 600 MHz nuclear magnetic resonance spectrometer (Bruker, Germany), using CDCl\u003csub\u003e3\u003c/sub\u003e as the solvent and tetramethylsilane (TMS) as the internal standard at 298 K.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e2.3.3 Performance evaluation of single-surfactant systems\u003c/h2\u003e \u003cp\u003eThe interfacial tension of the single-surfactant systems was measured by the spinning-drop method. The test temperature was 45\u0026deg;C, the measurement duration was 1 h, the rotational speed was 5000 r/min, and the oil-drop volume was 1 \u0026micro;L. Interfacial tension was recorded every 10 min, and both the quasi-equilibrium interfacial tension and the minimum interfacial tension observed during the measurement were reported.\u003c/p\u003e \u003cp\u003ePhase behavior and emulsion stability were preliminarily evaluated by salt-concentration scanning. Specifically, 2 mL of surfactant solution with a concentration of 0.3 wt% was prepared, with the NaCl concentration set at 0.4%, 0.8%, 1.2%, 1.6%, 2.0%, 2.5%, and 3.0%, respectively. Then, 2 mL of simulated oil was added to each sample to obtain an oil/water volume ratio of 1:1. The samples were mixed on a rotating mixer at 2 r/min for 24 h and then allowed to stand. The phase behavior and oil-water separation were recorded after 0, 1, 3, 7, 15, 30, and 60 d.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Screening of the alkali-free formulated flooding system\u003c/h2\u003e \u003cp\u003eThe formulation was screened by taking ultralow interfacial tension as a necessary prerequisite, while further considering emulsion stability, water-separation behavior, TSI value, and surfactant dosage. In this way, the selected system was not determined by the minimum interfacial tension alone, but by a combined evaluation of interfacial activity and emulsion-related performance. First, the interfacial and phase behaviors of the individual n-C12P9EnS (n\u0026thinsp;=\u0026thinsp;3, 6, 9) were evaluated to identify the target extended surfactant. The selected extended surfactant was then blended with WPS at different ratios, followed by a multifactorial screening of salinity and total surfactant concentration. During the screening process, under the premise of maintaining low interfacial tension, priority was given to systems with a relatively wide emulsification window, higher emulsion stability, and lower total surfactant dosage, and the optimal formulation was determined accordingly.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Comparison of the flooding system performance\u003c/h2\u003e \u003cp\u003eTo ensure comparability among different flooding systems, five types of flooding formulations were prepared in this study: a polymer-only system, a single C12P9E3S system, a single WPS system, an alkali-free WPS/C12P9E3S formulated system, and a weak-alkali ASP system. In all systems except the polymer-only system, the HPAM concentration was fixed at 1600 mg/L. In the polymer-only system, the polymer concentration was adjusted to 1400 mg/L to maintain a comparable viscosity. At 45\u0026deg;C, the viscosity of all systems was controlled at approximately 38 mPa\u0026middot;s. The polymer protection slug was prepared at a polymer concentration of 1400 mg/L, with a viscosity of approximately 38 mPa\u0026middot;s. These five systems were mainly used for staged contribution analysis in core flooding experiments to distinguish the respective contributions of mobility control, the effect of individual surfactants, and the synergistic effect of surfactant formulation to oil recovery. Among them, the alkali-free WPS/C12P9E3S formulated system and the weak-alkali ASP system were further selected for representative comparison in terms of interfacial performance, emulsion stability, and anti-adsorption behavior. The detailed formulations are listed in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical compositions of the different flooding systems.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003esystem\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSurfactant composition\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePolymer (HPAM)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSalinity / alkali\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003epolymer-only system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1400 mg/L\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.6 wt% NaCl\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSingle C12P9E3S system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.3 wt% C12P9E3S\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1600 mg/L HPAM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.6 wt% NaCl\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSingle WPS system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.3 wt% WPS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1600 mg/L HPAM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.6 wt% NaCl\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAlkali-free WPS/C12P9E3S formulated system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.2 wt% WPS\u0026thinsp;+\u0026thinsp;0.1 wt% C12P9E3S\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1600 mg/L HPAM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.6 wt% NaCl\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWeak-alkali ASP system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.3 wt% WPS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1600 mg/L HPAM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.2 wt% Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eNote: HPAM\u0026thinsp;=\u0026thinsp;partially hydrolyzed polyacrylamide.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e2.5.1 Formulations of the flooding systems\u003c/h2\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e2.5.2 Emulsion stability\u003c/h2\u003e \u003cp\u003eEach of the two system solutions was mixed with the simulated oil at a volume ratio of 1:1. The mixtures were rotated at 2 r/min for 24 h using a rotary mixer and then transferred into a 30 cm-high test bottle. The transmittance and backscattering profiles of the emulsions were analyzed using a Turbiscan Lab stability analyzer. The scanning time was 2 h, and the test temperature was maintained at 45\u0026deg;C. The Turbiscan Stability Index (TSI) was automatically calculated to evaluate emulsion stability. Each experiment was performed in triplicate.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e2.5.3 Interfacial tension measurement\u003c/h2\u003e \u003cp\u003eInterfacial tension was measured by the spinning-drop method at 45\u0026deg;C for 1 h. The quasi-equilibrium interfacial tension and the minimum instantaneous interfacial tension were selected as the effective evaluation parameters.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e \u003ch2\u003e2.5.4 Emulsification performance of the systems\u003c/h2\u003e \u003cp\u003eEach of the two system solutions was mixed with the simulated oil at a volume ratio of 1:1 and rotated at 2 r/min for 24 h using a rotary mixer. The mixtures were then placed in a constant-temperature oven at 45\u0026deg;C and allowed to stand. At fixed time intervals of 30 min, the volume of separated water was observed and recorded. The water separation ratio was then calculated based on the cumulative volume of separated water over time. Each experiment was conducted in triplicate.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e \u003ch2\u003e2.5.5 Zeta potential of the systems\u003c/h2\u003e \u003cp\u003eAn appropriate amount of the test system was added to the sample cell of the instrument, and the zeta potential of the emulsion was measured under the preset temperature and instrumental parameters. Each sample was measured three times, and the average value was taken as the final result.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e \u003ch2\u003e2.5.6 Anti-adsorption performance of the systems\u003c/h2\u003e \u003cp\u003eThe anti-adsorption performance was evaluated by a multiple adsorption test using oil sand. Specifically, 20 g of the target oil sand was weighed, and the test flooding system solution was added at a solid-to-liquid mass ratio of 1:9. After shaking at 45\u0026deg;C for 24 h, the supernatant was collected, and its interfacial tension against crude oil was measured. The entire supernatant was then transferred to an equal amount of fresh oil sand, and the above procedure was repeated. The variation in interfacial tension of the system after multiple adsorption cycles was used to characterize its anti-adsorption capacity.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Laboratory oil displacement experiments and produced-fluid analysis\u003c/h2\u003e \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e \u003ch2\u003e2.6.1 Laboratory oil displacement experiments\u003c/h2\u003e \u003cp\u003eEpoxy-resin-cast Berea cores from the same batch, with water-phase permeability in the range of 150\u0026ndash;200 mD, were used in the core flooding experiments. The cores were vacuumed for 8 h using a vacuum pump and then saturated with formation water from the target reservoir. The ionic composition of the formation water is listed in Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Simulated oil was injected at a constant rate of 0.1 mL/min until no water was produced from the outlet end of the core. The core was then aged in a constant-temperature oven at 45\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u0026deg;C for 24 h. Water flooding was subsequently conducted at a flow rate of 0.3 mL/min until the water cut at the outlet reached 100%. Thereafter, 0.3 PV of the main flooding slug and 0.2 PV of the polymer protection slug were injected, followed by subsequent water flooding until the water cut exceeded 98%. Each experiment was repeated three times, and the results are reported as the average of the three parallel runs.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section3\"\u003e \u003ch2\u003e2.6.2 Produced-fluid analysis\u003c/h2\u003e \u003cp\u003eThe produced-fluid analysis included measurements of salinity, polymer concentration, and surfactant concentration. For salinity determination, 1 mL of the lower aqueous phase of the produced fluid was diluted tenfold with 9 mL of distilled water and mixed thoroughly. The conductivity was then measured, and the measured conductivity value was multiplied by 10 and substituted into the NaCl conductivity calibration curve to determine the salt concentration. For polymer concentration determination, a 721 UV-Vis spectrophotometer was used to measure the absorbance at 470 nm. The measured absorbance value was multiplied by 10 and substituted into the polymer concentration\u0026ndash;absorbance calibration curve to obtain the polymer concentration. For surfactant concentration determination, the two-phase titration method was employed. At the titration endpoint, the lower phase appeared light gray. After the addition of two drops of Hyamine solution, the lower phase turned light blue. The volume of Hyamine solution consumed was recorded and used to calculate the active surfactant content.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3 Results","content":"\u003cp\u003e3.1 Determination of the EACN of the target crude oil and basis for the design of the target surfactant\u003c/p\u003e \u003cp\u003eTo achieve structural matching between the surfactant and the interfacial behavior of the target crude oil, the equivalent alkane carbon number (EACN) of the target crude oil was determined by an indirect method. Under the conditions of 45\u0026deg;C, 0.3 wt% anionic surfactant, and 1.5 wt% NaCl, the interfacial tension (IFT)-time curves of the target crude oil against n-alkanes from C8 to C15 showed that the system reached the lowest quasi-equilibrium IFT when the alkane carbon number was C12. Accordingly, the minimum IFT was obtained at C12, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea,b. According to measurements performed three times in accordance with GB/T 17282\u0026thinsp;\u0026minus;\u0026thinsp;2012, the average molecular weight of the crude oil was 453.5 g/mol. The EACN was further back-calculated using crude oil/n-alkane mixed oil samples. When the volume ratio of crude oil to n-pentadecane was 9:1, the IFT between the mixed oil sample and the aqueous phase reached a minimum value of 0.0033 mN/m, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c). Under this condition, (EACN)\u003csub\u003el\u003c/sub\u003e = 15, (EACN)\u003csub\u003emo\u003c/sub\u003e = 12, x\u003csub\u003el\u003c/sub\u003e = 0.1, x\u003csub\u003eo\u003c/sub\u003e = 0.9, x\u003csub\u003emo\u003c/sub\u003e = 1. Substitution into Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) gave an EACN value of approximately 11.67 (about 12). This result provided the basis for the subsequent design of extended surfactants using a C12-based hydrophobe.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec22\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Structural characterization of the extended surfactants\u003c/h2\u003e \u003cdiv id=\"Sec23\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 FTIR characterization of n-C12P9EnS (n\u0026thinsp;=\u0026thinsp;3, 6, 9)\u003c/h2\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), taking C12P9E3S as an example, the absorption peaks at approximately 2931.7cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 2846.8cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are assigned to the stretching vibrations of \u0026minus;\u0026thinsp;C\u0026minus;H, while the peak near 3493cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the stretching vibration of \u0026minus;\u0026thinsp;O\u0026minus;H. The peak at around 1092.4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to the stretching vibration of \u0026minus;\u0026thinsp;C\u0026minus;O\u0026thinsp;\u0026minus;\u0026thinsp;S\u0026minus;, and the peak near 1321.9cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is assigned to the stretching vibration of S\u0026thinsp;=\u0026thinsp;O, indicating that sulfation was successfully achieved. In addition, the peak near 721.1cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to the stretching vibration of S\u0026thinsp;\u0026minus;\u0026thinsp;O, further suggests that the target product, C12P9E3S was successfully synthesized.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b) and Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the actual numbers of added PO and EO units in the n-C12PmEnS series were generally consistent with the theoretical design values. Among the synthesized surfactants, n-C12P9E3S exhibited the closest agreement with the target structure, indicating relatively good structural controllability and supporting its use in the subsequent evaluation of interfacial and oil-displacement properties.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAverage numbers of added PO and EO units in n-C12PmEnS molecules.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003en-C12PmEnS\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNumber of PO units (m)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNumber of EO units (n)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003en-C12P9E3S\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003en-C12P9E6S\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003en-C12P9E9S\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec24\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Screening of the alkali-free formulated system\u003c/h2\u003e \u003cp\u003eUsing ultralow interfacial tension on the order of 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mN/m as a prerequisite, the formulation was screened comprehensively based on the width of the emulsification window, water separation ratio, TSI value, and total surfactant dosage. On this basis, the optimal formulation was determined to have a WPS/C12P9E3S mass ratio of 2:1 and an NaCl concentration of 1.6 wt%.\u003c/p\u003e \u003cdiv id=\"Sec25\" class=\"Section3\"\u003e \u003ch2\u003e3.3.1 Performance evaluation of the individual n-C12PmEnS(m\u0026thinsp;=\u0026thinsp;9, n\u0026thinsp;=\u0026thinsp;3, 6, 9)\u003c/h2\u003e \u003cp\u003eThe interfacial tension and phase behavior of the individual n-C12PmEnS (m\u0026thinsp;=\u0026thinsp;9, n\u0026thinsp;=\u0026thinsp;3, 6 ,9) were first evaluated. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, none of the three single-surfactant systems achieved ultralow interfacial tension on the order of 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mN/m, among them, the C12P9E3S system exhibited relatively lower interfacial tension over a relatively wide salinity range and showed the widest salinity range for emulsification. Therefore, C12P9E3S was selected as the target extended surfactant for subsequent formulation with WPS.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec26\" class=\"Section3\"\u003e \u003ch2\u003e3.3.2 Emulsification performance of the WPS/C12P9E3S formulated systems\u003c/h2\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, to examine whether formulation with C12P9E3S could compensate for the limited emulsification performance of WPS under alkali-free conditions, the salinity-scan phase behavior results of WPS and its mixtures with C12P9E3S at different ratios were compared. The results showed that, at an oil/water ratio of 1:1, the formulated systems with a WPS/C12P9E3S ratio below 2:1 exhibited a wider salinity range for emulsification and a higher emulsion volume fraction. In addition, the emulsification performance was further improved at lower WPS proportions. Therefore, the WPS/C12P9E3S formulated system was selected for subsequent investigation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec27\" class=\"Section3\"\u003e \u003ch2\u003e3.3.3 Effect of the WPS/C12P9E3S mixing ratio on the interfacial activity of the formulated systems\u003c/h2\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a), the alkali-free WPS/C12P9E3S system at a mixing ratio of 2:1 exhibited a relatively wide ultralow-interfacial-tension region. Interfacial tension on the order of 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mN/m was achieved over a total surfactant concentration range of 0.1\u0026ndash;0.3 wt% and an NaCl concentration range of 0.8\u0026ndash;2.0 wt%. By comparison, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b), the alkali-free system at a WPS/C12P9E3S ratio of 1:1 showed a narrower ultralow-interfacial-tension region, reaching the 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mN/m level only when the total surfactant concentration was 0.1\u0026ndash;0.3 wt% and the NaCl concentration was 1.8\u0026ndash;2.0 wt%. Considering both emulsification performance and interfacial tension, the optimal alkali-free formulated system was finally determined to be WPS/C12P9E3S\u0026thinsp;=\u0026thinsp;2:1 with an NaCl of 1.6 wt%.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec28\" class=\"Section3\"\u003e \u003ch2\u003e3.3.4 Micromorphology of the alkali-free WPS/C12P9E3S formulated system\u003c/h2\u003e \u003cp\u003eUnder the optimal conditions, the microstructure of the alkali-free WPS/C12P9E3S formulated system was further examined. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the upper phase was dominated by a water-in-oil structure, whereas the lower phase was dominated by an oil-in-water structure. The middle phase exhibited localized water-in-oil and oil-in-water morphologies simultaneously, together with bicontinuous characteristics. These observations suggest that the optimal formulated system has the potential to form a complex microemulsion structure at the corresponding salinity, which may provide a phase-behavior basis for the attainment of relatively low interfacial tension and relatively high emulsion stability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e3.4 Comparison of the interfacial and emulsification properties between the alkali-free formulated system and the weak-alkali ASP system\u003c/p\u003e \u003cp\u003eTo compare the alkali-free formulated system with the engineering baseline system, the interfacial performance, emulsion stability, and anti-adsorption behavior of the alkali-free WPS/C12P9E3S formulated system and the weak-alkali ASP system were comparatively analyzed under the same temperature, the same oil/water ratio, the same test methods, and comparable viscosity-matching conditions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec29\" class=\"Section3\"\u003e \u003ch2\u003e3.4.1 Comparison of the interfacial performance of the flooding systems\u003c/h2\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, comparison of the interfacial-tension windows of the two systems under the same temperature, oil/water ratio, and viscosity-matching conditions indicates that the weak-alkali ASP system could achieve ultralow interfacial tension over a wider alkali-concentration range, and under some conditions the minimum interfacial tension reached the 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e mN/m level. In contrast, the alkali-free WPS/C12P9E3S formulated system mainly achieved ultralow interfacial tension on the order of 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mN/m. These results suggest that, in terms of interfacial tension reduction alone, the weak-alkali ASP system still has a certain advantage. Accordingly, if the alkali-free formulated system shows superior oil-displacement performance overall, its advantage is more likely to arise from other factors, such as emulsion stability and transport behavior in porous media, rather than being determined solely by the minimum interfacial tension.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec30\" class=\"Section3\"\u003e \u003ch2\u003e3.4.2 Comparison of the emulsion stability of the flooding systems\u003c/h2\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(a-c), the results of water separation ratio, TSI, and zeta potential indicate that the emulsion stability of the alkali-free WPS/C12P9E3S formulated system was clearly superior to that of the weak-alkali ASP system. Within 300 min, the water separation ratio of the formulated system increased from 0% to 49%, which was markedly lower than the 74% observed for the weak-alkali ASP system. Meanwhile, the formulated system exhibited consistently lower TSI values throughout the test period, indicating stronger macroscopic stability. In addition, the formulated system showed a larger absolute zeta potential, suggesting stronger electrostatic repulsion between emulsion droplets and a greater resistance to coalescence. These results indicate that although the alkali-free formulated system did not show an advantage in terms of minimum interfacial tension, it exhibited a clear advantage in emulsion stability, which may provide an important basis for sustained emulsification and oil transport during subsequent displacement.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec31\" class=\"Section3\"\u003e \u003ch2\u003e3.4.3 Comparison of the anti-adsorption performance of the flooding systems\u003c/h2\u003e \u003cp\u003eAs shown in Table\u0026nbsp;\u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e5\u003c/span\u003e, after five adsorption cycles on oil sand, the interfacial tension (IFT) of both systems increased to the 10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e mN/m level, and the difference between the two systems became relatively small after the fifth adsorption cycle. The IFT of the alkali-free WPS/C12P9E3S formulated system was 1.904\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e mN/m, compared with 1.688\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e mN/m for the weak-alkali ASP system. These results indicate that the alkali-free WPS/C12P9E3S formulated system and the weak-alkali ASP system showed only a limited difference in resistance to adsorption on oil sand. This suggests that the improvement in oil-displacement efficiency of the alkali-free WPS/C12P9E3S formulated system was not mainly attributable to enhanced anti-adsorption performance, but was more likely associated with its superior emulsion stability, weaker chromatographic separation, and more favorable matching between emulsion droplets and pore throats.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab6\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eInterfacial tension after repeated adsorption on oil sand.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eFlooding system\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"6\" nameend=\"c7\" namest=\"c2\"\u003e \u003cp\u003eInterfacial tension (mN/m)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBefore adsorption\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAfter 1 adsorption cycle\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAfter 2 adsorption cycles\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAfter 3 adsorption cycles\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAfter 4 adsorption cycles\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eAfter 5 adsorption cycles\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAlkali-free WPS/C12P9E3S formulated system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e1.38\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c3\"\u003e \u003cp\u003e2.8\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c4\"\u003e \u003cp\u003e1.87\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c5\"\u003e \u003cp\u003e5.7\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c6\"\u003e \u003cp\u003e8.5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c7\"\u003e \u003cp\u003e1.9\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWeak-alkali ASP system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e6.7\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c3\"\u003e \u003cp\u003e9\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c4\"\u003e \u003cp\u003e2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c5\"\u003e \u003cp\u003e6.8\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c6\"\u003e \u003cp\u003e7.6\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c7\"\u003e \u003cp\u003e1.62\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec32\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Core flooding and produced-fluid characterization\u003c/h2\u003e \u003cdiv id=\"Sec33\" class=\"Section3\"\u003e \u003ch2\u003e3.5.1 Results of the core flooding experiments\u003c/h2\u003e \u003cp\u003eTo distinguish the respective contributions of mobility control, individual surfactant action, and formulation synergy to oil recovery, the core flooding results of the polymer-only system, the single C12P9E3S system, the alkali-free formulated system, and the weak-alkali ASP system were further compared. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e6\u003c/span\u003e, all four systems were able to further improve oil recovery after water flooding, although the incremental recovery beyond water flooding differed markedly among the systems.\u003c/p\u003e \u003cp\u003eAs summarized in Table\u0026nbsp;\u003cspan refid=\"Tab7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the polymer-only system provided an average incremental recovery of 11.9% over water flooding, indicating that mobility control plays a fundamental role in enlarging the swept volume. The single C12P9E3S system yielded an average incremental recovery of 20.0% over water flooding, representing a further increase of 8.1 percentage points compared with the polymer-only system. The single WPS system gave an average incremental recovery of 21.15% over water flooding, which was 9.24 percentage points higher than that of the polymer-only system. These results indicate that the action of an individual surfactant can further mobilize part of the residual oil on the basis of mobility control, although the magnitude of the improvement remains limited. By contrast, the alkali-free WPS/C12P9E3S formulated system increased oil recovery by an additional 16.72 percentage points relative to the single C12P9E3S system and by 15.57 percentage points relative to the single WPS system. This suggests a pronounced synergistic effect in the formulated system, and its oil-displacement performance cannot be explained by any single mechanism alone.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab7\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 7\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eIncremental oil recovery and staged contribution analysis for different flooding systems.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFlooding system\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eChemical-flooding incremental recovery (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAverage chemical flooding recovery (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRelative to polymer flooding (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eRelative to the single C12P9E3S system (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eRelative to the single WPS system (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003ePolymer-only system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e11.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e11.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e11.86\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eSingle C12P9E3S system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e8.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e19.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eSingle WPS system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e21.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e9.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e21.23\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e21.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eAlkali-free WPS/C12P9E3S formulated system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e36.72\u0026thinsp;\u0026plusmn;\u0026thinsp;0.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e24.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e16.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e15.57\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e35.88\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37.03\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eWeak-alkali ASP system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e30.92\u0026thinsp;\u0026plusmn;\u0026thinsp;0.69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e19.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e10.92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e9.77\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e31.58\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eIn addition, the alkali-free WPS/C12P9E3S formulated system achieved a further 5.80 percentage-point increase in oil recovery compared with the weak-alkali ASP system. This result indicates that, even under alkali-free conditions, the formulation of petroleum sulfonate with an extended surfactant can still deliver substantial incremental oil recovery and shows the potential to compete with the conventional weak-alkali ASP system.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec34\" class=\"Section3\"\u003e \u003ch2\u003e3.5.2 Injection pressure and flow-resistance characteristics\u003c/h2\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(a), the injection-pressure curves indicate that the polymer-only system exhibited the highest peak pressure, followed by the weak-alkali ASP system, whereas the alkali-free WPS/C12P9E3S formulated system showed the lowest and most stable pressure throughout the flooding process. During the entire displacement stage, the injection pressure of the WPS/C12P9E3S\u0026thinsp;=\u0026thinsp;2:1 system remained consistently lower than that of the weak-alkali ASP system. The maximum pressure of the former during flooding was 0.4 MPa, which was 0.5 MPa lower than that of the latter.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(b) and Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e, the water-cut curves and observations of the produced fluids further indicate that the alkali-free WPS/C12P9E3S formulated system maintained a longer low-water-cut stage and a more sustained emulsification-and-transport process. Its minimum water cut was 50.91%, approximately 15% lower than that of the weak-alkali ASP system. In addition, during the subsequent waterflooding stage, emulsification in the formulated system persisted for about 0.32 PV, compared with about 0.24 PV for the weak-alkali ASP system, corresponding to an extension of approximately 0.08 PV in emulsification-assisted transport. No obvious emulsification was observed in the polymer-only system, the single C12P9E3S system, or the single WPS system.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThese results suggest that the alkali-free WPS/C12P9E3S formulated system was more favorable for maintaining oil droplets in a dispersed state and for stable transport by the aqueous phase over a wider pore-volume range, thereby prolonging the effective window for mobilizing residual oil. Combined with the subsequent droplet-size analysis, it may be inferred that the lower injection pressure of this system is related to a more favorable matching relationship between emulsion droplets and pore throats.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec35\" class=\"Section3\"\u003e \u003ch2\u003e3.5.3 Relative concentration distribution of produced fluids\u003c/h2\u003e \u003cp\u003eThe relative concentration profiles of the chemical components in the produced fluids indicate that, in the alkali-free WPS/C12P9E3S\u0026thinsp;=\u0026thinsp;2:1 system, the peak positions of the different components were closer to one another, suggesting a smaller difference in component retardation during transport through porous media. A smaller difference in the PV corresponding to the concentration peaks generally indicates better component synchrony and a greater likelihood of maintaining an effective formulation ratio. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e(a), for the WPS/C12P9E3S formulated system, the concentration peaks of polymer, surfactant, and salt appeared at 1.04, 1.12, and 0.97 PV, respectively, with a maximum difference of 0.15 PV. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e(b), in contrast, the corresponding peak positions in the weak-alkali ASP system differed by approximately 0.24 PV. This suggests that the alkali-free WPS/C12P9E3S formulated system may exhibit a smaller retardation difference among components. In addition, the interfacial tension of the produced fluid from the formulated system decreased to the 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mN/m level over the range of 0.9\u0026ndash;1.2 PV, whereas the lowest value observed for the weak-alkali ASP system reached only the 10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e mN/m level. This result suggests that the formulated system may be more favorable for maintaining an effective component ratio over a wider pore-volume range.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4 Discussion","content":"\u003cp\u003eBased on the above comparative results, the displacement advantage of the alkali-free system is discussed here from a coupled interfacial-transport perspective. In particular, four mutually related aspects are considered: interfacial adsorption behavior, emulsification enhancement, droplet-size-related transport behavior, and differences in component migration.\u003c/p\u003e \u003cdiv id=\"Sec37\" class=\"Section3\"\u003e \u003cdiv class=\"Heading\"\u003e3.6.1 Proposed interfacial adsorption scenario\u003c/div\u003e \u003cp\u003eOne important reason why the alkali-free WPS/C12P9E3S formulated system may achieve enhanced oil-displacement efficiency is that it appears to exhibit interfacial behavior at the oil-water interface that differs from that of the weak-alkali ASP system\u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e, WPS possesses strong interfacial activity owing to the pronounced hydrophilicity of its sulfonate groups and the lipophilicity of its hydrocarbon chains, allowing it to adsorb rapidly at the oil-water interface and thereby produce an initial reduction in interfacial tension. However, because petroleum sulfonate is compositionally complex, the adsorption film formed at the interface is often relatively loose and of limited stability\u003csup\u003e[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e. C12P9E3S, as a typical extended surfactant, contains both PO and EO segments in its molecular structure. The intermediate PO/EO chains are conducive to regulating the molecular packing at the interface, thereby improving the structure of the interfacial layer\u003csup\u003e[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e. Previous studies have shown that extended surfactants can exhibit adsorption behavior at the oil-water interface that differs from that of conventional anionic surfactants because of their distinctive molecular architecture, and can achieve relatively low interfacial tension under suitable salinity and oil-phase conditions \u003csup\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e. However, the comparative results of this study show that although the weak-alkali ASP system has an advantage in achieving lower interfacial tension, the alkali-free WPS/C12P9E3S composite system exhibits higher oil recovery and better emulsion stability. This indicates that, for the systems investigated in this work, the oil-displacement performance is not determined solely by the minimum interfacial tension. Unlike many previous studies that have treated ultralow interfacial tension as the primary evaluation criterion for alkali-free systems, the present results further suggest that differences in interfacial behavior and their subsequent effects on emulsification and displacement processes deserve particular attention.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec38\" class=\"Section3\"\u003e \u003cdiv class=\"Heading\"\u003e3.6.2 Emulsification Enhancement and Crude Oil Entrapment\u0026ndash;Transport\u003c/div\u003e \u003cp\u003eIn addition to interfacial tension reduction, emulsification also plays a key role in the oil-displacement process of composite flooding systems\u003csup\u003e[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e. The formation and stability of emulsions are influenced by multiple factors, including surfactant structure, polymer presence, and flow conditions\u003csup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e. Recent pore-scale visualization studies have further shown that in situ emulsification is not merely a static bottle-test phenomenon, but directly participates in flow redistribution and residual oil mobilization in porous media\u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, the water separation ratio and TSI value of the alkali-free WPS/C12P9E3S composite system are significantly lower than those of the weak-alkali ASP system, indicating stronger emulsion stability. Meanwhile, its higher absolute zeta potential suggests stronger electrostatic repulsion between emulsion droplets, thereby effectively inhibiting droplet coalescence. In this system, crude oil can be dispersed into fine droplets and remain stably suspended in the aqueous phase, enabling a continuous process of \u0026ldquo;emulsification\u0026ndash;entrainment\u0026ndash;transport\u0026rdquo;\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. In addition, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e, the emulsification duration of the alkali-free WPS/C12P9E3S composite system is approximately 0.32 PV, which is clearly longer than the 0.24 PV observed for the weak-alkali ASP system. This suggests that, compared with studies limited to static comparisons of emulsification performance, the present work further relates emulsion stability to dynamic displacement responses. In other words, improved emulsion stability is reflected not only in slower water separation or lower TSI values in bottle tests, but may also translate into a more sustained emulsification-assisted transport process during flooding, thereby helping maintain crude oil dispersion and migration over a longer displacement interval.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec39\" class=\"Section3\"\u003e \u003cdiv class=\"Heading\"\u003e3.6.3 Relationship Between Produced-Fluid Emulsion Droplet Size and Pore-Throat Matching\u003c/div\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e, the emulsion droplet-size statistics were obtained from microscopic image analysis of the produced fluids. One microscope image was selected at each PV point, and 50 droplets were counted in each image. Previous studies have shown that particle transport behavior in porous media depends on the matching relationship between particle size and pore-throat size, giving rise to different modes such as complete blockage, deformation-assisted passage, and ineffective passage\u003csup\u003e[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e. More recent studies on emulsion transport in porous media have further demonstrated that the relationship between droplet size and pore-throat scale can significantly affect droplet passage, retention, and local flow resistance\u003csup\u003e[\u003cspan additionalcitationids=\"CR43\" citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e.When the droplet size exceeds the pore-throat scale, flow resistance increases markedly; in contrast, droplets whose size is better matched to the pore throats are more likely to pass through stably. During the subsequent waterflooding stage of the alkali-free WPS/C12P9E3S composite system, the average size of emulsified oil droplets was mainly concentrated around 30 \u0026micro;m, with a standard deviation of 7 \u0026micro;m. By comparison, the emulsified oil droplets in the weak-alkali ASP system were mostly distributed around 73 \u0026micro;m, with a standard deviation of 8 \u0026micro;m. According to the representative pore-throat radius estimated by the Winland r35 equation\u003csup\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/sup\u003e, Winland r35\u0026thinsp;\u0026asymp;\u0026thinsp;15.1 \u0026micro;m, corresponding to a representative pore-throat diameter of approximately 30 \u0026micro;m. Based on the matching relationship between droplet size and pore-throat diameter, the ratio of droplet diameter to pore-throat diameter in the WPS/C12P9E3S system (D/d\u0026thinsp;\u0026asymp;\u0026thinsp;1) is close to unity, suggesting that the droplets can undergo elastic deformation and pass through pore throats under a relatively low pressure difference, thereby enabling continuous transport. In contrast, for the weak-alkali ASP system, D/d\u0026thinsp;\u0026asymp;\u0026thinsp;2.1, indicating that the droplet size is substantially larger than the pore-throat diameter. Such droplets are more likely to induce pore-throat blockage, leading to additional injection-pressure drop, as shown in \u003cb\u003eFigure\u003c/b\u003e. It can therefore be inferred that the former system is more likely to undergo deformation-assisted passage and continuous transport at lower flow resistance, whereas the latter is more prone to generating additional flow resistance. It should be emphasized that this study did not further compare displacement performance under conditions where droplet size is substantially smaller than the pore-throat scale. Therefore, the contribution of this work does not lie in establishing an optimal droplet-to-throat size ratio, but rather in extending droplet-size analysis from a comparison of absolute droplet size to an interpretation based on its relationship with the representative pore-throat scale, and in indicating that droplet sizes significantly larger than the pore-throat scale may be unfavorable for displacement efficiency.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec40\" class=\"Section3\"\u003e \u003cdiv class=\"Heading\"\u003e3.6.4 Synergistic Transport of Components and the Mechanism of Sustained Interfacial Activity\u003c/div\u003e \u003cp\u003eIn multicomponent chemical flooding systems, the transport behavior of individual functional components in porous media is jointly affected by differences in molecular size, adsorption characteristics, and flow pathways. As a result, the migration velocities of different components may vary substantially, leading to spatial separation of components at the displacement front, i.e., the chromatographic separation effect\u003csup\u003e[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]\u003c/sup\u003e. In the present study, this effect played an important role in the flooding performance of the systems investigated. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e, the relative concentration profiles of the produced fluids indicate that, in the alkali-free WPS/C12P9E3S composite system, the peak positions of the polymer, surfactant, and salt concentrations are closer to one another, and the maximum difference among these peak positions is smaller than that in the weak-alkali ASP system. This suggests a smaller difference in component retardation in porous media, corresponding to a weaker chromatographic separation effect. Greater synchronization among the components is conducive to maintaining the synergistic action of surfactant-induced interfacial tension reduction and polymer-based mobility control over a wider pore-volume range, thereby enhancing the persistence of interfacial activity within the effective displacement interval. Combined with the interfacial tension evolution of the produced fluids, the results further show that the alkali-free WPS/C12P9E3S composite system maintained relatively low interfacial tension over a wider range of injected pore volumes. This may be one of the important reasons why it achieved higher oil recovery than the weak-alkali ASP system. The contribution of this study lies more specifically in proposing, through the correspondence between component concentration peak positions in the produced fluids and the variation in interfacial tension, a process-response-based interpretation: namely, that smaller differences in component retardation may be one of the key factors contributing to the higher oil recovery of the alkali-free composite system.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec41\" class=\"Section3\"\u003e \u003cdiv class=\"Heading\"\u003e3.6.5 Summary\u003c/div\u003e \u003cp\u003eIn summary, the enhanced oil recovery achieved by the alkali-free WPS/C12P9E3S composite system cannot be attributed to a single factor. Rather, based on the experimental results obtained in this study, the advantage of the alkali-free composite system can be interpreted not merely in terms of a \u0026ldquo;better formulation,\u0026rdquo; but more specifically in terms of stronger emulsion stability, a more suitable relationship between emulsion droplet size and pore-throat scale, and smaller differences in component retardation. It should be noted that the mechanistic analysis presented here is based mainly on macroscopic displacement behavior and indirect characterization results. The quantitative mechanisms governing the interfacial film microstructure and component transport behavior still require further verification through interfacial rheological measurements, microscopic visualization experiments, and numerical simulation.\u003c/p\u003e \u003c/div\u003e"},{"header":"5 Conclusion","content":"\u003cp\u003e(1) Based on the characterization of crude oil from the target block, the equivalent alkane carbon number (EACN) was determined to be approximately 12. Accordingly, a series of extended surfactants, n-C12P9EnS (n\u0026thinsp;=\u0026thinsp;3, 6, and 9), was synthesized, among which C12P9E3S showed the most favorable interfacial and emulsification performance.\u003c/p\u003e \u003cp\u003e(2) An alkali-free oil displacement system based on petroleum sulfonate and C12P9E3S was developed. Under optimal conditions (WPS/C12P9E3S\u0026thinsp;=\u0026thinsp;2:1, total surfactant concentration 0.3 wt%, NaCl concentration 1.6 wt%), the system achieved ultralow interfacial tension at the 10⁻\u0026sup3; mN/m level and exhibited good emulsification performance.\u003c/p\u003e \u003cp\u003e(3) Compared with the weak-alkali ASP system, the alkali-free system showed similar anti-adsorption performance but significantly improved emulsion stability, as reflected by lower water separation ratios, lower TSI values, higher absolute zeta potential, and smaller emulsion droplet size.\u003c/p\u003e \u003cp\u003e(4) Core flooding results showed that the alkali-free system achieved higher oil recovery. This improvement is mainly associated with enhanced emulsification and more favorable transport behavior in porous media, rather than the minimum interfacial tension alone.\u003c/p\u003e \u003cp\u003eThese results demonstrate that alkali-free systems can achieve effective oil displacement while reducing operational risks, showing good potential for field application.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work was supported by the Natural Science Foundation of Heilongjiang Province (Grant No. LH2023D013).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAuthor ContributionsJ.W. and Y.H. designed and performed the experiments; Y.Z. conceived the study, supervised the research, and wrote the manuscript; J.W. analyzed the data and prepared the figures; X.Y. contributed to data analysis and interpretation; L.C. and M.Z. provided materials and technical support; F.S. and Z.S. reviewed and edited the manuscript. All authors have read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGuo, H. et al. ASP flooding: Theory and practice progress in China. \u003cem\u003eJ. Chem.\u003c/em\u003e \u003cb\u003e2017\u003c/b\u003e, 2017(1), 8509563. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1155/2017/8509563\u003c/span\u003e\u003cspan address=\"10.1155/2017/8509563\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGuo, H. et al. Recent advances of alkali-surfactant-polymer flooding in China. In SPE Improved Oil Recovery Conference, ; SPE: D031S016R004. 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A\u003c/em\u003e. 610. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.colsurfa.2020.125642\u003c/span\u003e\u003cspan address=\"10.1016/j.colsurfa.2020.125642\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2021).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"extended surfactant, petroleum sulfonate, interfacial behavior, emulsification stability, enhanced oil recovery","lastPublishedDoi":"10.21203/rs.3.rs-9477351/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9477351/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo address the scaling and corrosion problems associated with long-term weak-alkali ASP flooding in Daqing Oilfield, an alkali-free oil-displacement system based on petroleum sulfonate (WPS) and an extended surfactant (C12P9E3S) was developed. The formulation was designed according to the equivalent alkane carbon number (EACN) of the target crude oil and optimized through experimental evaluation of interfacial tension and emulsification performance. The optimal system, with a WPS/C12P9E3S mass ratio of 2:1, total surfactant concentration of 0.3 wt%, and NaCl concentration of 1.6 wt%, achieved ultralow interfacial tension at the 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mN/m level and exhibited improved emulsion stability compared with the conventional weak-alkali ASP system. Core flooding experiments showed that the alkali-free system increased oil recovery by 36.72% after water flooding, which is 5.80 percentage points higher than that of the weak-alkali ASP system. In addition, the system demonstrated more stable displacement behavior and a longer effective flooding stage. The improved oil recovery performance is mainly associated with enhanced emulsification and better transport characteristics of the dispersed oil phase in porous media. The results indicate that the developed alkali-free system can effectively reduce alkali-related operational risks while maintaining favorable oil-displacement efficiency, providing a practical alternative for enhanced oil recovery in mature oilfields.\u003c/p\u003e","manuscriptTitle":"An Alkali-Free Surfactant System Based on Petroleum Sulfonate and Extended Surfactant for Enhanced Oil Recovery","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-18 06:14:29","doi":"10.21203/rs.3.rs-9477351/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"53740812498812532784467795818026512670","date":"2026-05-10T13:02:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"144067871677007938093485412007085991122","date":"2026-05-09T13:10:10+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-05-07T13:07:12+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-05-05T15:40:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-05-05T00:57:37+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-28T02:25:37+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2026-04-28T02:19:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"dd7b5d55-b821-49d0-b219-fb345f1a5efa","owner":[],"postedDate":"May 18th, 2026","published":true,"recentEditorialEvents":[{"type":"reviewerAgreed","content":"53740812498812532784467795818026512670","date":"2026-05-10T13:02:13+00:00","index":43,"fulltext":""},{"type":"reviewerAgreed","content":"144067871677007938093485412007085991122","date":"2026-05-09T13:10:10+00:00","index":42,"fulltext":""},{"type":"reviewersInvited","content":"11","date":"2026-05-07T13:07:12+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-05-05T15:40:44+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":68239254,"name":"Physical sciences/Chemistry"},{"id":68239255,"name":"Physical sciences/Energy science and technology"},{"id":68239256,"name":"Physical sciences/Engineering"},{"id":68239257,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2026-05-18T06:14:29+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-18 06:14:29","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9477351","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9477351","identity":"rs-9477351","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}