{"paper_id":"3bd83cb4-dac2-4bd6-8a25-681c5da6f0de","body_text":"Active ions’ impact in the enhanced oil recovery process: a microfluidic-based approach | 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 Research Article Active ions’ impact in the enhanced oil recovery process: a microfluidic-based approach Yajun Zhang, Menghao Chai, Yumeng Xie, Kunming Liang, Yiqiang Fan This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4258866/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 15 Oct, 2024 Read the published version in Microsystem Technologies → Version 1 posted 7 You are reading this latest preprint version Abstract More than 50% of the crude oil is trapped inside the pores of the rock after the primary and the secondary oil recovery stage, various methods have been currently used for enhanced oil recovery (EOR) to recover the trapped oil. Brine injection, as the most commonly used approach in EOR, was heavily influenced by the concentration of active ions like Ca 2+ , Mg 2+ , and SO 4 2− . In this study, two kinds of polydimethylsiloxane (PDMS)-based microfluidic devices were designed and fabricated to mimic the porous structure in order to study the active ion’s impact in the brine flooding process. Since the PDMS is transparent in the visible range, the fluid flow inside the fabricated porous structure can be observed directly during the brine flooding process. The effect of active ions including Ca 2+ , Mg 2+ , and SO 4 2− in the brine flooding process was studied in detail with the microfluidic devices. The proposed method could have wide application potential in the screening of flooding reagents in the oil industry. EOR brine flooding active ion microfluidics PDMS-based microfluidic device Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Introduction The oil recovery process is usually classified into three stages: the primary stage, the secondary stage, and the tertiary stage [ 1 ] . After the primary stage and the secondary stage, less than 50% of the total oil reserve is recovered. Thus, a further 5–35% of the total oil reserve is expected to be recovered in the tertiary stage. To further improve the recovery rate of oil, the chemical [ 2 , 3 ] , thermal [ 4 ] , polymer [ 5 , 6 ] , microorganism [ 7 , 8 ] , and particle gel [ 9 , 10 ] flooding methods are widely used in the tertiary stage, which is usually referred as the enhanced oil recovery (EOR) process. Brine flooding has been a popular method for EOR because of its high efficiency, low cost, and environmental friendly [ 11 ] . The brine salinity and ionic composition during the brine flooding process have a decisive impact on the recovery rate and have been attracting the interest of researchers worldwide. Evidence from laboratory studies and field tests suggests that the optimized ionic composition of the injected brine (especially active ions including Ca 2+ , Mg 2+ , and SO 4 2− ) can improve the oil recovery rate [ 12 ] . The active ions are capable of altering flow channel surface charges, releasing absorbed carboxylic oil components from the flow channel surface, changing the wettability of the flow channel from oil-wet to water-wet, and eventually improving oil recovery [ 13 – 15 ] . In recent years, microfluidics has been widely used in biological [ 16 ] , chemical [ 17 ] , micro-electro-mechanical [ 18 ], and energy field [ 19 , 20 ] and also was found to be a powerful tool in the EOR research as micromodel. Because microfluidic devices could provide direct visual observation of the oil displacement process, with highly controlled microstructures [ 21 – 23 ] . In this work, four kinds of brines with different active ionic (Ca 2+ , Mg 2+ , and SO 4 2− ) concentrations were prepared and used for the brine flooding process in the proposed microfluidic devices. The effect of the active ionic composition and concentration on the EOR process was studied in detail. Material and Methods In this study, the crude oil was sourced from Sulige oil field, Inner Mongolia, China. In order to reduce the viscosity of crude oil to enhance flowing, 40 wt % n-hexane was used for diluting the crude oil. The viscosimeter (NDJ-8S, Lichen Bangxi Instrument Technology Co., LTD, Shanghai, China) was used to measure the viscosity of crude oil, n-hexane, and experimental oil. The measured viscosities are shown in Table 1 . Table 1 Crude oil properties Type Viscosity(mPa·s) Crude oil 60 n-hexane 5 Experimental oil (Crude oil mixed with 40 wt% n-hexane) 6.99 Custom-made simulated salinity water (SW) was prepared by different combinations of active ions: NaCl, MgCl 2 , CaCl 2 , Na 2 SO 4 (Sinopharm Chemical Reagent Co., LTD, Beijing, China). To obtain simulated water with different concentrations of active ions, the SW was prepared by changing the composition of active ions, and the following terminology was used: (1) SW (0NaCl), SW depleted NaCl; (2) SW (4Ca 2+ ), SW (0NaCl) with 4×Ca 2+ compared to the ordinary SW; (3) SW (4Mg 2+ ), SW (0NaCl) with 4×Mg 2+ ; (4) SW (4SO 4 2− ), SW (0NaCl) with 4×SO 4 2− . The composition of different brines is listed in Table 2 . Table 2 Brine Compositions (mol/L) ions SW (mol/L) SW(0NaCl) (mol/L) SW(4Ca 2+ ) (mol/L) SW(4Mg 2+ ) (mol/L) SW(4SO 4 2− ) (mol/L) Cl − 0.44 0.04 0.10 0.10 0.04 Na + 0.42 0.02 0.02 0.02 0.08 Mg 2+ 0.01 0.01 0.01 0.04 0.01 Ca 2+ 0.01 0.01 0.04 0.01 0.01 SO 4 2− 0.01 0.01 0.01 0.01 0.04 ionic 0.89 0.09 0.18 0.18 0.18 The experimental platform (shown in Fig. 1 ) used in this work is composed of a syringe pump (SPLab02, Baoding Shenzhen Precision Pump Co., LTD, China), silicone tube, custom-made microfluidic device, CCD camera (FDX-906h, Shenzhen Fudingxi Co., LTD, Shenzhen, China) for capture images in the experimental process, LED light for chip illumination, and a computer. Device Design and Fabrication At present, there are four kinds of structures used for tertiary oil recovery study with the microfluidic approach, which are simulated natural reservoir structure [ 24 ] , regular structure [ 25 ] , semi-regular structure [ 26 ], and irregular structure [ 27 ] . Microfluidic devices with different structures are suitable for the experimental study of different oil reservoir types, different pore sizes, and different oil recovery technologies (flooding reagents). In this study, two kinds of devices with natural reservoir and regular structures were designed (shown in Fig. 2 ). The simulated natural reservoir structure (with pore structure sourced from sandstone) was shown in Fig. 2 a. The regular pillar array structure was shown in Fig. 2 b. The device was fabricated with a conventional soft lithography technique based on PDMS (DOW CORNING, USA), the device fabrication process is shown in Fig. 3 . Firstly, the mold was fabricated with a photolithography approach with SU-8 negative photoresist (MicroChem. USA) on a silicon wafer, the detailed process is shown in Fig. 3 a- 3 e, including spin coating the SU-8, prebake, ultraviolet exposure, post bake and developing. Then, a thoroughly mixed and degassed liquid PDMS mixture consisting of 10:1 silicone elastomer and a curing agent was poured onto the SU-8 mold and allowed to solidify at 65°C for 4 h, as illustrated in Fig. 3 f. After the curing process, PDMS was peeled off from the mold, and inlet and outlet holes were made with a punch as shown in Fig. 3 g. Bonding was achieved by placing the PDMS replica onto a clean glass slide after surface treatment in oxygen plasma at 75W, 17s, as shown in Fig. 3 h. Results and Discussion With the help of PDMS-based microfluidic device, the crude oil displacement (recovery) process can be observed directly on microscale in this study. In the first section, the simulated natural reservoir device was used to investigate the effect of brine flooding with four different active ion combinations in the EOR process. At first, the SW was fed into the device at a rate of 10µL/min to simulate the conditions inside the core before oil intrudes into the core. Then, the crude oil was injected at a speed of 10µL/min from the inlet port and swept the whole volume of the channel to simulate the process of crude oil invasion into the core. Next, the brine flooding was conducted at a speed of 10µL/min for the EOR process and the image during the displacement process was recorded every 2s with the camera mounted on the microscope. As shown in Fig. 4 , a path was created around the center of the simulated reservoir by brine-flooding and significant amounts of crude oil was recovered during the process, as the brine-flooding continued, more oil was recovered until a steady state (after around 810s, almost no more crude oil was recovered at the outlet) was reached. The time-lapse image of the flooding over the flooding process is also shown in Fig. 5 , each color represents a captured flow regime at different stages of the flooding process. After the brine flooding experiments conducted with two different microstructures and four different combinations of ion concentrations (Ca 2+ , Mg 2+ , and SO 4 2− ), the oil recovery data (in the form of acquisitive images) were sent for statistical analysis with a custom-designed script in MATLAB. The script can process the images during oil recovery and finally calculate the recovery rate automatically since the crude oil and brine have different colors such can be distinguished. The oil recovery rate indicates the amount of crude oil displaced during the oil displacement process divided by the total amount of crude oil stored inside the pore structure initially. The results of the cumulative crude oil recovery rate in the simulated natural reservoir device during the displacement process with four different brine concentrations of active ions (Ca 2+ , Mg 2+ , and SO 4 2− ) are shown in Fig. 6 . It can be observed that the oil recovery rate with four different brine flooding is relatively low at the first 120s, and jumps quickly from 120s to 240s (due to the formation of through channel), then slowly increases from 240s to 840s (channel expansion) until it finally stabilizes. Figure 6 also indicates that the oil recovery rates with four different action ions combinations are SW (4Mg 2+ ) > SW (4Ca 2+ ) > SW (4SO 4 2− ) > SW (0NaCl) with the 840s long displacement process. In the second section, the regular pillar array device was used to investigate the effect of brine flooding with four different active ion concentrations in the EOR process. The displacement process of SW(0NaCl) is shown in Fig. 7 . The results of the cumulative crude oil recovery rate in the regular pillar array device were also acquired by processing the obtained images with the same methods. The results of the regular pillar array device can be observed in Fig. 8 , the oil recovery process experienced almost stall, rapid growing, slow down, and finally reached a stable state with the relationship of the oil recovery rate of SW (4Mg 2+ ) > SW (4Ca 2+ ) > SW (4SO 4 2− ) > SW (0NaCl). Experimental studies have indicated that, by manipulation of the amount of Ca 2+ , Mg 2+ , and SO 4 2− ions in brine, the crude oil recovery rate differs more than 10%. The differences in oil recovery rate with different combinations of active ions are attributed to the reactivity of key brine ions (Ca 2+ , Mg 2+, and SO 4 2− ) that can change rock surface charges, release absorbed carboxylic oil components from the rock surface, alter rock wettability, and eventually improve crude oil recovery [ 11 – 15 ] . In this work, the effect of active ions (Ca 2+ , Mg 2+ , and SO 4 2− ) on crude oil recovery with the proposed two microfluidic devices are shown in Fig. 9 . It can be observed from Fig. 9 that the crude oil recovery rate can be increased by more than 10% by increasing the concentration of active ions (Ca 2+ , Mg 2+ , and SO 4 2− ) in the simulated natural reservoir device comparing with SW(0NaCl), the increasing rate is 16% for Ca 2+ , 18% for Mg 2+ , 10% for SO 4 2− . However, the increase of oil recovery rate in the regular pillar array device is not as obvious as that in the simulated natural reservoir device, the increased oil recovery rate is 9% for Ca 2+ , 12% for Mg 2+ , and 3% for SO 4 2− . This is due to the simulated natural reservoir device have some narrower throats and pores structures that are sensitive to the change of active ion concentrations in the flooding process. It can also be observed that the oil recovery rate of four kinds of brine in the regular pillar array device is higher compared with the simulated natural reservoir device because of the higher area sweep efficiency due to the highly aligned pillar array. Conclusion In this work, two kinds of microfluidic devices to mimic porous structure were designed and fabricated with the soft lithography technique, and the visualization of the brine flooding process in the PDMS-based microfluidic devices was conducted to study the effect of the active ions (Ca 2+ , Mg 2+ , and SO 4 2− ) in EOR process. According to the experimental results obtained, based on our experimental model, the following findings were made: Firstly, the increase of active ion concentration could have a positive effect on EOR and can promote the production of crude oil; Secondly, different active ions have different effects on EOR, in our experimental setup, the order is Mg 2+ > Ca 2+ > SO 4 2− ; Thirdly, the positive effect on oil recovery in the simulated natural reservoir device is more obvious with the optimal combination of active ions, while the promote on displacement efficiency of the regular pillar array device is less obvious. This study demonstrates the feasibility of using microfluidic approaches on tertiary oil recovery with a focus on active ion concertation, the proposed method could have wide application potential in screening the flooding reagents. Declarations Conflict of Internet The authors declare that we have no conflict of interest. Funding No funding was received to assist with the preparation of this manuscript. Author Contribution M.C. conducted the experimental work, Y.Z. is responsible for the idea and supervise, Y.X. and K.L. supplied experimental materials and equipment, Y.F. drafted the manuscript. Data availability Data are available on reasonable request. References Lake L, Johns RT, Rossen WR, Pope GA (2014) Fundamentals of enhanced oil recovery. Society of Petroleum Engineers Goudarzi A, Delshad M, Sepehrnoori K (2016) A chemical EOR benchmark study of different reservoir simulators. 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Cite Share Download PDF Status: Published Journal Publication published 15 Oct, 2024 Read the published version in Microsystem Technologies → Version 1 posted Editorial decision: Revision requested 02 Sep, 2024 Reviews received at journal 13 Aug, 2024 Reviewers agreed at journal 13 Aug, 2024 Reviewers invited by journal 19 May, 2024 Editor assigned by journal 08 May, 2024 Submission checks completed at journal 13 Apr, 2024 First submitted to journal 12 Apr, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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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-4258866\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":false,\"archivedVersions\":[],\"articleType\":\"Research Article\",\"associatedPublications\":[],\"authors\":[{\"id\":290739204,\"identity\":\"394d5da7-0e84-43c3-acd2-604f5ee1a7d0\",\"order_by\":0,\"name\":\"Yajun Zhang\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Beijing University of Chemical Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Yajun\",\"middleName\":\"\",\"lastName\":\"Zhang\",\"suffix\":\"\"},{\"id\":290739205,\"identity\":\"706e6d98-6d91-4a61-aefd-3457412055bd\",\"order_by\":1,\"name\":\"Menghao Chai\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Beijing University of Chemical Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Menghao\",\"middleName\":\"\",\"lastName\":\"Chai\",\"suffix\":\"\"},{\"id\":290739206,\"identity\":\"560a9887-4f5f-4bdb-827a-69bd31a0d6da\",\"order_by\":2,\"name\":\"Yumeng Xie\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"LK Injection Molding Machine Co\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Yumeng\",\"middleName\":\"\",\"lastName\":\"Xie\",\"suffix\":\"\"},{\"id\":290739207,\"identity\":\"957f214b-a808-49fe-835f-36faa181274d\",\"order_by\":3,\"name\":\"Kunming Liang\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"LK Injection Molding Machine Co\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Kunming\",\"middleName\":\"\",\"lastName\":\"Liang\",\"suffix\":\"\"},{\"id\":290739208,\"identity\":\"61b17759-0b8a-4d93-8bdc-30cdcf8a1613\",\"order_by\":4,\"name\":\"Yiqiang Fan\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABBUlEQVRIie3PPUvDQBjA8ecINMtzzdoivnyEk4KrH8Slm0tOHDOk4abrorsFqV/BIgTHCweXJdIP4FJwrRC3dhC8K3ZM0lHw/sO9cb/hAfD5/mIBgHJ7KCCsx4l7CcRhBBX0oK7cC+kg+xwhM+mOHYSVwUJtXyeA4Zv5oPPJVTS1ZJPkzUT3bov7qgTEm+sRzUv+oIkgd9V7C0GmqDRwCTE7ornhwpKAyHZSfFuC0dqSR8OfDiGayhRwEF8MZyLlz11kaGfRx1IhDtYjVhvFF5YUbbP0l/rl61NmJxjF56txmvH5UherTdJMzhQwu2n8vevdqhr/207FjmT7e9by1+fz+f5rPzU/X0B6gSoiAAAAAElFTkSuQmCC\",\"orcid\":\"\",\"institution\":\"Beijing University of Chemical Technology\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Yiqiang\",\"middleName\":\"\",\"lastName\":\"Fan\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2024-04-12 16:14:35\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-4258866/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-4258866/v1\",\"draftVersion\":[],\"editorialEvents\":[{\"content\":\"https://doi.org/10.1007/s00542-024-05788-8\",\"type\":\"published\",\"date\":\"2024-10-15T15:57:38+00:00\"}],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":54852915,\"identity\":\"cc387829-a3a9-4aeb-a820-02f8182bc62e\",\"added_by\":\"auto\",\"created_at\":\"2024-04-17 17:02:26\",\"extension\":\"jpeg\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":106176,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eAssembled experimental platform\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage1.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4258866/v1/1fcc6ca9706bfb4a588f38ea.jpeg\"},{\"id\":54852636,\"identity\":\"c5d8a376-20f5-4201-8b15-c2a0bb1943f6\",\"added_by\":\"auto\",\"created_at\":\"2024-04-17 16:54:26\",\"extension\":\"jpeg\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":141553,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eDevice structure design. 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16:54:26\",\"extension\":\"jpeg\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":278919,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe process of oil displacement in the simulated natural reservoir device with SW brine\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage4.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4258866/v1/4ee6924543547280cca099bb.jpeg\"},{\"id\":54852642,\"identity\":\"cad34677-6e4a-48e8-9f24-224481d40120\",\"added_by\":\"auto\",\"created_at\":\"2024-04-17 16:54:26\",\"extension\":\"jpeg\",\"order_by\":5,\"title\":\"Figure 5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":152883,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eTime-lapse image in the oil recovery process\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage5.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4258866/v1/09c3d2b5803e2fd02890672e.jpeg\"},{\"id\":54852917,\"identity\":\"2b4a040a-ab03-4e63-8ea0-c2dc288b5a90\",\"added_by\":\"auto\",\"created_at\":\"2024-04-17 17:02:26\",\"extension\":\"png\",\"order_by\":6,\"title\":\"Figure 6\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":7493,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eRecovery rate of brine flooding with different combinations of active ions in the simulated natural reservoir device\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Onlinefloatimage6.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4258866/v1/20a4e82f5b7e262432704ccf.png\"},{\"id\":54852640,\"identity\":\"d294ad18-ca7c-4b3d-8111-b1be8fece1c6\",\"added_by\":\"auto\",\"created_at\":\"2024-04-17 16:54:26\",\"extension\":\"jpeg\",\"order_by\":7,\"title\":\"Figure 7\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":223128,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe process of displacement in the regular pillar array device, the flooding regent used in this process is SW (0NaCl)\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage7.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4258866/v1/487b3fa63852a44b729804a6.jpeg\"},{\"id\":54852644,\"identity\":\"e2829224-34f7-4b85-994d-daebde97b9dc\",\"added_by\":\"auto\",\"created_at\":\"2024-04-17 16:54:26\",\"extension\":\"png\",\"order_by\":8,\"title\":\"Figure 8\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":7473,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eRecovery rate of different brine flooding in the regular pillar array device\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Onlinefloatimage8.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4258866/v1/b510c0200fb5c5dd67e8b980.png\"},{\"id\":54852639,\"identity\":\"1c115931-95f5-49b8-87c2-e615f6ef4236\",\"added_by\":\"auto\",\"created_at\":\"2024-04-17 16:54:26\",\"extension\":\"png\",\"order_by\":9,\"title\":\"Figure 9\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":10379,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eSteady-state oil recovery of two kinds of devices\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Onlinefloatimage9.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4258866/v1/b9afedf0099dfa60d582e7df.png\"},{\"id\":67149268,\"identity\":\"aea74215-be0e-4b81-919e-02427c61ac49\",\"added_by\":\"auto\",\"created_at\":\"2024-10-21 16:12:56\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":1373031,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4258866/v1/dc83fc83-f958-4f4b-94cd-53879fc073d8.pdf\"}],\"financialInterests\":\"No competing interests reported.\",\"formattedTitle\":\"Active ions’ impact in the enhanced oil recovery process: a microfluidic-based approach\",\"fulltext\":[{\"header\":\"Introduction\",\"content\":\"\\u003cp\\u003eThe oil recovery process is usually classified into three stages: the primary stage, the secondary stage, and the tertiary stage\\u003csup\\u003e[\\u003cspan citationid=\\\"CR1\\\" class=\\\"CitationRef\\\"\\u003e1\\u003c/span\\u003e]\\u003c/sup\\u003e. After the primary stage and the secondary stage, less than 50% of the total oil reserve is recovered. Thus, a further 5\\u0026ndash;35% of the total oil reserve is expected to be recovered in the tertiary stage. To further improve the recovery rate of oil, the chemical\\u003csup\\u003e[\\u003cspan citationid=\\\"CR2\\\" class=\\\"CitationRef\\\"\\u003e2\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR3\\\" class=\\\"CitationRef\\\"\\u003e3\\u003c/span\\u003e]\\u003c/sup\\u003e, thermal\\u003csup\\u003e[\\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e4\\u003c/span\\u003e]\\u003c/sup\\u003e, polymer\\u003csup\\u003e[\\u003cspan citationid=\\\"CR5\\\" class=\\\"CitationRef\\\"\\u003e5\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR6\\\" class=\\\"CitationRef\\\"\\u003e6\\u003c/span\\u003e]\\u003c/sup\\u003e, microorganism\\u003csup\\u003e[\\u003cspan citationid=\\\"CR7\\\" class=\\\"CitationRef\\\"\\u003e7\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR8\\\" class=\\\"CitationRef\\\"\\u003e8\\u003c/span\\u003e]\\u003c/sup\\u003e, and particle gel\\u003csup\\u003e[\\u003cspan citationid=\\\"CR9\\\" class=\\\"CitationRef\\\"\\u003e9\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR10\\\" class=\\\"CitationRef\\\"\\u003e10\\u003c/span\\u003e]\\u003c/sup\\u003e flooding methods are widely used in the tertiary stage, which is usually referred as the enhanced oil recovery (EOR) process.\\u003c/p\\u003e \\u003cp\\u003eBrine flooding has been a popular method for EOR because of its high efficiency, low cost, and environmental friendly\\u003csup\\u003e[\\u003cspan citationid=\\\"CR11\\\" class=\\\"CitationRef\\\"\\u003e11\\u003c/span\\u003e]\\u003c/sup\\u003e. The brine salinity and ionic composition during the brine flooding process have a decisive impact on the recovery rate and have been attracting the interest of researchers worldwide. Evidence from laboratory studies and field tests suggests that the optimized ionic composition of the injected brine (especially active ions including Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e ) can improve the oil recovery rate\\u003csup\\u003e[\\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e]\\u003c/sup\\u003e. The active ions are capable of altering flow channel surface charges, releasing absorbed carboxylic oil components from the flow channel surface, changing the wettability of the flow channel from oil-wet to water-wet, and eventually improving oil recovery\\u003csup\\u003e[\\u003cspan additionalcitationids=\\\"CR14\\\" citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR15\\\" class=\\\"CitationRef\\\"\\u003e15\\u003c/span\\u003e]\\u003c/sup\\u003e.\\u003c/p\\u003e \\u003cp\\u003eIn recent years, microfluidics has been widely used in biological\\u003csup\\u003e[\\u003cspan citationid=\\\"CR16\\\" class=\\\"CitationRef\\\"\\u003e16\\u003c/span\\u003e]\\u003c/sup\\u003e, chemical\\u003csup\\u003e[\\u003cspan citationid=\\\"CR17\\\" class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e]\\u003c/sup\\u003e, micro-electro-mechanical\\u003csup\\u003e[\\u003cspan citationid=\\\"CR18\\\" class=\\\"CitationRef\\\"\\u003e18\\u003c/span\\u003e],\\u003c/sup\\u003e and energy field\\u003csup\\u003e[\\u003cspan citationid=\\\"CR19\\\" class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR20\\\" class=\\\"CitationRef\\\"\\u003e20\\u003c/span\\u003e]\\u003c/sup\\u003e and also was found to be a powerful tool in the EOR research as micromodel. Because microfluidic devices could provide direct visual observation of the oil displacement process, with highly controlled microstructures\\u003csup\\u003e[\\u003cspan additionalcitationids=\\\"CR22\\\" citationid=\\\"CR21\\\" class=\\\"CitationRef\\\"\\u003e21\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR23\\\" class=\\\"CitationRef\\\"\\u003e23\\u003c/span\\u003e]\\u003c/sup\\u003e. In this work, four kinds of brines with different active ionic (Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e) concentrations were prepared and used for the brine flooding process in the proposed microfluidic devices. The effect of the active ionic composition and concentration on the EOR process was studied in detail.\\u003c/p\\u003e\"},{\"header\":\"Material and Methods\",\"content\":\"\\u003cp\\u003eIn this study, the crude oil was sourced from Sulige oil field, Inner Mongolia, China. In order to reduce the viscosity of crude oil to enhance flowing, 40 wt % n-hexane was used for diluting the crude oil. The viscosimeter (NDJ-8S, Lichen Bangxi Instrument Technology Co., LTD, Shanghai, China) was used to measure the viscosity of crude oil, n-hexane, and experimental oil. The measured viscosities are shown in Table\\u0026nbsp;\\u003cspan refid=\\\"Tab1\\\" class=\\\"InternalRef\\\"\\u003e1\\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\\u003eCrude oil properties\\u003c/p\\u003e \\u003c/div\\u003e \\u003c/caption\\u003e \\u003ccolgroup cols=\\\"2\\\"\\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 \\u003cthead\\u003e \\u003ctr\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eType\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003eViscosity(mPa\\u0026middot;s)\\u003c/p\\u003e \\u003c/th\\u003e \\u003c/tr\\u003e \\u003c/thead\\u003e \\u003ctbody\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eCrude oil\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e60\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003en-hexane\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e5\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eExperimental oil (Crude oil mixed with 40 wt% n-hexane)\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e6.99\\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\\u003eCustom-made simulated salinity water (SW) was prepared by different combinations of active ions: NaCl, MgCl\\u003csub\\u003e2\\u003c/sub\\u003e, CaCl\\u003csub\\u003e2\\u003c/sub\\u003e, Na\\u003csub\\u003e2\\u003c/sub\\u003eSO\\u003csub\\u003e4\\u003c/sub\\u003e (Sinopharm Chemical Reagent Co., LTD, Beijing, China). To obtain simulated water with different concentrations of active ions, the SW was prepared by changing the composition of active ions, and the following terminology was used: (1) SW (0NaCl), SW depleted NaCl; (2) SW (4Ca\\u003csup\\u003e2+\\u003c/sup\\u003e), SW (0NaCl) with 4\\u0026times;Ca\\u003csup\\u003e2+\\u003c/sup\\u003e compared to the ordinary SW; (3) SW (4Mg\\u003csup\\u003e2+\\u003c/sup\\u003e), SW (0NaCl) with 4\\u0026times;Mg\\u003csup\\u003e2+\\u003c/sup\\u003e; (4) SW (4SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e), SW (0NaCl) with 4\\u0026times;SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e. The composition of different brines is listed in Table \\u003cspan refid=\\\"Tab2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\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\\u003eBrine Compositions (mol/L)\\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=\\\".\\\" 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 \\u003cthead\\u003e \\u003ctr\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eions\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003eSW\\u003c/p\\u003e \\u003cp\\u003e(mol/L)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003eSW(0NaCl)\\u003c/p\\u003e \\u003cp\\u003e(mol/L)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003eSW(4Ca\\u003csup\\u003e2+\\u003c/sup\\u003e)\\u003c/p\\u003e \\u003cp\\u003e(mol/L)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003eSW(4Mg\\u003csup\\u003e2+\\u003c/sup\\u003e)\\u003c/p\\u003e \\u003cp\\u003e(mol/L)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003eSW(4SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e)\\u003c/p\\u003e \\u003cp\\u003e(mol/L)\\u003c/p\\u003e \\u003c/th\\u003e \\u003c/tr\\u003e \\u003c/thead\\u003e \\u003ctbody\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eCl\\u003csup\\u003e\\u0026minus;\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.44\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e0.04\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e0.10\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e0.10\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e0.04\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eNa\\u003csup\\u003e+\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.42\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e0.02\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e0.02\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e0.02\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e0.08\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eMg\\u003csup\\u003e2+\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e0.04\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eCa\\u003csup\\u003e2+\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e0.04\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eSO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e0.01\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e0.04\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eionic\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.89\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e0.09\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e0.18\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e0.18\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e0.18\\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\\u003eThe experimental platform (shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e) used in this work is composed of a syringe pump (SPLab02, Baoding Shenzhen Precision Pump Co., LTD, China), silicone tube, custom-made microfluidic device, CCD camera (FDX-906h, Shenzhen Fudingxi Co., LTD, Shenzhen, China) for capture images in the experimental process, LED light for chip illumination, and a computer.\\u003c/p\\u003e\\u003cp\\u003eDevice Design and Fabrication\\u003c/p\\u003e \\u003cp\\u003eAt present, there are four kinds of structures used for tertiary oil recovery study with the microfluidic approach, which are simulated natural reservoir structure\\u003csup\\u003e[\\u003cspan citationid=\\\"CR24\\\" class=\\\"CitationRef\\\"\\u003e24\\u003c/span\\u003e]\\u003c/sup\\u003e, regular structure\\u003csup\\u003e[\\u003cspan citationid=\\\"CR25\\\" class=\\\"CitationRef\\\"\\u003e25\\u003c/span\\u003e]\\u003c/sup\\u003e, semi-regular structure\\u003csup\\u003e[\\u003cspan citationid=\\\"CR26\\\" class=\\\"CitationRef\\\"\\u003e26\\u003c/span\\u003e],\\u003c/sup\\u003e and irregular structure\\u003csup\\u003e[\\u003cspan citationid=\\\"CR27\\\" class=\\\"CitationRef\\\"\\u003e27\\u003c/span\\u003e]\\u003c/sup\\u003e. Microfluidic devices with different structures are suitable for the experimental study of different oil reservoir types, different pore sizes, and different oil recovery technologies (flooding reagents). In this study, two kinds of devices with natural reservoir and regular structures were designed (shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e). The simulated natural reservoir structure (with pore structure sourced from sandstone) was shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ea. The regular pillar array structure was shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eb.\\u003c/p\\u003e \\u003cp\\u003eThe device was fabricated with a conventional soft lithography technique based on PDMS (DOW CORNING, USA), the device fabrication process is shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e. Firstly, the mold was fabricated with a photolithography approach with SU-8 negative photoresist (MicroChem. USA) on a silicon wafer, the detailed process is shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ea-\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ee, including spin coating the SU-8, prebake, ultraviolet exposure, post bake and developing. Then, a thoroughly mixed and degassed liquid PDMS mixture consisting of 10:1 silicone elastomer and a curing agent was poured onto the SU-8 mold and allowed to solidify at 65\\u0026deg;C for 4 h, as illustrated in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ef. After the curing process, PDMS was peeled off from the mold, and inlet and outlet holes were made with a punch as shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eg. Bonding was achieved by placing the PDMS replica onto a clean glass slide after surface treatment in oxygen plasma at 75W, 17s, as shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eh.\\u003c/p\\u003e \"},{\"header\":\"Results and Discussion\",\"content\":\"\\u003cp\\u003eWith the help of PDMS-based microfluidic device, the crude oil displacement (recovery) process can be observed directly on microscale in this study. In the first section, the simulated natural reservoir device was used to investigate the effect of brine flooding with four different active ion combinations in the EOR process. At first, the SW was fed into the device at a rate of 10\\u0026micro;L/min to simulate the conditions inside the core before oil intrudes into the core. Then, the crude oil was injected at a speed of 10\\u0026micro;L/min from the inlet port and swept the whole volume of the channel to simulate the process of crude oil invasion into the core. Next, the brine flooding was conducted at a speed of 10\\u0026micro;L/min for the EOR process and the image during the displacement process was recorded every 2s with the camera mounted on the microscope. As shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e, a path was created around the center of the simulated reservoir by brine-flooding and significant amounts of crude oil was recovered during the process, as the brine-flooding continued, more oil was recovered until a steady state (after around 810s, almost no more crude oil was recovered at the outlet) was reached. The time-lapse image of the flooding over the flooding process is also shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e, each color represents a captured flow regime at different stages of the flooding process.\\u003c/p\\u003e\\u003cp\\u003eAfter the brine flooding experiments conducted with two different microstructures and four different combinations of ion concentrations (Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e), the oil recovery data (in the form of acquisitive images) were sent for statistical analysis with a custom-designed script in MATLAB. The script can process the images during oil recovery and finally calculate the recovery rate automatically since the crude oil and brine have different colors such can be distinguished. The oil recovery rate indicates the amount of crude oil displaced during the oil displacement process divided by the total amount of crude oil stored inside the pore structure initially.\\u003c/p\\u003e \\u003cp\\u003eThe results of the cumulative crude oil recovery rate in the simulated natural reservoir device during the displacement process with four different brine concentrations of active ions (Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e) are shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e. It can be observed that the oil recovery rate with four different brine flooding is relatively low at the first 120s, and jumps quickly from 120s to 240s (due to the formation of through channel), then slowly increases from 240s to 840s (channel expansion) until it finally stabilizes. Figure\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e also indicates that the oil recovery rates with four different action ions combinations are SW (4Mg\\u003csup\\u003e2+\\u003c/sup\\u003e)\\u0026thinsp;\\u0026gt;\\u0026thinsp;SW (4Ca\\u003csup\\u003e2+\\u003c/sup\\u003e)\\u0026thinsp;\\u0026gt;\\u0026thinsp;SW (4SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e)\\u0026thinsp;\\u0026gt;\\u0026thinsp;SW (0NaCl) with the 840s long displacement process.\\u003c/p\\u003e\\u003cp\\u003eIn the second section, the regular pillar array device was used to investigate the effect of brine flooding with four different active ion concentrations in the EOR process. The displacement process of SW(0NaCl) is shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003e. The results of the cumulative crude oil recovery rate in the regular pillar array device were also acquired by processing the obtained images with the same methods. The results of the regular pillar array device can be observed in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003e, the oil recovery process experienced almost stall, rapid growing, slow down, and finally reached a stable state with the relationship of the oil recovery rate of SW (4Mg\\u003csup\\u003e2+\\u003c/sup\\u003e)\\u0026thinsp;\\u0026gt;\\u0026thinsp;SW (4Ca\\u003csup\\u003e2+\\u003c/sup\\u003e)\\u0026thinsp;\\u0026gt;\\u0026thinsp;SW (4SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e)\\u0026thinsp;\\u0026gt;\\u0026thinsp;SW (0NaCl).\\u003c/p\\u003e \\u003cp\\u003eExperimental studies have indicated that, by manipulation of the amount of Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e ions in brine, the crude oil recovery rate differs more than 10%. The differences in oil recovery rate with different combinations of active ions are attributed to the reactivity of key brine ions (Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+,\\u003c/sup\\u003e and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e) that can change rock surface charges, release absorbed carboxylic oil components from the rock surface, alter rock wettability, and eventually improve crude oil recovery\\u003csup\\u003e[\\u003cspan additionalcitationids=\\\"CR12 CR13 CR14\\\" citationid=\\\"CR11\\\" class=\\\"CitationRef\\\"\\u003e11\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR15\\\" class=\\\"CitationRef\\\"\\u003e15\\u003c/span\\u003e]\\u003c/sup\\u003e.\\u003c/p\\u003e \\u003cp\\u003eIn this work, the effect of active ions (Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e) on crude oil recovery with the proposed two microfluidic devices are shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig9\\\" class=\\\"InternalRef\\\"\\u003e9\\u003c/span\\u003e. It can be observed from Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig9\\\" class=\\\"InternalRef\\\"\\u003e9\\u003c/span\\u003e that the crude oil recovery rate can be increased by more than 10% by increasing the concentration of active ions (Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e) in the simulated natural reservoir device comparing with SW(0NaCl), the increasing rate is 16% for Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, 18% for Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, 10% for SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e. However, the increase of oil recovery rate in the regular pillar array device is not as obvious as that in the simulated natural reservoir device, the increased oil recovery rate is 9% for Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, 12% for Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and 3% for SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e. This is due to the simulated natural reservoir device have some narrower throats and pores structures that are sensitive to the change of active ion concentrations in the flooding process. It can also be observed that the oil recovery rate of four kinds of brine in the regular pillar array device is higher compared with the simulated natural reservoir device because of the higher area sweep efficiency due to the highly aligned pillar array.\\u003c/p\\u003e\"},{\"header\":\"Conclusion\",\"content\":\"\\u003cp\\u003eIn this work, two kinds of microfluidic devices to mimic porous structure were designed and fabricated with the soft lithography technique, and the visualization of the brine flooding process in the PDMS-based microfluidic devices was conducted to study the effect of the active ions (Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e) in EOR process. According to the experimental results obtained, based on our experimental model, the following findings were made: Firstly, the increase of active ion concentration could have a positive effect on EOR and can promote the production of crude oil; Secondly, different active ions have different effects on EOR, in our experimental setup, the order is Mg\\u003csup\\u003e2+\\u003c/sup\\u003e\\u0026gt; Ca\\u003csup\\u003e2+\\u003c/sup\\u003e\\u0026gt; SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e; Thirdly, the positive effect on oil recovery in the simulated natural reservoir device is more obvious with the optimal combination of active ions, while the promote on displacement efficiency of the regular pillar array device is less obvious. This study demonstrates the feasibility of using microfluidic approaches on tertiary oil recovery with a focus on active ion concertation, the proposed method could have wide application potential in screening the flooding reagents.\\u003c/p\\u003e \"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e \\u003ch2\\u003eConflict of Internet\\u003c/h2\\u003e The authors declare that we have no conflict of interest.\\u003c/p\\u003e \\u003c/p\\u003e\\u003ch2\\u003eFunding\\u003c/h2\\u003e \\u003cp\\u003eNo funding was received to assist with the preparation of this manuscript.\\u003c/p\\u003e\\u003ch2\\u003eAuthor Contribution\\u003c/h2\\u003e\\u003cp\\u003eM.C. conducted the experimental work, Y.Z. is responsible for the idea and supervise, Y.X. and K.L. supplied experimental materials and equipment, Y.F. drafted the manuscript.\\u003c/p\\u003e\\u003ch2\\u003eData availability\\u003c/h2\\u003e \\u003cp\\u003eData are available on reasonable request.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\u003cli\\u003e\\u003cspan\\u003eLake L, Johns RT, Rossen WR, Pope GA (2014) Fundamentals of enhanced oil recovery. 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J Petrol Sci Eng 176:612\\u0026ndash;620. \\u003cspan class=\\\"ExternalRef\\\"\\u003e\\u003cspan class=\\\"RefSource\\\"\\u003ehttps://doi.org/10.1016/j.petrol.2019.01.109\\u003c/span\\u003e\\u003cspan address=\\\"10.1016/j.petrol.2019.01.109\\\" targettype=\\\"DOI\\\" class=\\\"RefTarget\\\"\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\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\":true,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"microsystem-technologies\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"mite\",\"sideBox\":\"Learn more about [Microsystem Technologies](http://link.springer.com/journal/542)\",\"snPcode\":\"542\",\"submissionUrl\":\"https://submission.nature.com/new-submission/542/3\",\"title\":\"Microsystem Technologies\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"stoa\",\"reportingPortfolio\":\"Springer Hybrid\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":false},\"keywords\":\"EOR, brine flooding, active ion, microfluidics, PDMS-based microfluidic device\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-4258866/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-4258866/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eMore than 50% of the crude oil is trapped inside the pores of the rock after the primary and the secondary oil recovery stage, various methods have been currently used for enhanced oil recovery (EOR) to recover the trapped oil. Brine injection, as the most commonly used approach in EOR, was heavily influenced by the concentration of active ions like Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e. In this study, two kinds of polydimethylsiloxane (PDMS)-based microfluidic devices were designed and fabricated to mimic the porous structure in order to study the active ion\\u0026rsquo;s impact in the brine flooding process. Since the PDMS is transparent in the visible range, the fluid flow inside the fabricated porous structure can be observed directly during the brine flooding process. The effect of active ions including Ca\\u003csup\\u003e2+\\u003c/sup\\u003e, Mg\\u003csup\\u003e2+\\u003c/sup\\u003e, and SO\\u003csub\\u003e4\\u003c/sub\\u003e\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e in the brine flooding process was studied in detail with the microfluidic devices. The proposed method could have wide application potential in the screening of flooding reagents in the oil industry.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Active ions’ impact in the enhanced oil recovery process: a microfluidic-based approach\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2024-04-17 16:54:21\",\"doi\":\"10.21203/rs.3.rs-4258866/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0},{\"type\":\"decision\",\"content\":\"Revision requested\",\"date\":\"2024-09-02T08:51:58+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"editorInvitedReview\",\"content\":\"\",\"date\":\"2024-08-13T16:48:02+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"reviewerAgreed\",\"content\":\"92125402172293540980697378156313288370\",\"date\":\"2024-08-13T15:28:07+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"reviewersInvited\",\"content\":\"\",\"date\":\"2024-05-19T12:48:39+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"editorAssigned\",\"content\":\"\",\"date\":\"2024-05-08T11:57:00+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"checksComplete\",\"content\":\"\",\"date\":\"2024-04-13T05:56:21+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"submitted\",\"content\":\"Microsystem Technologies\",\"date\":\"2024-04-12T16:11:37+00:00\",\"index\":\"\",\"fulltext\":\"\"}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"microsystem-technologies\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"mite\",\"sideBox\":\"Learn more about [Microsystem Technologies](http://link.springer.com/journal/542)\",\"snPcode\":\"542\",\"submissionUrl\":\"https://submission.nature.com/new-submission/542/3\",\"title\":\"Microsystem Technologies\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"stoa\",\"reportingPortfolio\":\"Springer Hybrid\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":false}}],\"origin\":\"\",\"ownerIdentity\":\"2a184f47-c378-4d04-ab06-98a75f2cd500\",\"owner\":[],\"postedDate\":\"April 17th, 2024\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"published-in-journal\",\"subjectAreas\":[],\"tags\":[],\"updatedAt\":\"2024-10-21T16:07:45+00:00\",\"versionOfRecord\":{\"articleIdentity\":\"rs-4258866\",\"link\":\"https://doi.org/10.1007/s00542-024-05788-8\",\"journal\":{\"identity\":\"microsystem-technologies\",\"isVorOnly\":false,\"title\":\"Microsystem Technologies\"},\"publishedOn\":\"2024-10-15 15:57:38\",\"publishedOnDateReadable\":\"October 15th, 2024\"},\"versionCreatedAt\":\"2024-04-17 16:54:21\",\"video\":\"\",\"vorDoi\":\"10.1007/s00542-024-05788-8\",\"vorDoiUrl\":\"https://doi.org/10.1007/s00542-024-05788-8\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-4258866\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-4258866\",\"identity\":\"rs-4258866\",\"version\":[\"v1\"]},\"buildId\":\"8U1c8b4HqxoKbykW_rLl7\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}