Quantitative characterization of pore structure and NMR fractal based on nuclear magnetic resonance and fractal theory

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The T 2 spectra of six groups of rock samples in saturated water and bound water were obtained by low field nuclear magnetic resonance experiment, and the complexity and heterogeneity of pore structure were quantitatively characterized by fractal dimension. The results show that the reservoir is characterized by low porosity and low permeability, but the microscopic pore structure is significantly different. Some samples are mainly medium-small pores, with good pore connectivity, high movable fluid saturation and good seepage capacity. Most of the samples are dominated by micropores, with dense structure and poor fluid mobility. The cut-off value of T 2 (T 2C ) and effective porosity further reveal the difference of fluid mobility in reservoirs, and high values usually correspond to high-quality reservoirs. In addition, through fractal dimension analysis, the essential difference in structural complexity between bound fluid pores ( D min average 2.144 ) and movable fluid pores ( D max average 2.959 ) is clarified, indicating that the seepage pore system has higher heterogeneity and tortuosity. This study verifies the effectiveness of the combination of NMR and fractal theory in the quantitative characterization of tight sandstone pore structure, and provides a scientific basis for reservoir evaluation and CO 2 geological storage potential prediction. Physical sciences/Energy science and technology Earth and environmental sciences/Solid earth sciences NMR fractal theory sandstone reservoir pore structure T2 cut-off value(T2C) Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1 Introduction Tight sandstone reservoir is an important part of unconventional oil and gas reservoir, and it is one of the research focuses in the field of reservoir [ 1 – 5 ] . Tight sandstone is composed of relatively dense clastic rocks, mainly including siltstone, siltstone-fine sandstone, fine sandstone and a small part of medium-coarse sandstone. Due to the complexity and strong heterogeneity of the microscopic pore structure of tight sandstone reservoirs, it is difficult to carry out reservoir characteristics research and reservoir evaluation. Reservoir classification evaluation is an important part of reservoir research, and the results of reservoir evaluation will affect the overall evaluation of the reservoir [ 6 – 9 ] . At present, there are two main methods for reservoir classification and evaluation : one is qualitative evaluation based on geology, logging, earthquake and other disciplines ; the other is quantitative evaluation based on mathematical methods, such as fuzzy synthesis, grey correlation degree, analytic hierarchy process, fractal dimension method and so on. In recent years, nuclear magnetic resonance (NMR) has been widely used in the characterization of pore and fracture structures in porous media due to its non-destructive, fast and accurate advantages. Yao et al. [ 10 ] used low field nuclear magnetic resonance and computed tomography (CT) scanning technology to describe the pore size distribution and pore fracture space configuration of coal. Based on low field nuclear magnetic resonance, Zhou et al. [ 11 ] developed a method to quantify the fractal dimension of pores and fractures. Combined with nuclear magnetic resonance, high pressure mercury injection and scanning electron microscopy, Chai et al.explored the pore characteristics of shale in complex structural areas. Liu et al. [ 12 ] studied the pore structure of high-rank coal with different bedding structures by means of nuclear magnetic resonance, and analyzed the influence of centrifugal force. Liang et al. [ 13 ] studied the pore characteristics of shale reservoirs in Shahezi Formation of Songliao Basin by box dimension method and NMR fractal theory. They believed that fractal dimension has a good correlation with physical parameters such as shale porosity and permeability, and can be used as an index to evaluate shale physical properties. Many scholars have carried out research on reservoir classification and evaluation based on fractal theory. Yan et al. [ 14 ] based on fractal theory, using nuclear magnetic resonance T 2 spectrum and other experiments to explore the relationship between fractal dimension and pore structure type, clear the pore structure type ; according to the casting thin section, mercury injection and other data, ES4 is finally divided into 3 categories and 5 subcategories. Based on different basic geological experimental analysis data, Liu et al. [ 15 ] used two saturation methods to calculate the fractal dimension of pore structure of tight reservoir in He 1 member of Hangjinqi area, and analyzed its relationship with reservoir physical properties. Zhu et al. [ 16 ] took the Chang 6 tight sandstone reservoir of the Yanchang Formation in the Yan 'an-Ganquan area of the southern Ordos Basin as an example. Through casting thin sections, scanning electron microscopy analysis, X-ray diffraction and high-pressure mercury injection experiments, combined with fractal theory, the fractal characteristics of the target reservoir were determined, and the relationship between fractal dimension and reservoir physical properties, pore characteristics and mineral composition was discussed. Li et al. [ 17 ] used a variety of experimental methods and theoretical methods to compare the differences between the NMR fractal dimension, the fractal dimension of the nitrogen adsorption method and the fractal dimension of the mercury intrusion method. It is believed that the NMR fractal dimension can more accurately reflect the pore structure characteristics of shale. In summary, low-field nuclear magnetic resonance technology has achieved a lot of research results in the characterization of porous media pore structure due to its unique advantages. However, the study of pore and fracture structure of deep sandstone in coal mine is the basis of mine water storage or CO 2 storage. Although low-field nuclear magnetic resonance technology has been widely used in the characterization of pore and fracture structure of coal rock, there are few reports on the characterization of pore and fracture structure of deep sandstone in coal mine by low-field nuclear magnetic resonance technology. Therefore, this paper takes the deep tight sandstone reservoir of Dahaize Coal Mine in Ordos Basin as the research object, and uses nuclear magnetic resonance ( NMR ) experimental methods to quantitatively characterize the microscopic pore structure characteristics of the reservoir on the basis of systematically obtaining the distribution of core T 2 spectrum. By introducing NMR fractal theory, the internal relationship between fractal dimension and pore structure parameters, physical properties and fluid mobility is established to reveal the heterogeneity and complexity of pore structure of tight sandstone reservoirs, and to clarify the key microscopic factors controlling fluid occurrence and migration. In order to provide a reliable theoretical basis and technical support for the efficient evaluation of tight sandstone reservoirs and the prediction of CO 2 geological storage potential. 2 Experimental methods 2.1 Source of sample The samples used in the test were taken from the sandstone of Dahaize Coal Mine of China Coal Shaanxi Yulin Energy Chemical Co., Ltd. The Dahaize mine is located in the northwest of Yulin City, with a straight distance of about 50 km. The mine area is 280.03 km 2 , the total amount of coal resources is about 5.1 billion tons, and the production capacity is 20 million tons/year. The geological structure is simple. The Yan 'an Formation contains 7 coal seams, with an average total thickness of 15.41 m. Among them, the No.2 and No.3 coal seams with simple structure, recoverable and stable main coal seams in the whole area have an average total thickness of 10.30 m. Due to the large amount of mine water inflow in Dahaize Coal Mine, the drainage water of coal seam roof and the water in goaf are deeply transferred and sealed, which preliminarily realizes the deep, stable and safe storage of roof water in Dahaize Coal Mine in deep strata, without any environmental and safety risks. Sandstones of different depths were taken to the ground by drilling coring, and the depth was in the range of 2500 m-2600 m. The sandstones were processed into cylinders of the same size of Φ25 mm × 50 mm for nuclear magnetic resonance (NMR) experiments, and samples without obvious structural damage were selected to ensure the experimental accuracy. 2.2 Test principle Nuclear magnetic resonance (NMR) is a non-destructive technology widely used in the field of rock physics. Based on the magnetic properties of atomic nuclei, the pore structure, fluid properties and distribution of rocks can be obtained by analyzing the transition of hydrogen nuclei in rocks in magnetic field. The nuclear magnetic resonance T 2 spectrum can reflect the attenuation of the atomic nuclear magnetic resonance signal in the core sample in time. Through the analysis of several spectra, the information of different pore diameter and distribution in the rock can be obtained. Through nuclear magnetic resonance experiments, the T 2 relaxation time and distribution characteristics of fluid in rock samples can be obtained, and then the occurrence state of fluid in rock pores can be analyzed. The length of the T 2 relaxation time of the fluid is related to the strength of the surface force of the pore solid. When the fluid is subjected to a strong force, the T 2 relaxation time is very short, and the fluid is in a stagnant state ; when the fluid is subjected to a weak force, the T 2 relaxation time is longer and the fluid is movable. Large pores correspond to longer relaxation time, and small pores correspond to shorter relaxation time. 2.3 Experimental samples and process The sandstone reservoir of Dahaize Coal Mine in Yulin area of Shaanxi Province, China was selected as the research object. Six cylindrical specimens of Φ25 mm × 50 mm with the same size were processed ( as shown in Fig. 1 ). The nuclear magnetic resonance equipment was MacroMR 12–150 H-I nuclear magnetic resonance test system produced by Suzhou Newmai, China, as shown in Fig. 2 . The test was carried out based on the oil and gas industry standard 'Determination method of relative permeability of two-phase fluid in rock' (GB/T 28912 − 2012) and 'Laboratory measurement specification of nuclear magnetic resonance parameters of rock samples' (SY/T 6490 − 2014). The test parameters were set as follows : CPMG sequence of nuclear magnetic resonance, waiting time TW = 5000ms, echo interval time TE = 0.1ms ( minimum 0.06ms of equipment ), echo number NE = 15000, scanning times 32. The experimental method is as follows : first, the core holder is vacuumized with a vacuum pump to achieve the ideal vacuum condition, and then the core is saturated with distilled water for 12h under the excess pore pressure of 20MPa to ensure that the core is completely saturated. The saturated weight m 1 is weighed, and the CPMG sequence test is performed to obtain the T 2 distribution of the saturated core. Then, the water-saturated core was centrifuged at a speed of 6000r/min to remove the free water in the core plunger, and the weight m 2 after centrifugation was measured again to obtain the NMR T 2 in the centrifugal state. 3 Test results The results of NMR experiments are shown in Table 1 . It can be seen that the permeability distribution range of sandstone reservoir core samples in Dahaize Coal Mine is mainly concentrated in (0.13 ~ 14.13)×10 − 3 um 2 ; the nuclear magnetic porosity is between 1.854%~9.416%. The range of movable fluid saturation is 47.769%~93.05%. The geometric mean of saturated T 2 is mainly distributed in the range of 0.3 ~ 8.418 ms, the geometric mean of centrifugal T2 is mainly distributed in the range of 0.268 ~ 1.012 ms, and the cut-off value of T 2 is mainly distributed in the range of 0.252 ~ 1.163 ms. Table 1 Saturation-centrifugal analysis results of NMR test NO. Deep/m Horizon Permeability/10 − 3 µm 2 Porosity/% Movable fluid saturation/% Saturated T 2 geometric mean/ms Centrifugal T 2 geometric mean/ms T 2C /ms 3–3 up 1845.05 Zhifang Fm 14.13 9.416 93.05 8.418 1.012 1.163 3–3 down 1847.62 Zhifang Fm 0.96 1.949 53.667 0.3 0.268 0.265 3–8 1963.89 Heshanggou Fm 0.44 2.16 58.82 0.361 0.441 0.252 3–30 2417.81 Liujiagou Fm 7.44 2.95 75.82 1.39 0.73 0.365 3–35 2562.76 Shiqianfeng Fm 0.13 1.854 50.712 0.478 0.479 0.371 3–36 2566.76 Shiqianfeng Fm 0.14 2.03 47.769 0.399 0.296 0.312 Through the low field nuclear magnetic resonance experiment, the T2 spectrum distribution of 6 groups of samples under saturated water and bound water conditions was obtained, as shown in Fig. 3 . According to the pore classification method in Reference [ 18 ] , the pores with T 2 relaxation time between 0.01 ms and 1 ms are defined as micropores, the pores with T 2 relaxation time between 1 ms and 10 ms are defined as micropores, the pores with T 2 relaxation time between 10 ms and 100 ms are defined as mesopores, and the pores with T 2 relaxation time between 100 ms and 10000 ms are defined as macropores and microcracks. According to the analysis of T 2 spectrum morphology and pore structure distribution in Fig. 3 , it can be seen that the micropores, small pores and mesopores of specimen 3-3up and 3–30 all have a good degree of development. Among them, the micropores, small pores, mesopores and macropores or microfractures of specimen 3-3up account for 6.52%, 50.41%, 37.09% and 5.98%, respectively, while the micropores, small pores, mesopores and macropores or microfractures of specimen 3–30 account for 49.07%, 35.51%, 12.7% and 2.73%, respectively, and the pore structure connectivity of the two specimens is good. Combined with the pore structure distribution curve in Fig. 3 , it can be seen that the small pores and mesopores of the specimen 3-3up account for a large proportion, 87.5%, and the micropores and small pores of the specimen 3–30 account for a large proportion, 84.58%, indicating that the specimen 3-3up is mainly composed of small pores and mesopores, while the specimen 3–30 is mainly composed of micropores and small pores. The T 2 spectrum morphology and pore structure distribution of other specimens 3-3down, 3–8,3–35 and 3–36 are similar, all of which show a relatively high proportion of micropores. The area of micropores in specimen 3-3down is even as high as 96.11%, and from the analysis of pore structure distribution, the connectivity between micropores and micropores, mesopores and macropores or microcracks of these four specimens is poor, but the connectivity between micropores, mesopores and macropores or microcracks is better. 4 Results and analysis 4.1 Comparison of pore fracture characteristics The pore fracture structure is the storage space and migration channel of CO 2 or water in the reservoir, which has an important influence on the migration of CO 2 or water. In order to explore the pore fracture structure characteristics of each sandstone specimen, the proportion of different scale pores (micropores, small pores, mesopores and macropores) before and after centrifugation was quantitatively analyzed, as shown in Fig. 4 . According to Fig. 3 , the pore structure of specimen 3-3up is more complex than that of other specimens, and the pore size distribution range is wider. The proportion of micropores, small pores, mesopores and macropores is 5.60%, 54.65%, 35.27% and 4.48%, respectively.Among them, the distribution of small pores and mesopores is more extensive, and the sum of the two accounts for 89.92%. Therefore, the structural characteristics show that 3-3up not only has large pore space, but also has excellent pore connectivity and high fluid mobility. It is a typical high-porosity and high-permeability high-quality reservoir. The specimens 3-3up and 3-3down were compared and analyzed. The two specimens were taken from adjacent layers, but their physical properties were different. 3-3up is dominated by macropores, while 3-3down is dominated by small pores, which reveals the extreme heterogeneity of the reservoir at the microscopic scale. This difference may be due to: (1) Sedimentary microfacies changes, the two may be in different sedimentary rhythm layers; (2) The diagenetic transformation is uneven, 3-3up may have experienced stronger dissolution to form secondary pores, while 3-3down may be subjected to stronger compaction or cementation; (3) The development of micro-fractures is different, 3-3up may contain more developed micro-fracture system, which greatly improves its permeability. By comparing the test pieces 3-3down, 3–8,3–30,3–35 and 3–36, it can be seen that they all show similar pore structure characteristics, but they are completely different from the test piece 3-3up. Their signal contribution mainly comes from micropores and small holes. In the test pieces 3-3down, 3–8,3–35 and 3–36, the cumulative signal of micropores and small holes accounts for more than 90%. This tiny pore-based structure means that the rock is very dense, the capillary binding force is strong, the fluid is difficult to flow, and the permeability is extremely low. Therefore, these specimens belong to typical low porosity and low permeability tight reservoirs. 4.2 T 2 cutoff value and porosity analysis The T 2 cutoff value(T 2C ) is defined as the boundary threshold between the movable fluid and the bound fluid, and the porosity directly reflects the total development of the pores and fractures of the specimen. These two parameters play an important role in the migration of the reservoir. The cumulative porosity curve can be obtained by accumulating the porosity of the T 2 spectrum of the specimen under saturated and bound water conditions. The maximum cumulative porosity represents the total porosity and residual porosity, respectively. The effective porosity of the sample can be obtained by the difference between the total porosity and the residual porosity. In order to determine the boundary value of movable fluid and bound fluid, the horizontal line can be made to the left through the residual porosity, and the abscissa value of the intersection point with the cumulative porosity of saturated water is the T 2C value of T 2 . After calculation, the T 2C value and porosity evolution of the six specimens are shown in Fig. 5 . According to the analysis of Fig. 5 , the nuclear magnetic resonance (NMR) T 2 relaxation spectrum system of the six specimens reveals the microscopic pore structure heterogeneity and fluid occurrence state of the core samples of low porosity and low permeability reservoirs. The total porosity of all samples is in the low value range of 1.949% -9.416%, indicating that it belongs to the typical low porosity reservoir category. However, the detailed T 2 spectrum analysis reveals significant differences in its internal structure : the spectrum clearly divides the movable fluid and the bound fluid interval through the T 2C value, and exhibits two distinct pore configurations. Among them, the T 2 spectrum of Fig. 5 (a) shows a right-skewed unimodal shape, and the main peak T 2 value is greater than 1 millisecond, indicating that the pore system is dominated by small pores and mesopores, and contains micropores and macropores. The effective porosity is 8.77%, the pore structure is complex, and the effective porosity accounts for a high proportion, so the fluid mobility is good. Although Fig. 5 (d) also shows a right-skewed single-peak shape, the main peak T 2 value is less than 1 millisecond, indicating that the pore system is dominated by micropores and contains small pores, mesopores and macropores. The effective porosity is 2.24%, the pore structure is complex, and the effective porosity accounts for a high proportion, so the fluid mobility is relatively good. In sharp contrast, the T 2 spectra of Fig. 5 (b), (c), (e) and (f) generally show bimodal characteristics, and the effective porosity is small, ranging from 0.94% to 1.27%. There is a clear signal in the interval where the T 2 value is less than 1 millisecond, which reflects that the micropores in the sample are very well developed and the specific surface area is large, which leads to the enhancement of capillary binding force and the significant increase of irreducible water saturation, which seriously restricts the mobility of the fluid. This difference in microscopic pore structure directly determines the macroscopic seepage capacity and storage potential of the reservoir. The samples with macropores have better productivity prospects, while the samples with micropores have certain storage space, but it is difficult and requires more complex storage measures. 4.3 NMR fractal theory and its characteristics Through NMR experiments, the T 2 spectrum distribution of the sample can be obtained, reflecting the pore size distribution characteristics of the sample. Then fractal theory is applied to further study the complexity of pore structure. The more complex the pore structure is, the larger the fractal dimension is, and vice versa. Zhang et al. [ 19 ] discussed in detail the method of calculating NMR fractal dimension by using NMR T2 spectrum curve. The calculation formula is as follows: $$\:{\text{S}}_{\text{V}}\text{=}{\left(\frac{{\text{T}}_{\text{2}\text{max}}}{{\text{T}}_{\text{2}}}\right)}^{\text{D}\text{−3}}$$ 1 After taking the logarithm on both sides, it can be derived that $$\:\text{lg}\left({\text{S}}_{\text{V}}\right)\text{=(3−}\text{D}\text{)}\text{lg}\text{(}{\text{T}}_{\text{2}}\text{)+(}\text{D}\text{−3)}\text{lg}\text{(}{\text{T}}_{\text{2max}}\text{)}$$ 2 Among them, S V is the percentage of cumulative pore volume to total pore volume corresponding to pores with relaxation time less than T 2 ; d is the fractal dimension ; T 2max is the maximum relaxation time. If the sample conforms to the NMR fractal characteristics, then the formula (2) has a linear relationship. By calculating the slope λ, the fractal dimension (D) can be calculated. $$\:\text{D=3}\text{−}\lambda$$ 3 The NMR fractal dimensions of the six reservoir specimens are shown in Fig. 6 .It can be clearly seen that the curves show two-stage characteristics and can be well described by linear relationship, indicating that the pore structure of the reservoir has certain fractal characteristics. In order to study the complexity of pore structure more accurately, the curve is divided into two sections with T 2C as the boundary point of relaxation time. It is considered that the pore structure with relaxation time less than T 2C is bound fluid pore, and the pore structure with relaxation time greater than T2C is movable fluid pore. The two curves are linearly fitted respectively, and the fractal dimension (D min ) of the bound fluid pores and the fractal dimension (D max ) of the movable fluid pores are obtained by using formula(3), as shown in Table 3. It can be found that the correlation coefficient of the fitting line of the two fractal dimension curves is high, indicating that the NMR fractal is reasonable and can be used to characterize the pore structure. The results show that the fractal dimension (D min ) of the bound fluid pores of the six specimens is 1.909 ~ 2.286, with an average value of 2.144, and the fractal dimension (D max ) of the movable fluid pores is 2.873 ~ 2.989, with an average value of 2.959. Therefore, it is shown that the micro pore system structure occupied by the bound fluid is relatively simple, the pore surface tends to be smoother, and the pore size distribution is more uniform, while the seepage pore system structure occupied by the movable fluid is extremely complex, with a high degree of spatial irregularity and strong heterogeneity. The pore surface is rough and highly tortuous. An intricate strong fractal pore network is formed. This significant difference clearly reveals the fundamental difference in structural complexity between the bound fluid and the movable fluid pore system in this medium from the perspective of fractal dimension, and provides a key structural parameter basis for understanding the mechanism of fluid occurrence and migration. Table 2 Calculation results of NMR fractal dimension NO. T 2 T 2C NMR regression equation D min correlation coefficient R 2 NMR regression equation D max correlation coefficient R 2 3–3 up y = 1.091x + 0.835 1.909 0.982 y = 0.127x + 1.547 2.873 0.811 3–3 down y = 0.863x + 3.657 2.137 0.982 y = 0.011x + 2.83 2.989 0.702 3–8 y = 0.923x + 2.819 2.077 0.985 y = 0.017x + 1.94 2.983 0.741 3–30 y = 0.811x + 2.246 2.189 0.98 y = 0.058x + 2.077 2.942 0.735 3–35 y = 0.714x + 2.511 2.286 0.975 y = 0.017x + 1.94 2.983 0.792 3–36 y = 0.736x + 2.66 2.264 0.976 y = 0.015x + 1.948 2.985 0.727 5 Discussions and conclusions 5.1 Discussion In the study, the T 2 spectrum distribution of six groups of sandstone cores in saturated water and bound water was obtained by low field nuclear magnetic resonance experimental system, and the pore structure was quantitatively characterized by fractal theory. The experimental results show that the sandstone reservoir in Dahaize Coal Mine has typical characteristics of low porosity and low permeability, but its microscopic pore structure shows significant heterogeneity. The following three aspects of the research results are discussed in depth: (1) Relationship between pore structure heterogeneity and fluid mobility The results of nuclear magnetic resonance T 2 spectrum morphology and pore classification show that the specimens 3-3up and 3–30 have better pore connectivity and higher movable fluid saturation ( 93.05% and 75.82%, respectively). The T 2 spectrum shows a right-skewed single-peak shape, and the main peak is located in the middle-small pore interval, reflecting better seepage capacity. In contrast, the samples 3-3down, 3–8, 3–35 and 3–36 are dominated by micropores, and the T 2 spectrum is bimodal or left-skewed, and the movable fluid saturation is less than 60%, indicating that the capillary binding effect in the pore system is significant and the fluid mobility is poor. This difference is mainly controlled by the heterogeneity of sedimentary microfacies and diagenesis. For example, dissolution enhances pore connectivity, while compaction and cementation lead to densification of pore structure. (2) The reservoir significance of T 2C value and effective porosity As a key parameter to distinguish the movable fluid from the bound fluid, the cutoff value of T 2 is between 0.252 and 1.163 ms in this study. Combined with the cumulative porosity curve analysis, the effective porosity of specimen 3-3up is as high as 8.77%, which is much higher than that of other samples ( 0.94% -2.24%), further confirming its high-quality reservoir properties. It is worth noting that although the total porosity of some samples is similar ( such as 3-3down and 3–36 ), there is a difference between the T 2C value and the effective porosity, indicating that a single porosity index is difficult to fully reflect the reservoir quality, and it must be combined with T 2 spectrum morphology and movable fluid saturation for comprehensive judgment. (3) The ability of NMR fractal dimension to characterize the complexity of pore structure The fractal dimension calculated based on T 2 spectrum shows that the relationship between lgS V and lgT 2 of all samples shows a good two-stage linear feature, indicating that the pore structure has a clear fractal behavior. The fractal dimension D min of the bound fluid pores is between 1.909 and 2.286, with an average of 2.144, which reflects that the structure of the micro-pore system is relatively simple and the surface is smooth. The fractal dimension D max of movable fluid pores is between 2.873 and 2.989, with an average of 2.959, indicating that the seepage pore system has high complexity and heterogeneity, and the pore surface is rough and tortuous. This result reveals the structural differences between the two types of pore systems in tight sandstone from the perspective of fractal dimension, and provides an important structural parameter basis for understanding the mechanism of fluid occurrence and migration. 5.2 Conclusion In the study, the microscopic pore structure of tight sandstone reservoirs in Dahaize Coal Mine was systematically characterized by integrating nuclear magnetic resonance experiments and fractal theory, and the following main conclusions were drawn: (1) The sandstone reservoir in the study area belongs to the type of low porosity and low permeability, but the microscopic pore structure has significant heterogeneity. The samples can be divided into two categories : one is mainly composed of medium-small pores, with good pore connectivity, high movable fluid saturation and good seepage capacity; the other is mainly composed of small pores, with good pore connectivity, high movable fluid saturation and good seepage capacity. The other type is dominated by micropores, with dense pore structure and poor fluid mobility. (2) T 2C value and effective porosity are the key parameters to evaluate the mobility of reservoir fluid. High T 2C value and high effective porosity usually correspond to good reservoir quality, while low value reflects strong capillary confinement and poor development potential. (3) Nuclear magnetic resonance fractal dimension can effectively quantify the complexity of pore structure. The significant difference between D min and D max reveals the essential difference in structural characteristics between bound pores and seepage pores, which provides a new quantitative index for the classification and evaluation of tight reservoirs. The study verifies the applicability and advantages of the combination of nuclear magnetic resonance and fractal theory in the characterization of tight sandstone reservoirs. This method can provide a scientific basis for pore structure analysis, fluid mobility evaluation and CO 2 geological storage potential prediction of similar reservoirs. Declarations Funding The Innovation Capability Support Plan in Shaanxi Province of China(NO. 2024RS-CXTD-54); Research and engineering demonstration project on water depth transfer and storage of coal seam roof strata in Shaanxi Province (NO. 2024SF-YBXM-603). Author Contribution Shuhui Zhang completed analyzed the data, and wrote the draft paper. Baoyan Zhi and Binhu Xiao provided the paper ideas. 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Fractal characteristics of reservoir rock pore structure based on NMR T 2 distribution[J]. Journal of Oil and Gas Technology,2007,29(4):80-86，166. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-7846824","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":540142654,"identity":"97900f0a-163f-4eb6-9b35-b41712604836","order_by":0,"name":"Baoyan Zhi","email":"","orcid":"","institution":"China Coal Shaanxi Energy \u0026 Chemical Group Co.,Ltd","correspondingAuthor":false,"prefix":"","firstName":"Baoyan","middleName":"","lastName":"Zhi","suffix":""},{"id":540142655,"identity":"d4cdc8d9-0099-4d0b-a9ca-0fcded481d43","order_by":1,"name":"Binhu Xiao","email":"","orcid":"","institution":"China Coal Shaanxi Energy \u0026 Chemical Group Co.,Ltd","correspondingAuthor":false,"prefix":"","firstName":"Binhu","middleName":"","lastName":"Xiao","suffix":""},{"id":540142656,"identity":"e8eecdc5-fc0f-480e-94a4-a75ac545e7f4","order_by":2,"name":"Lin Bai","email":"","orcid":"","institution":"China Coal Shaanxi Energy \u0026 Chemical Group Co.,Ltd","correspondingAuthor":false,"prefix":"","firstName":"Lin","middleName":"","lastName":"Bai","suffix":""},{"id":540142657,"identity":"41931595-265d-4127-aba0-64721dd3da04","order_by":3,"name":"Hongbo Guo","email":"","orcid":"","institution":"China Coal Shaanxi Energy \u0026 Chemical Group Co.,Ltd","correspondingAuthor":false,"prefix":"","firstName":"Hongbo","middleName":"","lastName":"Guo","suffix":""},{"id":540142658,"identity":"6d402c54-1b60-455f-8c15-350bd93aac1d","order_by":4,"name":"Heping Li","email":"","orcid":"","institution":"China Coal Shaanxi Energy \u0026 Chemical Group 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1","display":"","copyAsset":false,"role":"figure","size":43969,"visible":true,"origin":"","legend":"\u003cp\u003eSandstone sample\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7846824/v1/02c992f5b87ede1803e02f57.jpeg"},{"id":95386761,"identity":"1536ea50-6a3b-4fdd-a8a2-46e501ea72b5","added_by":"auto","created_at":"2025-11-07 13:07:53","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":263961,"visible":true,"origin":"","legend":"\u003cp\u003eNMR experimental system\u003c/p\u003e\n\u003cp\u003e①Liquid and gas injection equipment; ②Pressure control equipment; ③Temperature and pressure control equipment; ④CO\u003csub\u003e2\u003c/sub\u003e displacement equipment; ⑤NMR equipment; ⑥Back pressure equipment; ⑦Data analysis system; ⑧Rock core gripper\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7846824/v1/58b1e7f7c53686c30ca62b79.png"},{"id":95386766,"identity":"489118c4-e1ce-4df7-a027-7dd7586d439e","added_by":"auto","created_at":"2025-11-07 13:07:53","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":504137,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003e2 \u003c/sub\u003espectra of saturated water samples and bound samples\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7846824/v1/e5ff9dbebfc836898494fc33.jpeg"},{"id":95526564,"identity":"b9103131-fbea-459e-8405-0d3beeef2de0","added_by":"auto","created_at":"2025-11-10 10:07:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":152671,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of pore fracture composition of each specimen\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7846824/v1/07e9c9685c47872036c8efdd.png"},{"id":95386776,"identity":"8c6212f8-b40d-4516-9c45-7a88984be37a","added_by":"auto","created_at":"2025-11-07 13:07:54","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":581781,"visible":true,"origin":"","legend":"\u003cp\u003eT\u003csub\u003e2C\u003c/sub\u003e and effective porosity of each specimen\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7846824/v1/270d1d3d818363b82ef7ecd4.jpeg"},{"id":95386771,"identity":"0d58f8e3-2879-426c-aef0-15fbd84d3b6b","added_by":"auto","created_at":"2025-11-07 13:07:54","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":427851,"visible":true,"origin":"","legend":"\u003cp\u003ePore fractal dimension fitting\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7846824/v1/577c85f7066d5f9db7c88c07.jpeg"},{"id":95657986,"identity":"28d725b5-2857-4904-9287-2ce6ff778e33","added_by":"auto","created_at":"2025-11-11 16:22:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2861093,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7846824/v1/6a7b922b-74ba-468f-8e4b-f0266807818a.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Quantitative characterization of pore structure and NMR fractal based on nuclear magnetic resonance and fractal theory","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eTight sandstone reservoir is an important part of unconventional oil and gas reservoir, and it is one of the research focuses in the field of reservoir \u003csup\u003e[\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e. Tight sandstone is composed of relatively dense clastic rocks, mainly including siltstone, siltstone-fine sandstone, fine sandstone and a small part of medium-coarse sandstone. Due to the complexity and strong heterogeneity of the microscopic pore structure of tight sandstone reservoirs, it is difficult to carry out reservoir characteristics research and reservoir evaluation.\u003c/p\u003e\u003cp\u003eReservoir classification evaluation is an important part of reservoir research, and the results of reservoir evaluation will affect the overall evaluation of the reservoir \u003csup\u003e[\u003cspan additionalcitationids=\"CR7 CR8\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. At present, there are two main methods for reservoir classification and evaluation : one is qualitative evaluation based on geology, logging, earthquake and other disciplines ; the other is quantitative evaluation based on mathematical methods, such as fuzzy synthesis, grey correlation degree, analytic hierarchy process, fractal dimension method and so on. In recent years, nuclear magnetic resonance (NMR) has been widely used in the characterization of pore and fracture structures in porous media due to its non-destructive, fast and accurate advantages. Yao et al.\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e used low field nuclear magnetic resonance and computed tomography (CT) scanning technology to describe the pore size distribution and pore fracture space configuration of coal. Based on low field nuclear magnetic resonance, Zhou et al.\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e developed a method to quantify the fractal dimension of pores and fractures. Combined with nuclear magnetic resonance, high pressure mercury injection and scanning electron microscopy, Chai et al.explored the pore characteristics of shale in complex structural areas. Liu et al.\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e studied the pore structure of high-rank coal with different bedding structures by means of nuclear magnetic resonance, and analyzed the influence of centrifugal force. Liang et al.\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e studied the pore characteristics of shale reservoirs in Shahezi Formation of Songliao Basin by box dimension method and NMR fractal theory. They believed that fractal dimension has a good correlation with physical parameters such as shale porosity and permeability, and can be used as an index to evaluate shale physical properties. Many scholars have carried out research on reservoir classification and evaluation based on fractal theory. Yan et al.\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e based on fractal theory, using nuclear magnetic resonance T\u003csub\u003e2\u003c/sub\u003e spectrum and other experiments to explore the relationship between fractal dimension and pore structure type, clear the pore structure type ; according to the casting thin section, mercury injection and other data, ES4 is finally divided into 3 categories and 5 subcategories. Based on different basic geological experimental analysis data, Liu et al.\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e used two saturation methods to calculate the fractal dimension of pore structure of tight reservoir in He 1 member of Hangjinqi area, and analyzed its relationship with reservoir physical properties. Zhu et al.\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e took the Chang 6 tight sandstone reservoir of the Yanchang Formation in the Yan 'an-Ganquan area of the southern Ordos Basin as an example. Through casting thin sections, scanning electron microscopy analysis, X-ray diffraction and high-pressure mercury injection experiments, combined with fractal theory, the fractal characteristics of the target reservoir were determined, and the relationship between fractal dimension and reservoir physical properties, pore characteristics and mineral composition was discussed. Li et al.\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e used a variety of experimental methods and theoretical methods to compare the differences between the NMR fractal dimension, the fractal dimension of the nitrogen adsorption method and the fractal dimension of the mercury intrusion method. It is believed that the NMR fractal dimension can more accurately reflect the pore structure characteristics of shale.\u003c/p\u003e\u003cp\u003eIn summary, low-field nuclear magnetic resonance technology has achieved a lot of research results in the characterization of porous media pore structure due to its unique advantages. However, the study of pore and fracture structure of deep sandstone in coal mine is the basis of mine water storage or CO\u003csub\u003e2\u003c/sub\u003e storage. Although low-field nuclear magnetic resonance technology has been widely used in the characterization of pore and fracture structure of coal rock, there are few reports on the characterization of pore and fracture structure of deep sandstone in coal mine by low-field nuclear magnetic resonance technology. Therefore, this paper takes the deep tight sandstone reservoir of Dahaize Coal Mine in Ordos Basin as the research object, and uses nuclear magnetic resonance ( NMR ) experimental methods to quantitatively characterize the microscopic pore structure characteristics of the reservoir on the basis of systematically obtaining the distribution of core T\u003csub\u003e2\u003c/sub\u003e spectrum. By introducing NMR fractal theory, the internal relationship between fractal dimension and pore structure parameters, physical properties and fluid mobility is established to reveal the heterogeneity and complexity of pore structure of tight sandstone reservoirs, and to clarify the key microscopic factors controlling fluid occurrence and migration. In order to provide a reliable theoretical basis and technical support for the efficient evaluation of tight sandstone reservoirs and the prediction of CO\u003csub\u003e2\u003c/sub\u003e geological storage potential.\u003c/p\u003e"},{"header":"2 Experimental methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Source of sample\u003c/h2\u003e\u003cp\u003eThe samples used in the test were taken from the sandstone of Dahaize Coal Mine of China Coal Shaanxi Yulin Energy Chemical Co., Ltd. The Dahaize mine is located in the northwest of Yulin City, with a straight distance of about 50 km. The mine area is 280.03 km\u003csup\u003e2\u003c/sup\u003e, the total amount of coal resources is about 5.1\u0026nbsp;billion tons, and the production capacity is 20\u0026nbsp;million tons/year. The geological structure is simple. The Yan 'an Formation contains 7 coal seams, with an average total thickness of 15.41 m. Among them, the No.2 and No.3 coal seams with simple structure, recoverable and stable main coal seams in the whole area have an average total thickness of 10.30 m. Due to the large amount of mine water inflow in Dahaize Coal Mine, the drainage water of coal seam roof and the water in goaf are deeply transferred and sealed, which preliminarily realizes the deep, stable and safe storage of roof water in Dahaize Coal Mine in deep strata, without any environmental and safety risks. Sandstones of different depths were taken to the ground by drilling coring, and the depth was in the range of 2500 m-2600 m. The sandstones were processed into cylinders of the same size of Φ25 mm \u0026times; 50 mm for nuclear magnetic resonance (NMR) experiments, and samples without obvious structural damage were selected to ensure the experimental accuracy.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Test principle\u003c/h2\u003e\u003cp\u003eNuclear magnetic resonance (NMR) is a non-destructive technology widely used in the field of rock physics. Based on the magnetic properties of atomic nuclei, the pore structure, fluid properties and distribution of rocks can be obtained by analyzing the transition of hydrogen nuclei in rocks in magnetic field. The nuclear magnetic resonance T\u003csub\u003e2\u003c/sub\u003e spectrum can reflect the attenuation of the atomic nuclear magnetic resonance signal in the core sample in time. Through the analysis of several spectra, the information of different pore diameter and distribution in the rock can be obtained. Through nuclear magnetic resonance experiments, the T\u003csub\u003e2\u003c/sub\u003e relaxation time and distribution characteristics of fluid in rock samples can be obtained, and then the occurrence state of fluid in rock pores can be analyzed. The length of the T\u003csub\u003e2\u003c/sub\u003e relaxation time of the fluid is related to the strength of the surface force of the pore solid. When the fluid is subjected to a strong force, the T\u003csub\u003e2\u003c/sub\u003e relaxation time is very short, and the fluid is in a stagnant state ; when the fluid is subjected to a weak force, the T\u003csub\u003e2\u003c/sub\u003e relaxation time is longer and the fluid is movable. Large pores correspond to longer relaxation time, and small pores correspond to shorter relaxation time.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Experimental samples and process\u003c/h2\u003e\u003cp\u003eThe sandstone reservoir of Dahaize Coal Mine in Yulin area of Shaanxi Province, China was selected as the research object. Six cylindrical specimens of Φ25 mm \u0026times; 50 mm with the same size were processed ( as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e ). The nuclear magnetic resonance equipment was MacroMR 12\u0026ndash;150 H-I nuclear magnetic resonance test system produced by Suzhou Newmai, China, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The test was carried out based on the oil and gas industry standard 'Determination method of relative permeability of two-phase fluid in rock' (GB/T 28912\u0026thinsp;\u0026minus;\u0026thinsp;2012) and 'Laboratory measurement specification of nuclear magnetic resonance parameters of rock samples' (SY/T 6490\u0026thinsp;\u0026minus;\u0026thinsp;2014). The test parameters were set as follows : CPMG sequence of nuclear magnetic resonance, waiting time TW\u0026thinsp;=\u0026thinsp;5000ms, echo interval time TE\u0026thinsp;=\u0026thinsp;0.1ms ( minimum 0.06ms of equipment ), echo number NE\u0026thinsp;=\u0026thinsp;15000, scanning times 32. The experimental method is as follows : first, the core holder is vacuumized with a vacuum pump to achieve the ideal vacuum condition, and then the core is saturated with distilled water for 12h under the excess pore pressure of 20MPa to ensure that the core is completely saturated. The saturated weight m\u003csub\u003e1\u003c/sub\u003e is weighed, and the CPMG sequence test is performed to obtain the T\u003csub\u003e2\u003c/sub\u003e distribution of the saturated core. Then, the water-saturated core was centrifuged at a speed of 6000r/min to remove the free water in the core plunger, and the weight m\u003csub\u003e2\u003c/sub\u003e after centrifugation was measured again to obtain the NMR T\u003csub\u003e2\u003c/sub\u003e in the centrifugal state.\u003c/p\u003e"},{"header":"3 Test results","content":"\u003cp\u003eThe results of NMR experiments are shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. It can be seen that the permeability distribution range of sandstone reservoir core samples in Dahaize Coal Mine is mainly concentrated in (0.13\u0026thinsp;~\u0026thinsp;14.13)\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003eum\u003csup\u003e2\u003c/sup\u003e; the nuclear magnetic porosity is between 1.854%~9.416%. The range of movable fluid saturation is 47.769%~93.05%. The geometric mean of saturated T\u003csub\u003e2\u003c/sub\u003e is mainly distributed in the range of 0.3\u0026thinsp;~\u0026thinsp;8.418 ms, the geometric mean of centrifugal T2 is mainly distributed in the range of 0.268\u0026thinsp;~\u0026thinsp;1.012 ms, and the cut-off value of T\u003csub\u003e2\u003c/sub\u003e is mainly distributed in the range of 0.252\u0026thinsp;~\u0026thinsp;1.163 ms.\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\u003eSaturation-centrifugal analysis results of NMR test\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=\"char\" char=\".\" 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=\"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=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNO.\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDeep/m\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHorizon\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003ePermeability/10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u0026micro;m\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003ePorosity/%\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eMovable fluid saturation/%\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSaturated T\u003csub\u003e2\u003c/sub\u003e geometric mean/ms\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eCentrifugal T\u003csub\u003e2\u003c/sub\u003e geometric mean/ms\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c9\"\u003e\u003cp\u003eT\u003csub\u003e2C\u003c/sub\u003e/ms\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;3 up\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1845.05\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eZhifang Fm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e14.13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e9.416\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e93.05\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e8.418\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e1.012\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e1.163\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;3 down\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1847.62\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eZhifang Fm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.96\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e1.949\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e53.667\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.268\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e0.265\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1963.89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHeshanggou Fm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.44\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e2.16\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e58.82\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.361\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.441\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e0.252\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2417.81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLiujiagou Fm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e7.44\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e2.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e75.82\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e1.39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.73\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e0.365\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;35\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2562.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eShiqianfeng Fm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e1.854\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e50.712\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.478\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.479\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e0.371\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2566.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eShiqianfeng Fm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e2.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e47.769\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.399\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.296\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e0.312\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\u003eThrough the low field nuclear magnetic resonance experiment, the T2 spectrum distribution of 6 groups of samples under saturated water and bound water conditions was obtained, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. According to the pore classification method in Reference\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e, the pores with T\u003csub\u003e2\u003c/sub\u003e relaxation time between 0.01 ms and 1 ms are defined as micropores, the pores with T\u003csub\u003e2\u003c/sub\u003e relaxation time between 1 ms and 10 ms are defined as micropores, the pores with T\u003csub\u003e2\u003c/sub\u003e relaxation time between 10 ms and 100 ms are defined as mesopores, and the pores with T\u003csub\u003e2\u003c/sub\u003e relaxation time between 100 ms and 10000 ms are defined as macropores and microcracks.\u003c/p\u003e\u003cp\u003eAccording to the analysis of T\u003csub\u003e2\u003c/sub\u003e spectrum morphology and pore structure distribution in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, it can be seen that the micropores, small pores and mesopores of specimen 3-3up and 3\u0026ndash;30 all have a good degree of development. Among them, the micropores, small pores, mesopores and macropores or microfractures of specimen 3-3up account for 6.52%, 50.41%, 37.09% and 5.98%, respectively, while the micropores, small pores, mesopores and macropores or microfractures of specimen 3\u0026ndash;30 account for 49.07%, 35.51%, 12.7% and 2.73%, respectively, and the pore structure connectivity of the two specimens is good. Combined with the pore structure distribution curve in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, it can be seen that the small pores and mesopores of the specimen 3-3up account for a large proportion, 87.5%, and the micropores and small pores of the specimen 3\u0026ndash;30 account for a large proportion, 84.58%, indicating that the specimen 3-3up is mainly composed of small pores and mesopores, while the specimen 3\u0026ndash;30 is mainly composed of micropores and small pores. The T\u003csub\u003e2\u003c/sub\u003e spectrum morphology and pore structure distribution of other specimens 3-3down, 3\u0026ndash;8,3\u0026ndash;35 and 3\u0026ndash;36 are similar, all of which show a relatively high proportion of micropores. The area of micropores in specimen 3-3down is even as high as 96.11%, and from the analysis of pore structure distribution, the connectivity between micropores and micropores, mesopores and macropores or microcracks of these four specimens is poor, but the connectivity between micropores, mesopores and macropores or microcracks is better.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"4 Results and analysis","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e4.1 Comparison of pore fracture characteristics\u003c/h2\u003e\u003cp\u003eThe pore fracture structure is the storage space and migration channel of CO\u003csub\u003e2\u003c/sub\u003e or water in the reservoir, which has an important influence on the migration of CO\u003csub\u003e2\u003c/sub\u003e or water. In order to explore the pore fracture structure characteristics of each sandstone specimen, the proportion of different scale pores (micropores, small pores, mesopores and macropores) before and after centrifugation was quantitatively analyzed, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eAccording to Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the pore structure of specimen 3-3up is more complex than that of other specimens, and the pore size distribution range is wider. The proportion of micropores, small pores, mesopores and macropores is 5.60%, 54.65%, 35.27% and 4.48%, respectively.Among them, the distribution of small pores and mesopores is more extensive, and the sum of the two accounts for 89.92%. Therefore, the structural characteristics show that 3-3up not only has large pore space, but also has excellent pore connectivity and high fluid mobility. It is a typical high-porosity and high-permeability high-quality reservoir. The specimens 3-3up and 3-3down were compared and analyzed. The two specimens were taken from adjacent layers, but their physical properties were different. 3-3up is dominated by macropores, while 3-3down is dominated by small pores, which reveals the extreme heterogeneity of the reservoir at the microscopic scale. This difference may be due to: (1) Sedimentary microfacies changes, the two may be in different sedimentary rhythm layers; (2) The diagenetic transformation is uneven, 3-3up may have experienced stronger dissolution to form secondary pores, while 3-3down may be subjected to stronger compaction or cementation; (3) The development of micro-fractures is different, 3-3up may contain more developed micro-fracture system, which greatly improves its permeability. By comparing the test pieces 3-3down, 3\u0026ndash;8,3\u0026ndash;30,3\u0026ndash;35 and 3\u0026ndash;36, it can be seen that they all show similar pore structure characteristics, but they are completely different from the test piece 3-3up. Their signal contribution mainly comes from micropores and small holes. In the test pieces 3-3down, 3\u0026ndash;8,3\u0026ndash;35 and 3\u0026ndash;36, the cumulative signal of micropores and small holes accounts for more than 90%. This tiny pore-based structure means that the rock is very dense, the capillary binding force is strong, the fluid is difficult to flow, and the permeability is extremely low. Therefore, these specimens belong to typical low porosity and low permeability tight reservoirs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e4.2 T\u003csub\u003e2\u003c/sub\u003e cutoff value and porosity analysis\u003c/h2\u003e\u003cp\u003eThe T\u003csub\u003e2\u003c/sub\u003e cutoff value(T\u003csub\u003e2C\u003c/sub\u003e) is defined as the boundary threshold between the movable fluid and the bound fluid, and the porosity directly reflects the total development of the pores and fractures of the specimen. These two parameters play an important role in the migration of the reservoir. The cumulative porosity curve can be obtained by accumulating the porosity of the T\u003csub\u003e2\u003c/sub\u003e spectrum of the specimen under saturated and bound water conditions. The maximum cumulative porosity represents the total porosity and residual porosity, respectively. The effective porosity of the sample can be obtained by the difference between the total porosity and the residual porosity. In order to determine the boundary value of movable fluid and bound fluid, the horizontal line can be made to the left through the residual porosity, and the abscissa value of the intersection point with the cumulative porosity of saturated water is the T\u003csub\u003e2C\u003c/sub\u003e value of T\u003csub\u003e2\u003c/sub\u003e. After calculation, the T\u003csub\u003e2C\u003c/sub\u003e value and porosity evolution of the six specimens are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eAccording to the analysis of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the nuclear magnetic resonance (NMR) T\u003csub\u003e2\u003c/sub\u003e relaxation spectrum system of the six specimens reveals the microscopic pore structure heterogeneity and fluid occurrence state of the core samples of low porosity and low permeability reservoirs. The total porosity of all samples is in the low value range of 1.949% -9.416%, indicating that it belongs to the typical low porosity reservoir category. However, the detailed T\u003csub\u003e2\u003c/sub\u003e spectrum analysis reveals significant differences in its internal structure : the spectrum clearly divides the movable fluid and the bound fluid interval through the T\u003csub\u003e2C\u003c/sub\u003e value, and exhibits two distinct pore configurations. Among them, the T\u003csub\u003e2\u003c/sub\u003e spectrum of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (a) shows a right-skewed unimodal shape, and the main peak T\u003csub\u003e2\u003c/sub\u003e value is greater than 1 millisecond, indicating that the pore system is dominated by small pores and mesopores, and contains micropores and macropores. The effective porosity is 8.77%, the pore structure is complex, and the effective porosity accounts for a high proportion, so the fluid mobility is good. Although Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (d) also shows a right-skewed single-peak shape, the main peak T\u003csub\u003e2\u003c/sub\u003e value is less than 1 millisecond, indicating that the pore system is dominated by micropores and contains small pores, mesopores and macropores. The effective porosity is 2.24%, the pore structure is complex, and the effective porosity accounts for a high proportion, so the fluid mobility is relatively good.\u003c/p\u003e\u003cp\u003eIn sharp contrast, the T\u003csub\u003e2\u003c/sub\u003e spectra of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (b), (c), (e) and (f) generally show bimodal characteristics, and the effective porosity is small, ranging from 0.94% to 1.27%. There is a clear signal in the interval where the T\u003csub\u003e2\u003c/sub\u003e value is less than 1 millisecond, which reflects that the micropores in the sample are very well developed and the specific surface area is large, which leads to the enhancement of capillary binding force and the significant increase of irreducible water saturation, which seriously restricts the mobility of the fluid. This difference in microscopic pore structure directly determines the macroscopic seepage capacity and storage potential of the reservoir. The samples with macropores have better productivity prospects, while the samples with micropores have certain storage space, but it is difficult and requires more complex storage measures.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e4.3 NMR fractal theory and its characteristics\u003c/h2\u003e\u003cp\u003eThrough NMR experiments, the T\u003csub\u003e2\u003c/sub\u003e spectrum distribution of the sample can be obtained, reflecting the pore size distribution characteristics of the sample. Then fractal theory is applied to further study the complexity of pore structure. The more complex the pore structure is, the larger the fractal dimension is, and vice versa. Zhang et al.\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e discussed in detail the method of calculating NMR fractal dimension by using NMR T2 spectrum curve. The calculation formula is as follows:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{\\text{S}}_{\\text{V}}\\text{=}{\\left(\\frac{{\\text{T}}_{\\text{2}\\text{max}}}{{\\text{T}}_{\\text{2}}}\\right)}^{\\text{D}\\text{\u0026minus;3}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eAfter taking the logarithm on both sides, it can be derived that\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:\\text{lg}\\left({\\text{S}}_{\\text{V}}\\right)\\text{=(3\u0026minus;}\\text{D}\\text{)}\\text{lg}\\text{(}{\\text{T}}_{\\text{2}}\\text{)+(}\\text{D}\\text{\u0026minus;3)}\\text{lg}\\text{(}{\\text{T}}_{\\text{2max}}\\text{)}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eAmong them, S\u003csub\u003eV\u003c/sub\u003e is the percentage of cumulative pore volume to total pore volume corresponding to pores with relaxation time less than T\u003csub\u003e2\u003c/sub\u003e; d is the fractal dimension ; T\u003csub\u003e2max\u003c/sub\u003e is the maximum relaxation time.\u003c/p\u003e\u003cp\u003eIf the sample conforms to the NMR fractal characteristics, then the formula (2) has a linear relationship. By calculating the slope λ, the fractal dimension (D) can be calculated.\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:\\text{D=3}\\text{\u0026minus;}\\lambda$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe NMR fractal dimensions of the six reservoir specimens are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.It can be clearly seen that the curves show two-stage characteristics and can be well described by linear relationship, indicating that the pore structure of the reservoir has certain fractal characteristics. In order to study the complexity of pore structure more accurately, the curve is divided into two sections with T\u003csub\u003e2C\u003c/sub\u003e as the boundary point of relaxation time. It is considered that the pore structure with relaxation time less than T\u003csub\u003e2C\u003c/sub\u003e is bound fluid pore, and the pore structure with relaxation time greater than T2C is movable fluid pore. The two curves are linearly fitted respectively, and the fractal dimension (D\u003csub\u003emin\u003c/sub\u003e) of the bound fluid pores and the fractal dimension (D\u003csub\u003emax\u003c/sub\u003e) of the movable fluid pores are obtained by using formula(3), as shown in Table\u0026nbsp;3.\u003c/p\u003e\u003cp\u003eIt can be found that the correlation coefficient of the fitting line of the two fractal dimension curves is high, indicating that the NMR fractal is reasonable and can be used to characterize the pore structure. The results show that the fractal dimension (D\u003csub\u003emin\u003c/sub\u003e) of the bound fluid pores of the six specimens is 1.909\u0026thinsp;~\u0026thinsp;2.286, with an average value of 2.144, and the fractal dimension (D\u003csub\u003emax\u003c/sub\u003e) of the movable fluid pores is 2.873\u0026thinsp;~\u0026thinsp;2.989, with an average value of 2.959. Therefore, it is shown that the micro pore system structure occupied by the bound fluid is relatively simple, the pore surface tends to be smoother, and the pore size distribution is more uniform, while the seepage pore system structure occupied by the movable fluid is extremely complex, with a high degree of spatial irregularity and strong heterogeneity. The pore surface is rough and highly tortuous. An intricate strong fractal pore network is formed. This significant difference clearly reveals the fundamental difference in structural complexity between the bound fluid and the movable fluid pore system in this medium from the perspective of fractal dimension, and provides a key structural parameter basis for understanding the mechanism of fluid occurrence and migration.\u003c/p\u003e\u003cp\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\u003eCalculation results of NMR fractal dimension\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=\"left\" 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=\"left\" 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=\"char\" char=\".\" 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\u003eNO.\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003eT\u003csub\u003e2\u003c/sub\u003e\u0026lt;T\u003csub\u003e2C\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e\u003cp\u003eT\u003csub\u003e2\u003c/sub\u003e\u0026gt;T\u003csub\u003e2C\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNMR regression equation\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eD\u003csub\u003emin\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003ecorrelation coefficient R\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eNMR regression equation\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eD\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003ecorrelation coefficient R\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;3 up\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;1.091x\u0026thinsp;+\u0026thinsp;0.835\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.909\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.982\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.127x\u0026thinsp;+\u0026thinsp;1.547\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.873\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.811\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;3 down\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.863x\u0026thinsp;+\u0026thinsp;3.657\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2.137\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.982\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.011x\u0026thinsp;+\u0026thinsp;2.83\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.989\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.702\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.923x\u0026thinsp;+\u0026thinsp;2.819\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2.077\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.985\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.017x\u0026thinsp;+\u0026thinsp;1.94\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.983\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.741\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.811x\u0026thinsp;+\u0026thinsp;2.246\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2.189\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.98\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.058x\u0026thinsp;+\u0026thinsp;2.077\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.942\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.735\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;35\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.714x\u0026thinsp;+\u0026thinsp;2.511\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2.286\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.975\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.017x\u0026thinsp;+\u0026thinsp;1.94\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.983\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.792\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u0026ndash;36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.736x\u0026thinsp;+\u0026thinsp;2.66\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2.264\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.976\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003ey\u0026thinsp;=\u0026thinsp;0.015x\u0026thinsp;+\u0026thinsp;1.948\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.985\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.727\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"},{"header":"5 Discussions and conclusions","content":"\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e5.1 Discussion\u003c/h2\u003e\u003cp\u003eIn the study, the T\u003csub\u003e2\u003c/sub\u003e spectrum distribution of six groups of sandstone cores in saturated water and bound water was obtained by low field nuclear magnetic resonance experimental system, and the pore structure was quantitatively characterized by fractal theory. The experimental results show that the sandstone reservoir in Dahaize Coal Mine has typical characteristics of low porosity and low permeability, but its microscopic pore structure shows significant heterogeneity. The following three aspects of the research results are discussed in depth:\u003c/p\u003e\u003cp\u003e(1) Relationship between pore structure heterogeneity and fluid mobility\u003c/p\u003e\u003cp\u003eThe results of nuclear magnetic resonance T\u003csub\u003e2\u003c/sub\u003e spectrum morphology and pore classification show that the specimens 3-3up and 3\u0026ndash;30 have better pore connectivity and higher movable fluid saturation ( 93.05% and 75.82%, respectively). The T\u003csub\u003e2\u003c/sub\u003e spectrum shows a right-skewed single-peak shape, and the main peak is located in the middle-small pore interval, reflecting better seepage capacity. In contrast, the samples 3-3down, 3\u0026ndash;8, 3\u0026ndash;35 and 3\u0026ndash;36 are dominated by micropores, and the T\u003csub\u003e2\u003c/sub\u003e spectrum is bimodal or left-skewed, and the movable fluid saturation is less than 60%, indicating that the capillary binding effect in the pore system is significant and the fluid mobility is poor. This difference is mainly controlled by the heterogeneity of sedimentary microfacies and diagenesis. For example, dissolution enhances pore connectivity, while compaction and cementation lead to densification of pore structure.\u003c/p\u003e\u003cp\u003e(2) The reservoir significance of T\u003csub\u003e2C\u003c/sub\u003e value and effective porosity\u003c/p\u003e\u003cp\u003eAs a key parameter to distinguish the movable fluid from the bound fluid, the cutoff value of T\u003csub\u003e2\u003c/sub\u003e is between 0.252 and 1.163 ms in this study. Combined with the cumulative porosity curve analysis, the effective porosity of specimen 3-3up is as high as 8.77%, which is much higher than that of other samples ( 0.94% -2.24%), further confirming its high-quality reservoir properties. It is worth noting that although the total porosity of some samples is similar ( such as 3-3down and 3\u0026ndash;36 ), there is a difference between the T\u003csub\u003e2C\u003c/sub\u003e value and the effective porosity, indicating that a single porosity index is difficult to fully reflect the reservoir quality, and it must be combined with T\u003csub\u003e2\u003c/sub\u003e spectrum morphology and movable fluid saturation for comprehensive judgment.\u003c/p\u003e\u003cp\u003e(3) The ability of NMR fractal dimension to characterize the complexity of pore structure\u003c/p\u003e\u003cp\u003eThe fractal dimension calculated based on T\u003csub\u003e2\u003c/sub\u003e spectrum shows that the relationship between lgS\u003csub\u003eV\u003c/sub\u003e and lgT\u003csub\u003e2\u003c/sub\u003e of all samples shows a good two-stage linear feature, indicating that the pore structure has a clear fractal behavior. The fractal dimension D\u003csub\u003emin\u003c/sub\u003e of the bound fluid pores is between 1.909 and 2.286, with an average of 2.144, which reflects that the structure of the micro-pore system is relatively simple and the surface is smooth. The fractal dimension D\u003csub\u003emax\u003c/sub\u003e of movable fluid pores is between 2.873 and 2.989, with an average of 2.959, indicating that the seepage pore system has high complexity and heterogeneity, and the pore surface is rough and tortuous. This result reveals the structural differences between the two types of pore systems in tight sandstone from the perspective of fractal dimension, and provides an important structural parameter basis for understanding the mechanism of fluid occurrence and migration.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e5.2 Conclusion\u003c/h2\u003e\u003cp\u003eIn the study, the microscopic pore structure of tight sandstone reservoirs in Dahaize Coal Mine was systematically characterized by integrating nuclear magnetic resonance experiments and fractal theory, and the following main conclusions were drawn:\u003c/p\u003e\u003cp\u003e(1) The sandstone reservoir in the study area belongs to the type of low porosity and low permeability, but the microscopic pore structure has significant heterogeneity. The samples can be divided into two categories : one is mainly composed of medium-small pores, with good pore connectivity, high movable fluid saturation and good seepage capacity; the other is mainly composed of small pores, with good pore connectivity, high movable fluid saturation and good seepage capacity. The other type is dominated by micropores, with dense pore structure and poor fluid mobility.\u003c/p\u003e\u003cp\u003e(2) T\u003csub\u003e2C\u003c/sub\u003e value and effective porosity are the key parameters to evaluate the mobility of reservoir fluid. High T\u003csub\u003e2C\u003c/sub\u003e value and high effective porosity usually correspond to good reservoir quality, while low value reflects strong capillary confinement and poor development potential.\u003c/p\u003e\u003cp\u003e(3) Nuclear magnetic resonance fractal dimension can effectively quantify the complexity of pore structure. The significant difference between D\u003csub\u003emin\u003c/sub\u003e and D\u003csub\u003emax\u003c/sub\u003e reveals the essential difference in structural characteristics between bound pores and seepage pores, which provides a new quantitative index for the classification and evaluation of tight reservoirs.\u003c/p\u003e\u003cp\u003eThe study verifies the applicability and advantages of the combination of nuclear magnetic resonance and fractal theory in the characterization of tight sandstone reservoirs. This method can provide a scientific basis for pore structure analysis, fluid mobility evaluation and CO\u003csub\u003e2\u003c/sub\u003e geological storage potential prediction of similar reservoirs.\u003c/p\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eThe Innovation Capability Support Plan in Shaanxi Province of China(NO. 2024RS-CXTD-54); Research and engineering demonstration project on water depth transfer and storage of coal seam roof strata in Shaanxi Province (NO. 2024SF-YBXM-603).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eShuhui Zhang completed analyzed the data, and wrote the draft paper. Baoyan Zhi and Binhu Xiao provided the paper ideas. Lin Bai and Hongbo Guo collated data. Heping Li collated and analyzed data.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe datasets used and/or analysed during the current study available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eFeng R H，Balling N，Grana D．\u0026nbsp;Lithofacies classification of a geothermal reservoir in Denmark and its facies-dependent porosity estimation from seismic inversion[J]．\u0026nbsp;Geothermics，2020，87 ( 9 ) : 1-11．\u003c/li\u003e\n \u003cli\u003eYi Zhenlin，Zhang Lei．\u0026nbsp;Classification of extra-low permeability reservoir based on microscopic pore structure: a case study of south Gulong area in Daqing oil field[J]．\u0026nbsp;Fault-Block Oil and Gas Field，\u0026nbsp;2020，27( 1) : 40-44．\u003c/li\u003e\n \u003cli\u003eYu H Y，Wang Z L，Wen F G，et al．\u0026nbsp;Reservoir and lithofacies shale classification based on NMR logging[J]．\u0026nbsp;Petroleum Research，2020，5( 3) : 202-209.\u003c/li\u003e\n \u003cli\u003eZhang Shuhui, Du Song, Zhu Guangpei, et al. Analysis on the Research Trend of Carbon Capture, Utilization and Storage (CCUS) Technology Based on Bibliometrics[J]. GeoStorage, 2025, 1(1): 80-90.\u003c/li\u003e\n \u003cli\u003eWang Wentao，Liu Pengchao，Li Biao，et al．\u0026nbsp;Fuzzy comprehensive evaluation for productivity of ultra low permeability reservoir[J]．\u0026nbsp;Special Oil and Gas Reservoirs，2019，26( 2) : 127-131．\u003c/li\u003e\n \u003cli\u003eZhang Wenkai，Shi Zejin，Tian Yaming，et al．\u0026nbsp;The combination of high-pressure mercury injection and rate-controlled mercury injection to characterize the pore-throat structure in tight sandstone reservoirs[J]．\u0026nbsp;Fault-Block Oil and Gas Field，2021，28( 1) : 14-20．\u003c/li\u003e\n \u003cli\u003eLi Kui，Han Yanlong，Jia Yuan，et al．\u0026nbsp;Study on the microstructure and fractal characteristics of sandstone in complex tectonic mining areas[J]．\u0026nbsp;China Mining Magazine，2024，33( S1) : 414-420．\u003c/li\u003e\n \u003cli\u003eYan Jianping，He Xu，Geng Bin，et al．\u0026nbsp;Evaluation method of pore structure in low-permeability sandstone reservoirs based on fractal theory[J]．\u0026nbsp;Logging Technology，2017，41( 3) : 345-352．\u003c/li\u003e\n \u003cli\u003eZhu Yangqi，Zhang Hui，Yao Zhigang，et al．\u0026nbsp;Fractal characteristics and controlling factors of tight sandstone reservoirs based on high-pressure mercury injection experiments[J]．\u0026nbsp;Science Technology and Engineering，2024，24( 20) : 8419-8428．\u003c/li\u003e\n \u003cli\u003eYao Yanbin, Liu Dameng, Cai Yidong，et al. 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Journal of Oil and Gas Technology,2007,29(4):80-86，166.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"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":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"NMR, fractal theory, sandstone reservoir, pore structure, T2 cut-off value(T2C)","lastPublishedDoi":"10.21203/rs.3.rs-7846824/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7846824/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eBased on nuclear magnetic resonance (NMR) technology and fractal theory, this paper systematically studies the microscopic pore structure of tight sandstone reservoirs from Dahaize Coal Mine and its influence on fluid occurrence and migration. The T\u003csub\u003e2\u003c/sub\u003e spectra of six groups of rock samples in saturated water and bound water were obtained by low field nuclear magnetic resonance experiment, and the complexity and heterogeneity of pore structure were quantitatively characterized by fractal dimension. The results show that the reservoir is characterized by low porosity and low permeability, but the microscopic pore structure is significantly different. Some samples are mainly medium-small pores, with good pore connectivity, high movable fluid saturation and good seepage capacity. Most of the samples are dominated by micropores, with dense structure and poor fluid mobility. The cut-off value of T\u003csub\u003e2\u003c/sub\u003e (T\u003csub\u003e2C\u003c/sub\u003e) and effective porosity further reveal the difference of fluid mobility in reservoirs, and high values usually correspond to high-quality reservoirs. In addition, through fractal dimension analysis, the essential difference in structural complexity between bound fluid pores ( D\u003csub\u003emin\u003c/sub\u003e average 2.144 ) and movable fluid pores ( D\u003csub\u003emax\u003c/sub\u003e average 2.959 ) is clarified, indicating that the seepage pore system has higher heterogeneity and tortuosity. This study verifies the effectiveness of the combination of NMR and fractal theory in the quantitative characterization of tight sandstone pore structure, and provides a scientific basis for reservoir evaluation and CO\u003csub\u003e2\u003c/sub\u003e geological storage potential prediction.\u003c/p\u003e","manuscriptTitle":"Quantitative characterization of pore structure and NMR fractal based on nuclear magnetic resonance and fractal theory","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-07 13:07:49","doi":"10.21203/rs.3.rs-7846824/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7bf7d374-4b7d-41f5-8767-eade0fcf445e","owner":[],"postedDate":"November 7th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":57455220,"name":"Physical sciences/Energy science and technology"},{"id":57455221,"name":"Earth and environmental sciences/Solid earth sciences"}],"tags":[],"updatedAt":"2025-11-11T09:54:31+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-07 13:07:49","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7846824","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7846824","identity":"rs-7846824","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}