有效压力条件下岩石结构对页岩油藏物理性质的影响

IF 1.8 4区 地球科学 Q3 GEOCHEMISTRY & GEOPHYSICS
Yu Ma, Suping Yao, Ning Zhu, Huimin Liu, Junliang Li, Weiqing Wang
{"title":"有效压力条件下岩石结构对页岩油藏物理性质的影响","authors":"Yu Ma, Suping Yao, Ning Zhu, Huimin Liu, Junliang Li, Weiqing Wang","doi":"10.2113/2024/lithosphere_2023_338","DOIUrl":null,"url":null,"abstract":"The physical properties of shale oil reservoirs under overburden pressure are of great significance for reservoir prediction and evaluation during exploration and development. Based on core, thin section, and SEM observations, as well as test data such as XRD, TOC, and porosity and permeability under pressure conditions, this study systematically analyzes the variation of physical properties of different lithofacies shales in the Jiyang depression and the influence of rock fabric on the physical variation under pressure. The porosity and permeability of shale samples significantly decrease under pressure. According to the phased reduction in porosity and permeability, the pressurization process is divided into three pressure stages: low pressure (<8 MPa), medium pressure (8–15 MPa), and high pressure (>15 MPa). The reduction of porosity is fastest in the low-pressure stage and slowest in the medium-pressure stage. The reduction of permeability is fastest in the low-pressure stage and the slowest in the high-pressure stage. The rock fabric has a significant impact on porosity and permeability under pressure conditions. The permeability of laminated shale and bedded shale is higher than that of massive shale under pressure, and the permeability loss rate is lower than that of massive shales. Especially under lower pressure, the difference can be 10–20 times. In addition, the reduction rate of porosity and permeability under pressure is negatively correlated with felsic minerals content, which is positively correlated with carbonate minerals content and clay minerals content. The contribution of clay minerals to the porosity reduction rate is dominant, followed by carbonate minerals. The contribution of carbonate minerals to the permeability reduction rate is dominant, followed by clay minerals. The TOC content has no significant impact on the porosity and permeability of shales under pressure in the study due to the low maturity.With the change in global energy structure, shale oil and gas has become the core growth point of China’s oil and gas resources [1-4]. In the past decade, a series of important progresses have been made in the exploration and development of shale oil and gas in China, including breakthroughs in the exploration of shale oil in Junggar Basin, Ordos Basin, Jianghan Basin, Songliao Basin, and Bohai Bay Basin [5-8]. However, due to the heterogeneity of shales and the complexity of geological conditions in China, the prediction and evaluation of shale oil reservoirs still face many challenges [3, 9, 10].Many studies have shown that rock fabric, such as laminated structure and mineral composition, has a significant influence on the pore development and physical properties of shale oil reservoirs [10-14]. However, most of these studies were conducted under unpressurized conditions, and there are some errors with the formation conditions, which affect the prediction and evaluation of shale oil desserts. To recover the real physical parameters under formation conditions, some scholars carried out pressure experiments and found that both porosity and permeability of shale decreased gradually with the increase of effective pressure, and proposed exponential function, binomial, and other functional models to describe the change of porosity and permeability with pressure during pressurization [15-22]. However, for individual shale samples, the differences in porosity and permeability between different samples under the same pressure conditions are large [17-19], and the reasons for these differences are still unclear and need to be further explored. In this study, the Es3x-Es4s shales from the Jiyang depression were selected. Through petrology, mineralogy, porosity and permeability experiments under effective pressure, and FE-SEM observation, the physical characteristics of the shale reservoirs under effective pressure and the influence of rock fabric on porosity and permeability were discussed to provide a basis for the prediction and evaluation of the shale oil desserts in the Paleocene shale of the Jiyang depression.The Jiyang depression is located in the southeast of the Bohai Bay Basin, China, with four sub-depressions from south to north, Dongying, Huimin, Zhanhua, and Chezhen (Figure 1(a)) [23]. It has made great breakthroughs in multiple layers and types of shale oil in Dongying, Zhanhua, and other sags during the past ten years of exploration, showing a good exploration prospect of shale oil in Jiyang depression [24-26]. Currently, Es4s, Es3x, and Es1 are the main exploration intervals for shale oil in Jiyang depression (Figure 1(b)), characterized by high carbonate content, with an average content of more than 50% [23, 27, 28]. In addition, the laminae are well developed with alternating light laminae and dark laminae [23, 29, 30].As the main exploration intervals for shale oil in Jiyang depression, there are many industrial oil flow wells and sampling wells in Es4s-Es3x. The samples in this study were collected from Well BYP5, F201, and FYP1 (Figure 1(a)), and the basic information is shown in Table 1.Before experiments, the samples were treated in different ways as illustrated in Figure 2. First, a plug sample (diameter of 25 mm and length of 60 mm) was drilled parallel to the bedding from the core sample. Then, a 2 cm high micro plug was cut from one side of the plug, which was used for thin section and scanning electron microscopy observations. The remaining plug sample was used for the measurement of porosity and permeability under effective pressure. After completing the measurement of porosity and permeability, the plug sample was crushed for TOC and mineral composition analysis.A Zeiss microscope was used to observe the thin sections of rocks with a thickness of 30 μm to clarify the development characteristics and mineral distribution of the laminae. A high-resolution field emission scanning electron microscope (Zeiss Sigma 500) was used to observe the morphology, size, and distribution of pores. The surface of the samples (10 × 10 μm) was polished by hand grinding and argon ion polishing (Leica EM 3X) to achieve better image quality.Vitrinite reflectance (Ro%) measurements were undertaken using a Zeiss microscope equipped with a J&M MSP200 microphotometer. The total organic carbon (TOC) contents were determined using a Vario MICRO cube elemental analyzer. Before testing, the shale samples were ground to less than 200 mesh and reacted with dilute HCL for 72 hours to remove carbonate minerals. The X-ray diffraction (XRD) analysis was conducted using a Dmax IIIa diffractometer with a Cu x-ray tube (40 kV, 30 mA). The separation and XRD tests of clay minerals were carried out according to the Chinese petroleum industry standard (SY/T 5163–2018). The above experiments were carried out in the Key Laboratory of Surficial Geochemistry (KLSG), Ministry of Education, Nanjing University.The porosity and permeability under effective pressure conditions were measured by PoroPerm-200 automatic porosity-permeability instrument. The realization of overpressure conditions mainly relies on a core holding system, which uses gas as the pressurized medium to mimic the pressurization of hydraulic fracturing, thus realizing the online testing of porosity permeability under overpressure conditions. During the experiment, high-pressure air was used for the pressurized gas source and high-purity helium was used for the test. To avoid the influence of slip effect on the permeability of shale samples, the driving pressure was kept constant during the pressurization process. The effective pressure points set in this experiment were kept constant during the pressurization process. The effective pressure points set in this experiment were 2.41, 4.48, 6.55, 8.62, 10.69, 12.07, 20, 30, and 50 MPa, respectively. The measurements were kept at each pressure point for 30 minutes, and then the porosity and permeability under the corresponding pressures were measured after stabilization. Before the overpressure experiments, the shale samples need to be extracted (72 hours) with dichloromethane and methanol (93:7) to remove the residual oil in the samples.The shale is characterized by laminations in the Jiyang depression [29-31]. According to the thickness of the lamination, the shale samples were divided into laminated shale (<2 mm), bedded shale (2 mm to 2 cm), and massive shale (>2 cm) by combing core and thin section observations. Except for massive shales, the laminations of laminated shale and bedded shale are well-developed, showing as alternating light lamination and dark lamination (Figure 3).In addition, there is a significant difference in the mineral composition between light lamination and dark lamination. The main mineral of the light lamination is calcite, with small amounts of quartz, feldspar, and dolomite floating between mud crystalline calcite and sparry calcite; the mineral composition of dark lamination is more complex, and is dominated by clay minerals, also contains some calcite, dolomite, quartz, feldspar, and organic matter (OM). The difference in the mineral composition of lamination results in strong heterogeneity in the distribution of minerals in laminated shales and bedded shales. In contrast, massive shales lack laminae and thus have a relatively homogeneous distribution of minerals.The mean vitrinite reflectance (Ro%), TOC content, and mineral composition of shale samples are presented in Table 2. The shale samples in this study have lower thermal maturity (0.78% Ro to 1.12% Ro) and moderate TOC content (2.85 to 6.26 wt%, avg. 3.51 wt%). The minerals include calcite (7 to 69 wt%, avg. 37 wt%), clay minerals (4 to 30 wt%, avg. 22%), quartz (10 to 34 wt%, avg. 19 wt%), feldspar (4 to 22 wt%, avg. 11 wt%), dolomite (3 to 22 wt%, avg. 10 wt%) and a small amount of pyrite (1 to 3 wt%, average <2 wt%). Among clay minerals, illite/smectite mixed layer (20 to 66 wt%, avg. 51 wt%) is dominated, followed by illite (25 to 72 wt%, avg. 46 wt%), with a small amount of kaolinite and chlorite.Based on scanning electron microscopy observations and previous studies [32, 33], four types of pores are found in the Jiyang depression: interparticle (InterP) pores, intraparticle (IntraP) pores, fracture pores, and a few OM pores.Fracture pores, a major pore type in the Es3x-Es4s shales, mainly include interlayer fractures, tectonic fractures, and irregular fractures. The interlayer fractures are mostly developed at the interface of the laminae in laminated shales and bedded shales. The main body of the interlayer fractures is nearly horizontal and relatively stable in extension (Figure 4(a–c)). Tectonic fractures usually form an angle with the direction of the laminae and span multiple layers. Irregular fractures are mostly related to shrinkage by hydrocarbon generation and the dehydration of clay minerals [34], and usually extend around rigid particles (Figure 4(d)), with poor continuity.IntraP pores occur inside mineral particles/crystals and are controlled by mineral types, including the pores in clay aggregates (Figure 4(e–f)), dissolution pores in calcite and dolomite (Figure 4(e–f)), pores in feldspar cleavages, and pores in pyrite aggregates (Figure 4(g)). IntraP pores are mostly isolated and have poor connectivity. InterP pores occur between mineral particles/crystals or at the edges of mineral particles/crystals and mainly include the pores between calcite granules (Figure 4(i–j)), between clay minerals (Figure 4(h)), and between calcite and dolomite, quartz, feldspar (Figure 4(k)) in the research area. Compared with IntraP pores, the InterP pores generally have good connectivity. Most of the InterP pores are wedge-shaped (5–10 μm) or slit-shaped in calcite laminae of laminated shales and bedded shales (Figure 4(i–j)), which are related to calcite recrystallization and have reformed by late dissolution [35]. The shape of InterP pores is irregular in massive shales and argillaceous laminae of laminated shales and bedded shales, with diameters ranging from 500 nm to 2 μm. OM pores are not developed in this study, and only a small amount of elongated slit-shaped OM pores were observed inside or at the edges of the OM (Figure 4(l)).The porosity and permeability of shale samples at different effective pressures are shown in online Supplementary Material Tables S1 and S2. It can be seen that the porosity and permeability of shale samples under effective pressure are significantly lower than the initial porosity and initial permeability. During the pressurization process, the porosity and permeability of the samples decreased with the increase of the effective pressure, and the corresponding porosity loss and permeability loss continued to increase (Figure 5). When the effective pressure reaches the maximum value (50 MPa) of this experiment, the porosity and permeability of the samples are the lowest, and the corresponding loss rate also reaches the maximum value. At this time, the porosity loss rate of shale samples was 9.48%–54.32%, and the permeability loss rate was 95.18% to 99.5%. Compared with porosity, the permeability loss of shale samples is generally greater.From the viewpoint of the whole pressurization process, the decrease in porosity and permeability shows an obvious stage with increasing effective pressure. This phasing is specifically manifested in the permeability as follows: at lower pressure, the permeability decreases rapidly, and the permeability loss rate increases rapidly; at medium pressure, the decrease in permeability slows down, and the increase in the permeability loss rate slows down; and at high pressure, the decrease in permeability is very slow, and the increase in the permeability loss rate is also very slow. Thus, based on the differential decrease in porosity and permeability during pressurization process, the pressurization process can be divided into three stages: low-pressure stage (<8 MPa), medium-pressure stage (8–15 MPa), and high-pressure stage (>15 MPa).Within each stage, the decrease in porosity and permeability with effective pressure can be fitted with a linear function (Figure 6), which exhibits a higher fitting degree than overall fitting. The absolute values of the slopes of the porosity-effective pressure and permeability-effective pressure fitting curves are defined as the porosity reduction rate and permeability reduction rate under effective pressure, respectively. According to the fitting results (Figure 6), the porosity reduction rate (%/MPa) of shale samples is 0.0093–0.2620 at the low-pressure stage, which is 0.0027–0.0562 at the medium-pressure stage, and 0.0045–0.0564 at the high-pressure stage. In other words, the porosity reduction rate is the highest at the low-pressure stage, and lowest at the medium-pressure stage, with a difference ranging 2–15 times. The reduction of permeability with effective pressure exhibits similar phased characteristics (Figure 6). The permeability reduction rate (mD/MPa) of shale samples is 0.0002–16.1060 at the low-pressure stage, which is 0.000003–1.9931 at the medium-pressure stage, and 0.0000004–0.0912 at the high-pressure stage. Thus, the permeability reduction rate is the highest in the low-pressure stage, and lowest in the high-pressure stage, with a difference of two orders of magnitude. In conclusion, with the increase of effective pressure, the reduction of porosity goes through three stages of rapid-slow-rapid changes in sequence, and the reduction of permeability goes through three stages of rapid–slow–basically stable changes in sequence.Previous studies have shown that the permeability of laminated shales and bedded shales is generally higher than that of massive shales [36, 37]. The shale samples in this study exhibit the same characteristics: the permeability (mD) of the laminated samples ranges from 0.0156 to 5.1990, and that of the bedded samples ranges from 0.0026 to 110.7375, which is much higher than that of the massive sample (0.0014 mD). This may be related to the differential development of fractures in different lithofacies. As mentioned in section 3.3, the interface of laminae in laminated shales and bedded shales is prone to the development of interlayer fractures, while only a small number of small-scale irregular fractures are present in massive shales. Fractures can greatly improve the permeability of shale reservoirs [38], thus the permeability of laminated shales and bedded shales is generally higher than that of massive shales.The permeability of laminated shales and bedded shales remains higher than that of massive shales under effective pressure, while the permeability loss of massive shale isgreater, especially under lower effective pressure (online Supplementary Material Tables S3 and S4). When the effective pressure is 2.41 MPa, the permeability of the massive shale sample decreases by 85.17%, compared to a decrease of 2.79%–4.48% for the laminated samples and 1.42% ~ 7.50% for the bedded samples. As the effective pressure increases, the difference in permeability loss rate gradually decreases, and the permeability loss rate of laminated shales and bedded shales is always lower than that of massive shales. When the effective pressure reaches 4.48 MPa, the permeability of the massive shale decreases by 91.31%, compared to a decrease of 52.75% ~ 69.92% for the laminated samples and 32.82%–73.48% for the bedded samples. When the effective pressure reaches 20 MPa, the permeability loss of shale samples is more than 96%.The reduction of permeability can be attributed to the following factors: (1) compression and closure of fractures, (2) the reduction of pores’ size and closure of matrix pores, and (3) loss of connectivity between pores [15, 16, 39]. For laminated shales and bedded shales containing a large number of interlayer fractures, the reduction of permeability is mainly attributed to the closure of fractures, followed by the compression and closure of matrix pores. For massive shales dominated by matrix pores, the reduction of permeability is mainly related to matrix pores (Figure 7). Massive shales have small pore size and poor connectivity, so the reduction of pores’ size and closure of pores lead to higher permeability reduction at lower pressure. The fractures in laminated shales and bedded shales are also compressed but not completely closed, so the permeability reduction is relatively low. In addition, the relatively low permeability reduction of laminated shales and bedded shales may also be related to uneven compression. The distribution of pores is relatively uniform in massive shales, while it varies significantly due to mineral composition in laminated shales and bedded shale. Argillaceous laminae are rich in clay minerals which are plastic, and have strong compressibility. However, the compressibility of slit-shaped and wedge-shaped intergranular pores in calcite laminae is weaker than pores in argillaceous laminae due to the support of rigid calcite particles. This is also the main reason why the porosity of laminated samples and bedded samples is generally higher than that of massive samples in this study.As mentioned in 3.3, mineral composition affects the type and development of pores [40, 41]. In addition, researches have shown that mineral composition also affects the compressive capacity of rocks to some extent [1, 42]. Felsic minerals have high hardness and strong anticompression ability, clay minerals have strong plasticity and are easy to be compressed under pressure, while carbonate minerals are in between [1, 42]. Therefore, the correlation coefficient method was adopted to analyze the contribution of individual mineral components to the porosity-permeability reduction at effective pressure in this study.As shown in Figure 8, there is a positive correlation between carbonate minerals content and porosity-permeability reduction rate. This positive correlation indicates that carbonate-related pores have compressibility to some extent. The correlation coefficient (R2) between carbonate minerals content and average porosity reduction rate is only 0.07, which is basically not correlated, while the R2 between carbonate minerals content and stage porosity reduction rate is 0.58, 0.35, and 0.29, respectively, indicating a good correlation (Figure 8). Permeability also shows similar characteristics (Figure 8). In other words, the correlation between carbonate minerals content and stage permeability reduction rates (R2=0.69, 0.88, 0.82) is higher than that between carbonate minerals content and average permeability reduction rate (R2=0.62). This indicates that the influence of carbonate minerals on porosity-permeability reduction rate has obvious stage characteristics under effective pressure.In addition, the correlation between carbonate minerals and permeability reduction rate is significantly better than that of porosity. This implies that the contribution of carbonate minerals to the permeability reduction rate is greater than that to porosity reduction rate. This may be related to the pore structure of shales. In terms of pore structure, porosity is related to the total volume of all pores, while permeability depends on the connectivity between pores. As mentioned in 3.3, the shales develop carbonate-related pores, clay-related pores, and felsic-related pores in this study. Intergranular pores between carbonate minerals and felsic minerals have good connectivity, while clay-related pores have poor connectivity. Therefore, carbonate minerals have a greater influence on permeability, which is consistent with Su et al [43].There is a weak negative correlation between clay minerals and porosity-permeability reduction rate (Figure 9). Obviously, this is inconsistent with the conventional understanding of the compressibility of clay minerals [42, 44]. Further analysis shows that two samples (F201-2 and FYP1-6) with low clay minerals content (<10%) interfere with the changing characteristics of the whole curve. After removing the two samples with low clay minerals content, there is a positive correlation between the clay minerals content and porosity-permeability reduction rate, which shows that the porosity-permeability reduction rate increases with the increase of clay minerals content (Figure 9). Consistent with carbonate minerals, the correlation between the content of clay minerals and stage porosity-permeability reduction rate is better than the average porosity-permeability reduction rate.In contrast to carbonate minerals, the correlation between clay minerals content and permeability reduction rate is lower than its correlation with porosity reduction rate. This means that the contribution of clay minerals content to porosity reduction rate is higher than its contribution to permeability reduction rate. This is consistent with the pore development characteristics of shales. In this study, the shale samples are widely developed with clay mineral-related pores, which are numerous but poorly connected to each other (Figure 7).Under effective pressure conditions, there is a negative correlation between felsic minerals content and porosity-permeability reduction rate to a certain extent (Figure 10), which is consistent with the conventional understanding of the strong anticompression ability of felsic minerals. As shown in Figure 10, the correlation between felsic minerals content and stage porosity-permeability reduction rate is significantly better than that between felsic minerals content and average porosity-permeability reduction rate, which is similar to carbonate minerals and clay minerals. In addition, the contribution of felsic minerals content to the stage permeability reduction rate (R2>0.87) is higher than to the stage porosity reduction rate (R2=0.34), which is consistent with carbonate minerals. As described in 3.2 and 3.3, felsic minerals are low in content and dispersed in carbonate minerals and clay minerals. As a result, felsic minerals-related pores have a low contribution to the total porosity. Compared with other minerals, felsic minerals have a stronger anticompression ability at effective pressure, thus greatly protecting the connectivity of the pore-fracture system. Therefore, the permeability reduction rate shows a trend of increasing with the increase of felsic minerals content.In summary, carbonate minerals, clay minerals, and felsic minerals all affect the reduction of porosity and permeability at effective pressure to varying degrees. Carbonate minerals content and clay minerals content are positively correlated with porosity-permeability reduction rate, while felsic minerals content is negatively correlated with porosity-permeability reduction rate. Consistent with the phased changes in porosity-permeability reduction rate, this correlation is also presented with phased characteristics (Figures 8 and 9, Figure 10). In other words, the contribution of minerals to the porosity-permeability reduction rate is different in different pressure stages. Overall, porosity reduction rate is mainly related to clay minerals content (R2=0.74, 0.51, 0.52), followed by carbonate minerals content (R2=0.58, 0.35, 0.29); permeability reduction rate is mainly affected by carbonate minerals content (R2=0.69, 0.88, 0.82), followed by clay minerals content (R2=0.27, 0.46, 0.30).Previous studies have shown that the TOC content (%) has significant effects on the development of pores in organic-rich shales [32, 45-47]. Compared to minerals, OM has stronger ductility and is easier to compress [50]. In this study, there is no significant correlation between TOC content and porosity-permeability reduction rate (Figure 11). This may be attributed to the lower maturity of the shale samples.A large number of OM pores are developed in medium-high maturity shales, and thus the higher the TOC content, the more developed the OM pores and the larger the total pore volume [47]. In contrast, the maturity of the shale samples in this study is relatively low, with vitrinite reflectance ranging from 0.82% to 1.12%. The lower maturity corresponds to undeveloped OM pores. As described in 3.3, the pore types of shales in the study area are dominated by fractures and inorganic mineral pores, and OM pores are not developed, thus making the effect of TOC content on porosity permeability under effective pressure insignificant.Under the effective pressure influence, the porosity and permeability of shale decrease with the increase of effective pressure, and the reduction rate is characterized by an obvious stage change. According to the stage changes of porosity-permeability reduction rate, the pressurization process can be divided into three pressure stages: low pressure (<8 MPa), medium pressure (8–15 MPa) and high pressure (>15 MPa). Porosity reduction rate is highest in the low-pressure stage and lowest in the medium-pressure stage, with a difference of 2–15 times. Permeability reduction rate is highest at low-pressure stage and lowest at high-pressure stage, with a difference of 2 orders of magnitude.Under the effective pressure influence, the rock fabric affects porosity and permeability by influencing mineral distribution, pore development, and differential compaction. The laminae and interlayer fractures are poorly developed, and the distribution of minerals and pores is relatively homogeneous in massive shales. The presence of laminae is accompanied by well-developed interlayer fractures and the differential distribution of minerals and pores in laminated shale and layered shale. the minerals are dominated by calcite in calcite laminae where the pores mainly occur between calcite crystals and at the edges of calcite crystals, with better connectivity; the minerals are dominated by clay minerals in argillaceous laminae where the pores are mainly in clay minerals, with poorer connectivity. The compressibility of clay minerals is stronger than calcite. As a result, the clay mineral-related pores in the argillaceous laminae are more likely to be damaged and lose connectivity under effective pressure conditions, while the calcite-related pores in calcite laminae are less compressed and retain a certain permeability. In addition, interlayer fractures have not been completely closed at lower pressure, thus still able to maintain a certain connectivity. Therefore, compared with massive shale, laminated shale, and bedded shale generally have lower permeability loss and relatively higher permeability under effective pressure.During pressurization process, the porosity and permeability reduction rates are negatively correlated with felsic minerals content and positively correlated with carbonate minerals content and clay minerals content. Porosity reduction rate is mainly related to clay minerals content (R2=0.74, 0.51, 0.52), followed by carbonate minerals content (R2=0.58, 0.35, 0.29). Permeability reduction rate is primarily related to carbonate minerals content (R2=0.69, 0.88, 0.82), followed by clay minerals content (R2=0.27, 0.46, 0.30). The TOC content has no significant effect on the porosity and permeability of the shale in the study area due to lower maturity.All relevant data are within the manuscript and supplementary materials.The authors declared that they have no conflicts of interest to this work.This study was financially supported by the National Natural Science Foundation of China (Grant No.42072152).Table S1: Measured porosity of shale samples at different pressures.Table S2: Measured permeability of shale samples at different pressures.Table S3: Porosity loss rate calculated based on the values of measured porosity and pressure.Table S4: Permeability loss rate calculated based on the values of measured permeability and pressure.Table S5: Porosity-permeability reduction rate at different pressure stages based on the results of segmentation fitting.","PeriodicalId":18147,"journal":{"name":"Lithosphere","volume":"23 1","pages":""},"PeriodicalIF":1.8000,"publicationDate":"2024-04-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Influence of Rock Fabric on Physical Properties of Shale Oil Reservoir Under Effective Pressure Conditions\",\"authors\":\"Yu Ma, Suping Yao, Ning Zhu, Huimin Liu, Junliang Li, Weiqing Wang\",\"doi\":\"10.2113/2024/lithosphere_2023_338\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"The physical properties of shale oil reservoirs under overburden pressure are of great significance for reservoir prediction and evaluation during exploration and development. Based on core, thin section, and SEM observations, as well as test data such as XRD, TOC, and porosity and permeability under pressure conditions, this study systematically analyzes the variation of physical properties of different lithofacies shales in the Jiyang depression and the influence of rock fabric on the physical variation under pressure. The porosity and permeability of shale samples significantly decrease under pressure. According to the phased reduction in porosity and permeability, the pressurization process is divided into three pressure stages: low pressure (<8 MPa), medium pressure (8–15 MPa), and high pressure (>15 MPa). The reduction of porosity is fastest in the low-pressure stage and slowest in the medium-pressure stage. The reduction of permeability is fastest in the low-pressure stage and the slowest in the high-pressure stage. The rock fabric has a significant impact on porosity and permeability under pressure conditions. The permeability of laminated shale and bedded shale is higher than that of massive shale under pressure, and the permeability loss rate is lower than that of massive shales. Especially under lower pressure, the difference can be 10–20 times. In addition, the reduction rate of porosity and permeability under pressure is negatively correlated with felsic minerals content, which is positively correlated with carbonate minerals content and clay minerals content. The contribution of clay minerals to the porosity reduction rate is dominant, followed by carbonate minerals. The contribution of carbonate minerals to the permeability reduction rate is dominant, followed by clay minerals. The TOC content has no significant impact on the porosity and permeability of shales under pressure in the study due to the low maturity.With the change in global energy structure, shale oil and gas has become the core growth point of China’s oil and gas resources [1-4]. In the past decade, a series of important progresses have been made in the exploration and development of shale oil and gas in China, including breakthroughs in the exploration of shale oil in Junggar Basin, Ordos Basin, Jianghan Basin, Songliao Basin, and Bohai Bay Basin [5-8]. However, due to the heterogeneity of shales and the complexity of geological conditions in China, the prediction and evaluation of shale oil reservoirs still face many challenges [3, 9, 10].Many studies have shown that rock fabric, such as laminated structure and mineral composition, has a significant influence on the pore development and physical properties of shale oil reservoirs [10-14]. However, most of these studies were conducted under unpressurized conditions, and there are some errors with the formation conditions, which affect the prediction and evaluation of shale oil desserts. To recover the real physical parameters under formation conditions, some scholars carried out pressure experiments and found that both porosity and permeability of shale decreased gradually with the increase of effective pressure, and proposed exponential function, binomial, and other functional models to describe the change of porosity and permeability with pressure during pressurization [15-22]. However, for individual shale samples, the differences in porosity and permeability between different samples under the same pressure conditions are large [17-19], and the reasons for these differences are still unclear and need to be further explored. In this study, the Es3x-Es4s shales from the Jiyang depression were selected. Through petrology, mineralogy, porosity and permeability experiments under effective pressure, and FE-SEM observation, the physical characteristics of the shale reservoirs under effective pressure and the influence of rock fabric on porosity and permeability were discussed to provide a basis for the prediction and evaluation of the shale oil desserts in the Paleocene shale of the Jiyang depression.The Jiyang depression is located in the southeast of the Bohai Bay Basin, China, with four sub-depressions from south to north, Dongying, Huimin, Zhanhua, and Chezhen (Figure 1(a)) [23]. It has made great breakthroughs in multiple layers and types of shale oil in Dongying, Zhanhua, and other sags during the past ten years of exploration, showing a good exploration prospect of shale oil in Jiyang depression [24-26]. Currently, Es4s, Es3x, and Es1 are the main exploration intervals for shale oil in Jiyang depression (Figure 1(b)), characterized by high carbonate content, with an average content of more than 50% [23, 27, 28]. In addition, the laminae are well developed with alternating light laminae and dark laminae [23, 29, 30].As the main exploration intervals for shale oil in Jiyang depression, there are many industrial oil flow wells and sampling wells in Es4s-Es3x. The samples in this study were collected from Well BYP5, F201, and FYP1 (Figure 1(a)), and the basic information is shown in Table 1.Before experiments, the samples were treated in different ways as illustrated in Figure 2. First, a plug sample (diameter of 25 mm and length of 60 mm) was drilled parallel to the bedding from the core sample. Then, a 2 cm high micro plug was cut from one side of the plug, which was used for thin section and scanning electron microscopy observations. The remaining plug sample was used for the measurement of porosity and permeability under effective pressure. After completing the measurement of porosity and permeability, the plug sample was crushed for TOC and mineral composition analysis.A Zeiss microscope was used to observe the thin sections of rocks with a thickness of 30 μm to clarify the development characteristics and mineral distribution of the laminae. A high-resolution field emission scanning electron microscope (Zeiss Sigma 500) was used to observe the morphology, size, and distribution of pores. The surface of the samples (10 × 10 μm) was polished by hand grinding and argon ion polishing (Leica EM 3X) to achieve better image quality.Vitrinite reflectance (Ro%) measurements were undertaken using a Zeiss microscope equipped with a J&M MSP200 microphotometer. The total organic carbon (TOC) contents were determined using a Vario MICRO cube elemental analyzer. Before testing, the shale samples were ground to less than 200 mesh and reacted with dilute HCL for 72 hours to remove carbonate minerals. The X-ray diffraction (XRD) analysis was conducted using a Dmax IIIa diffractometer with a Cu x-ray tube (40 kV, 30 mA). The separation and XRD tests of clay minerals were carried out according to the Chinese petroleum industry standard (SY/T 5163–2018). The above experiments were carried out in the Key Laboratory of Surficial Geochemistry (KLSG), Ministry of Education, Nanjing University.The porosity and permeability under effective pressure conditions were measured by PoroPerm-200 automatic porosity-permeability instrument. The realization of overpressure conditions mainly relies on a core holding system, which uses gas as the pressurized medium to mimic the pressurization of hydraulic fracturing, thus realizing the online testing of porosity permeability under overpressure conditions. During the experiment, high-pressure air was used for the pressurized gas source and high-purity helium was used for the test. To avoid the influence of slip effect on the permeability of shale samples, the driving pressure was kept constant during the pressurization process. The effective pressure points set in this experiment were kept constant during the pressurization process. The effective pressure points set in this experiment were 2.41, 4.48, 6.55, 8.62, 10.69, 12.07, 20, 30, and 50 MPa, respectively. The measurements were kept at each pressure point for 30 minutes, and then the porosity and permeability under the corresponding pressures were measured after stabilization. Before the overpressure experiments, the shale samples need to be extracted (72 hours) with dichloromethane and methanol (93:7) to remove the residual oil in the samples.The shale is characterized by laminations in the Jiyang depression [29-31]. According to the thickness of the lamination, the shale samples were divided into laminated shale (<2 mm), bedded shale (2 mm to 2 cm), and massive shale (>2 cm) by combing core and thin section observations. Except for massive shales, the laminations of laminated shale and bedded shale are well-developed, showing as alternating light lamination and dark lamination (Figure 3).In addition, there is a significant difference in the mineral composition between light lamination and dark lamination. The main mineral of the light lamination is calcite, with small amounts of quartz, feldspar, and dolomite floating between mud crystalline calcite and sparry calcite; the mineral composition of dark lamination is more complex, and is dominated by clay minerals, also contains some calcite, dolomite, quartz, feldspar, and organic matter (OM). The difference in the mineral composition of lamination results in strong heterogeneity in the distribution of minerals in laminated shales and bedded shales. In contrast, massive shales lack laminae and thus have a relatively homogeneous distribution of minerals.The mean vitrinite reflectance (Ro%), TOC content, and mineral composition of shale samples are presented in Table 2. The shale samples in this study have lower thermal maturity (0.78% Ro to 1.12% Ro) and moderate TOC content (2.85 to 6.26 wt%, avg. 3.51 wt%). The minerals include calcite (7 to 69 wt%, avg. 37 wt%), clay minerals (4 to 30 wt%, avg. 22%), quartz (10 to 34 wt%, avg. 19 wt%), feldspar (4 to 22 wt%, avg. 11 wt%), dolomite (3 to 22 wt%, avg. 10 wt%) and a small amount of pyrite (1 to 3 wt%, average <2 wt%). Among clay minerals, illite/smectite mixed layer (20 to 66 wt%, avg. 51 wt%) is dominated, followed by illite (25 to 72 wt%, avg. 46 wt%), with a small amount of kaolinite and chlorite.Based on scanning electron microscopy observations and previous studies [32, 33], four types of pores are found in the Jiyang depression: interparticle (InterP) pores, intraparticle (IntraP) pores, fracture pores, and a few OM pores.Fracture pores, a major pore type in the Es3x-Es4s shales, mainly include interlayer fractures, tectonic fractures, and irregular fractures. The interlayer fractures are mostly developed at the interface of the laminae in laminated shales and bedded shales. The main body of the interlayer fractures is nearly horizontal and relatively stable in extension (Figure 4(a–c)). Tectonic fractures usually form an angle with the direction of the laminae and span multiple layers. Irregular fractures are mostly related to shrinkage by hydrocarbon generation and the dehydration of clay minerals [34], and usually extend around rigid particles (Figure 4(d)), with poor continuity.IntraP pores occur inside mineral particles/crystals and are controlled by mineral types, including the pores in clay aggregates (Figure 4(e–f)), dissolution pores in calcite and dolomite (Figure 4(e–f)), pores in feldspar cleavages, and pores in pyrite aggregates (Figure 4(g)). IntraP pores are mostly isolated and have poor connectivity. InterP pores occur between mineral particles/crystals or at the edges of mineral particles/crystals and mainly include the pores between calcite granules (Figure 4(i–j)), between clay minerals (Figure 4(h)), and between calcite and dolomite, quartz, feldspar (Figure 4(k)) in the research area. Compared with IntraP pores, the InterP pores generally have good connectivity. Most of the InterP pores are wedge-shaped (5–10 μm) or slit-shaped in calcite laminae of laminated shales and bedded shales (Figure 4(i–j)), which are related to calcite recrystallization and have reformed by late dissolution [35]. The shape of InterP pores is irregular in massive shales and argillaceous laminae of laminated shales and bedded shales, with diameters ranging from 500 nm to 2 μm. OM pores are not developed in this study, and only a small amount of elongated slit-shaped OM pores were observed inside or at the edges of the OM (Figure 4(l)).The porosity and permeability of shale samples at different effective pressures are shown in online Supplementary Material Tables S1 and S2. It can be seen that the porosity and permeability of shale samples under effective pressure are significantly lower than the initial porosity and initial permeability. During the pressurization process, the porosity and permeability of the samples decreased with the increase of the effective pressure, and the corresponding porosity loss and permeability loss continued to increase (Figure 5). When the effective pressure reaches the maximum value (50 MPa) of this experiment, the porosity and permeability of the samples are the lowest, and the corresponding loss rate also reaches the maximum value. At this time, the porosity loss rate of shale samples was 9.48%–54.32%, and the permeability loss rate was 95.18% to 99.5%. Compared with porosity, the permeability loss of shale samples is generally greater.From the viewpoint of the whole pressurization process, the decrease in porosity and permeability shows an obvious stage with increasing effective pressure. This phasing is specifically manifested in the permeability as follows: at lower pressure, the permeability decreases rapidly, and the permeability loss rate increases rapidly; at medium pressure, the decrease in permeability slows down, and the increase in the permeability loss rate slows down; and at high pressure, the decrease in permeability is very slow, and the increase in the permeability loss rate is also very slow. Thus, based on the differential decrease in porosity and permeability during pressurization process, the pressurization process can be divided into three stages: low-pressure stage (<8 MPa), medium-pressure stage (8–15 MPa), and high-pressure stage (>15 MPa).Within each stage, the decrease in porosity and permeability with effective pressure can be fitted with a linear function (Figure 6), which exhibits a higher fitting degree than overall fitting. The absolute values of the slopes of the porosity-effective pressure and permeability-effective pressure fitting curves are defined as the porosity reduction rate and permeability reduction rate under effective pressure, respectively. According to the fitting results (Figure 6), the porosity reduction rate (%/MPa) of shale samples is 0.0093–0.2620 at the low-pressure stage, which is 0.0027–0.0562 at the medium-pressure stage, and 0.0045–0.0564 at the high-pressure stage. In other words, the porosity reduction rate is the highest at the low-pressure stage, and lowest at the medium-pressure stage, with a difference ranging 2–15 times. The reduction of permeability with effective pressure exhibits similar phased characteristics (Figure 6). The permeability reduction rate (mD/MPa) of shale samples is 0.0002–16.1060 at the low-pressure stage, which is 0.000003–1.9931 at the medium-pressure stage, and 0.0000004–0.0912 at the high-pressure stage. Thus, the permeability reduction rate is the highest in the low-pressure stage, and lowest in the high-pressure stage, with a difference of two orders of magnitude. In conclusion, with the increase of effective pressure, the reduction of porosity goes through three stages of rapid-slow-rapid changes in sequence, and the reduction of permeability goes through three stages of rapid–slow–basically stable changes in sequence.Previous studies have shown that the permeability of laminated shales and bedded shales is generally higher than that of massive shales [36, 37]. The shale samples in this study exhibit the same characteristics: the permeability (mD) of the laminated samples ranges from 0.0156 to 5.1990, and that of the bedded samples ranges from 0.0026 to 110.7375, which is much higher than that of the massive sample (0.0014 mD). This may be related to the differential development of fractures in different lithofacies. As mentioned in section 3.3, the interface of laminae in laminated shales and bedded shales is prone to the development of interlayer fractures, while only a small number of small-scale irregular fractures are present in massive shales. Fractures can greatly improve the permeability of shale reservoirs [38], thus the permeability of laminated shales and bedded shales is generally higher than that of massive shales.The permeability of laminated shales and bedded shales remains higher than that of massive shales under effective pressure, while the permeability loss of massive shale isgreater, especially under lower effective pressure (online Supplementary Material Tables S3 and S4). When the effective pressure is 2.41 MPa, the permeability of the massive shale sample decreases by 85.17%, compared to a decrease of 2.79%–4.48% for the laminated samples and 1.42% ~ 7.50% for the bedded samples. As the effective pressure increases, the difference in permeability loss rate gradually decreases, and the permeability loss rate of laminated shales and bedded shales is always lower than that of massive shales. When the effective pressure reaches 4.48 MPa, the permeability of the massive shale decreases by 91.31%, compared to a decrease of 52.75% ~ 69.92% for the laminated samples and 32.82%–73.48% for the bedded samples. When the effective pressure reaches 20 MPa, the permeability loss of shale samples is more than 96%.The reduction of permeability can be attributed to the following factors: (1) compression and closure of fractures, (2) the reduction of pores’ size and closure of matrix pores, and (3) loss of connectivity between pores [15, 16, 39]. For laminated shales and bedded shales containing a large number of interlayer fractures, the reduction of permeability is mainly attributed to the closure of fractures, followed by the compression and closure of matrix pores. For massive shales dominated by matrix pores, the reduction of permeability is mainly related to matrix pores (Figure 7). Massive shales have small pore size and poor connectivity, so the reduction of pores’ size and closure of pores lead to higher permeability reduction at lower pressure. The fractures in laminated shales and bedded shales are also compressed but not completely closed, so the permeability reduction is relatively low. In addition, the relatively low permeability reduction of laminated shales and bedded shales may also be related to uneven compression. The distribution of pores is relatively uniform in massive shales, while it varies significantly due to mineral composition in laminated shales and bedded shale. Argillaceous laminae are rich in clay minerals which are plastic, and have strong compressibility. However, the compressibility of slit-shaped and wedge-shaped intergranular pores in calcite laminae is weaker than pores in argillaceous laminae due to the support of rigid calcite particles. This is also the main reason why the porosity of laminated samples and bedded samples is generally higher than that of massive samples in this study.As mentioned in 3.3, mineral composition affects the type and development of pores [40, 41]. In addition, researches have shown that mineral composition also affects the compressive capacity of rocks to some extent [1, 42]. Felsic minerals have high hardness and strong anticompression ability, clay minerals have strong plasticity and are easy to be compressed under pressure, while carbonate minerals are in between [1, 42]. Therefore, the correlation coefficient method was adopted to analyze the contribution of individual mineral components to the porosity-permeability reduction at effective pressure in this study.As shown in Figure 8, there is a positive correlation between carbonate minerals content and porosity-permeability reduction rate. This positive correlation indicates that carbonate-related pores have compressibility to some extent. The correlation coefficient (R2) between carbonate minerals content and average porosity reduction rate is only 0.07, which is basically not correlated, while the R2 between carbonate minerals content and stage porosity reduction rate is 0.58, 0.35, and 0.29, respectively, indicating a good correlation (Figure 8). Permeability also shows similar characteristics (Figure 8). In other words, the correlation between carbonate minerals content and stage permeability reduction rates (R2=0.69, 0.88, 0.82) is higher than that between carbonate minerals content and average permeability reduction rate (R2=0.62). This indicates that the influence of carbonate minerals on porosity-permeability reduction rate has obvious stage characteristics under effective pressure.In addition, the correlation between carbonate minerals and permeability reduction rate is significantly better than that of porosity. This implies that the contribution of carbonate minerals to the permeability reduction rate is greater than that to porosity reduction rate. This may be related to the pore structure of shales. In terms of pore structure, porosity is related to the total volume of all pores, while permeability depends on the connectivity between pores. As mentioned in 3.3, the shales develop carbonate-related pores, clay-related pores, and felsic-related pores in this study. Intergranular pores between carbonate minerals and felsic minerals have good connectivity, while clay-related pores have poor connectivity. Therefore, carbonate minerals have a greater influence on permeability, which is consistent with Su et al [43].There is a weak negative correlation between clay minerals and porosity-permeability reduction rate (Figure 9). Obviously, this is inconsistent with the conventional understanding of the compressibility of clay minerals [42, 44]. Further analysis shows that two samples (F201-2 and FYP1-6) with low clay minerals content (<10%) interfere with the changing characteristics of the whole curve. After removing the two samples with low clay minerals content, there is a positive correlation between the clay minerals content and porosity-permeability reduction rate, which shows that the porosity-permeability reduction rate increases with the increase of clay minerals content (Figure 9). Consistent with carbonate minerals, the correlation between the content of clay minerals and stage porosity-permeability reduction rate is better than the average porosity-permeability reduction rate.In contrast to carbonate minerals, the correlation between clay minerals content and permeability reduction rate is lower than its correlation with porosity reduction rate. This means that the contribution of clay minerals content to porosity reduction rate is higher than its contribution to permeability reduction rate. This is consistent with the pore development characteristics of shales. In this study, the shale samples are widely developed with clay mineral-related pores, which are numerous but poorly connected to each other (Figure 7).Under effective pressure conditions, there is a negative correlation between felsic minerals content and porosity-permeability reduction rate to a certain extent (Figure 10), which is consistent with the conventional understanding of the strong anticompression ability of felsic minerals. As shown in Figure 10, the correlation between felsic minerals content and stage porosity-permeability reduction rate is significantly better than that between felsic minerals content and average porosity-permeability reduction rate, which is similar to carbonate minerals and clay minerals. In addition, the contribution of felsic minerals content to the stage permeability reduction rate (R2>0.87) is higher than to the stage porosity reduction rate (R2=0.34), which is consistent with carbonate minerals. As described in 3.2 and 3.3, felsic minerals are low in content and dispersed in carbonate minerals and clay minerals. As a result, felsic minerals-related pores have a low contribution to the total porosity. Compared with other minerals, felsic minerals have a stronger anticompression ability at effective pressure, thus greatly protecting the connectivity of the pore-fracture system. Therefore, the permeability reduction rate shows a trend of increasing with the increase of felsic minerals content.In summary, carbonate minerals, clay minerals, and felsic minerals all affect the reduction of porosity and permeability at effective pressure to varying degrees. Carbonate minerals content and clay minerals content are positively correlated with porosity-permeability reduction rate, while felsic minerals content is negatively correlated with porosity-permeability reduction rate. Consistent with the phased changes in porosity-permeability reduction rate, this correlation is also presented with phased characteristics (Figures 8 and 9, Figure 10). In other words, the contribution of minerals to the porosity-permeability reduction rate is different in different pressure stages. Overall, porosity reduction rate is mainly related to clay minerals content (R2=0.74, 0.51, 0.52), followed by carbonate minerals content (R2=0.58, 0.35, 0.29); permeability reduction rate is mainly affected by carbonate minerals content (R2=0.69, 0.88, 0.82), followed by clay minerals content (R2=0.27, 0.46, 0.30).Previous studies have shown that the TOC content (%) has significant effects on the development of pores in organic-rich shales [32, 45-47]. Compared to minerals, OM has stronger ductility and is easier to compress [50]. In this study, there is no significant correlation between TOC content and porosity-permeability reduction rate (Figure 11). This may be attributed to the lower maturity of the shale samples.A large number of OM pores are developed in medium-high maturity shales, and thus the higher the TOC content, the more developed the OM pores and the larger the total pore volume [47]. In contrast, the maturity of the shale samples in this study is relatively low, with vitrinite reflectance ranging from 0.82% to 1.12%. The lower maturity corresponds to undeveloped OM pores. As described in 3.3, the pore types of shales in the study area are dominated by fractures and inorganic mineral pores, and OM pores are not developed, thus making the effect of TOC content on porosity permeability under effective pressure insignificant.Under the effective pressure influence, the porosity and permeability of shale decrease with the increase of effective pressure, and the reduction rate is characterized by an obvious stage change. According to the stage changes of porosity-permeability reduction rate, the pressurization process can be divided into three pressure stages: low pressure (<8 MPa), medium pressure (8–15 MPa) and high pressure (>15 MPa). Porosity reduction rate is highest in the low-pressure stage and lowest in the medium-pressure stage, with a difference of 2–15 times. Permeability reduction rate is highest at low-pressure stage and lowest at high-pressure stage, with a difference of 2 orders of magnitude.Under the effective pressure influence, the rock fabric affects porosity and permeability by influencing mineral distribution, pore development, and differential compaction. The laminae and interlayer fractures are poorly developed, and the distribution of minerals and pores is relatively homogeneous in massive shales. The presence of laminae is accompanied by well-developed interlayer fractures and the differential distribution of minerals and pores in laminated shale and layered shale. the minerals are dominated by calcite in calcite laminae where the pores mainly occur between calcite crystals and at the edges of calcite crystals, with better connectivity; the minerals are dominated by clay minerals in argillaceous laminae where the pores are mainly in clay minerals, with poorer connectivity. The compressibility of clay minerals is stronger than calcite. As a result, the clay mineral-related pores in the argillaceous laminae are more likely to be damaged and lose connectivity under effective pressure conditions, while the calcite-related pores in calcite laminae are less compressed and retain a certain permeability. In addition, interlayer fractures have not been completely closed at lower pressure, thus still able to maintain a certain connectivity. Therefore, compared with massive shale, laminated shale, and bedded shale generally have lower permeability loss and relatively higher permeability under effective pressure.During pressurization process, the porosity and permeability reduction rates are negatively correlated with felsic minerals content and positively correlated with carbonate minerals content and clay minerals content. Porosity reduction rate is mainly related to clay minerals content (R2=0.74, 0.51, 0.52), followed by carbonate minerals content (R2=0.58, 0.35, 0.29). Permeability reduction rate is primarily related to carbonate minerals content (R2=0.69, 0.88, 0.82), followed by clay minerals content (R2=0.27, 0.46, 0.30). The TOC content has no significant effect on the porosity and permeability of the shale in the study area due to lower maturity.All relevant data are within the manuscript and supplementary materials.The authors declared that they have no conflicts of interest to this work.This study was financially supported by the National Natural Science Foundation of China (Grant No.42072152).Table S1: Measured porosity of shale samples at different pressures.Table S2: Measured permeability of shale samples at different pressures.Table S3: Porosity loss rate calculated based on the values of measured porosity and pressure.Table S4: Permeability loss rate calculated based on the values of measured permeability and pressure.Table S5: Porosity-permeability reduction rate at different pressure stages based on the results of segmentation fitting.\",\"PeriodicalId\":18147,\"journal\":{\"name\":\"Lithosphere\",\"volume\":\"23 1\",\"pages\":\"\"},\"PeriodicalIF\":1.8000,\"publicationDate\":\"2024-04-30\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Lithosphere\",\"FirstCategoryId\":\"89\",\"ListUrlMain\":\"https://doi.org/10.2113/2024/lithosphere_2023_338\",\"RegionNum\":4,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q3\",\"JCRName\":\"GEOCHEMISTRY & GEOPHYSICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Lithosphere","FirstCategoryId":"89","ListUrlMain":"https://doi.org/10.2113/2024/lithosphere_2023_338","RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
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摘要

页岩油藏在覆盖层压力下的物性对于勘探和开发过程中的储层预测和评价具有重要意义。本研究基于岩心、薄片和扫描电镜观察,以及XRD、TOC、压力条件下孔隙度和渗透率等测试数据,系统分析了济阳凹陷不同岩性页岩的物性变化,以及岩石结构对压力作用下物性变化的影响。在压力作用下,页岩样品的孔隙度和渗透率明显降低。根据孔隙度和渗透率的阶段性降低,加压过程分为三个压力阶段:低压(15 兆帕)。孔隙度在低压阶段降低最快,在中压阶段降低最慢。渗透率的降低在低压阶段最快,在高压阶段最慢。在压力条件下,岩石结构对孔隙度和渗透率有很大影响。在压力作用下,层状页岩和层状页岩的渗透率高于块状页岩,而渗透率损失率低于块状页岩。特别是在低压条件下,两者相差可达 10-20 倍。此外,压力下孔隙度和渗透率的降低率与长石矿物含量呈负相关,与碳酸盐矿物含量和粘土矿物含量呈正相关。粘土矿物对孔隙度降低率的贡献占主导地位,碳酸盐矿物次之。碳酸盐矿物对渗透率降低率的贡献最大,其次是粘土矿物。随着全球能源结构的变化,页岩油气已成为我国油气资源的核心增长点[1-4]。近十年来,我国页岩油气勘探开发取得了一系列重要进展,包括准噶尔盆地、鄂尔多斯盆地、江汉盆地、松辽盆地、渤海湾盆地等页岩油勘探取得突破[5-8]。然而,由于页岩的异质性和中国地质条件的复杂性,页岩油藏的预测和评价仍面临许多挑战[3, 9, 10]。许多研究表明,层状构造和矿物组成等岩石结构对页岩油藏的孔隙发育和物性有重要影响[10-14]。然而,这些研究大多是在无压条件下进行的,与地层条件存在一定误差,影响了页岩油甜点的预测和评价。为了恢复地层条件下的真实物理参数,一些学者进行了加压实验,发现页岩的孔隙度和渗透率都随着有效压力的增加而逐渐降低,并提出了指数函数、二项式等函数模型来描述加压过程中孔隙度和渗透率随压力的变化[15-22]。然而,对于单个页岩样本而言,在相同压力条件下,不同样本之间的孔隙度和渗透率差异较大[17-19],造成这些差异的原因尚不清楚,有待进一步探讨。本研究选取了济阳凹陷的 Es3x-Es4s 页岩。通过岩石学、矿物学、有效压力下的孔隙度和渗透率实验以及 FE-SEM 观察,探讨了有效压力下页岩储层的物理特征以及岩石结构对孔隙度和渗透率的影响,为预测和评价济阳凹陷古新统页岩油藏提供依据。济阳凹陷位于中国渤海湾盆地东南部,由南向北依次为东营、惠民、沾化和车镇四个子凹陷(图 1(a))[23]。在近十年的勘探中,东营、沾化等子凹陷多层次、多类型页岩油取得重大突破,显示出济阳凹陷页岩油良好的勘探前景[24-26]。目前,Es4s、Es3x 和 Es1 是济阳凹陷页岩油的主要勘探区间(图 1(b)),其特点是碳酸盐岩含量高,平均含量超过 50%[23、27、28]。作为济阳凹陷页岩油的主要勘探区带,Es4-Es3x有许多工业油流井和采样井。 本研究的样品采集自 BYP5 井、F201 井和 FYP1 井(图 1(a)),基本信息见表 1。首先,从岩心样品中钻出一个与基底平行的塞状样品(直径 25 毫米,长 60 毫米)。然后,从塞子的一侧切割出一个 2 厘米高的微型塞子,用于薄片和扫描电子显微镜观察。剩余的塞子样品用于测量有效压力下的孔隙度和渗透率。在完成孔隙度和渗透率测量后,将塞子样品粉碎,用于总有机碳和矿物成分分析。使用蔡司显微镜观察厚度为 30 μm 的岩石薄片,以明确层理的发育特征和矿物分布。使用高分辨率场发射扫描电子显微镜(蔡司 Sigma 500)观察孔隙的形态、大小和分布。使用配备 J&M MSP200 显微光度计的蔡司显微镜对样品表面(10 × 10 μm)进行手工研磨和氩离子抛光(Leica EM 3X),以获得更好的图像质量。总有机碳 (TOC) 含量是用 Vario MICRO 立方体元素分析仪测定的。测试前,将页岩样本研磨至小于 200 目,并与稀 HCL 反应 72 小时以去除碳酸盐矿物。X 射线衍射(XRD)分析是使用 Dmax IIIa 衍射仪和铜 X 射线管(40 千伏,30 毫安)进行的。粘土矿物的分离和 XRD 测试按照中国石油行业标准(SY/T 5163-2018)进行。上述实验在南京大学表层地球化学教育部重点实验室进行,有效压力条件下的孔隙度和渗透率采用 PoroPerm-200 全自动孔隙度-渗透率仪进行测量。超压条件的实现主要依靠岩心保持系统,该系统以气体为加压介质,模拟水力压裂加压,从而实现超压条件下孔隙渗透率的在线测试。实验过程中,加压气源采用高压空气,测试采用高纯度氦气。为避免滑移效应对页岩样本渗透率的影响,在加压过程中保持驱动压力恒定。实验中设定的有效压力点在加压过程中保持不变。实验中设定的有效压力点分别为 2.41、4.48、6.55、8.62、10.69、12.07、20、30 和 50 兆帕。在每个压力点保持测量 30 分钟,稳定后测量相应压力下的孔隙度和渗透率。过压实验前,页岩样品需要用二氯甲烷和甲醇(93:7)萃取(72 小时),以去除样品中的残余油。根据层理厚度,结合岩芯和薄片观察,将页岩样品划分为层理页岩(2 厘米)。除块状页岩外,层状页岩和层状页岩的层理均较发育,表现为浅色层理和深色层理交替出现(图 3)。浅色层理的主要矿物为方解石,泥晶方解石和疏松方解石之间漂浮着少量石英、长石和白云石;深色层理的矿物成分较为复杂,以粘土矿物为主,还含有一些方解石、白云石、石英、长石和有机质(OM)。层状矿物成分的不同导致层状页岩和层状页岩中矿物分布的强烈异质性。表 2 列出了页岩样本的平均玻璃光泽反射率(Ro%)、总有机碳含量和矿物成分。本研究中的页岩样本热成熟度较低(0.78% Ro 至 1.12% Ro),总有机碳含量适中(2.85% 至 6.26 wt%,平均 3.51 wt%)。矿物包括方解石(7-69 wt%,平均 37 wt%)、粘土矿物(4-30 wt%,平均 22%)、石英(10-34 wt%,平均 19 wt%)、长石(4-22 wt%,平均 11 wt%)、白云石(3-22 wt%,平均 10 wt%)和少量黄铁矿(1-3 wt%,平均 15 MPa)。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Influence of Rock Fabric on Physical Properties of Shale Oil Reservoir Under Effective Pressure Conditions
The physical properties of shale oil reservoirs under overburden pressure are of great significance for reservoir prediction and evaluation during exploration and development. Based on core, thin section, and SEM observations, as well as test data such as XRD, TOC, and porosity and permeability under pressure conditions, this study systematically analyzes the variation of physical properties of different lithofacies shales in the Jiyang depression and the influence of rock fabric on the physical variation under pressure. The porosity and permeability of shale samples significantly decrease under pressure. According to the phased reduction in porosity and permeability, the pressurization process is divided into three pressure stages: low pressure (<8 MPa), medium pressure (8–15 MPa), and high pressure (>15 MPa). The reduction of porosity is fastest in the low-pressure stage and slowest in the medium-pressure stage. The reduction of permeability is fastest in the low-pressure stage and the slowest in the high-pressure stage. The rock fabric has a significant impact on porosity and permeability under pressure conditions. The permeability of laminated shale and bedded shale is higher than that of massive shale under pressure, and the permeability loss rate is lower than that of massive shales. Especially under lower pressure, the difference can be 10–20 times. In addition, the reduction rate of porosity and permeability under pressure is negatively correlated with felsic minerals content, which is positively correlated with carbonate minerals content and clay minerals content. The contribution of clay minerals to the porosity reduction rate is dominant, followed by carbonate minerals. The contribution of carbonate minerals to the permeability reduction rate is dominant, followed by clay minerals. The TOC content has no significant impact on the porosity and permeability of shales under pressure in the study due to the low maturity.With the change in global energy structure, shale oil and gas has become the core growth point of China’s oil and gas resources [1-4]. In the past decade, a series of important progresses have been made in the exploration and development of shale oil and gas in China, including breakthroughs in the exploration of shale oil in Junggar Basin, Ordos Basin, Jianghan Basin, Songliao Basin, and Bohai Bay Basin [5-8]. However, due to the heterogeneity of shales and the complexity of geological conditions in China, the prediction and evaluation of shale oil reservoirs still face many challenges [3, 9, 10].Many studies have shown that rock fabric, such as laminated structure and mineral composition, has a significant influence on the pore development and physical properties of shale oil reservoirs [10-14]. However, most of these studies were conducted under unpressurized conditions, and there are some errors with the formation conditions, which affect the prediction and evaluation of shale oil desserts. To recover the real physical parameters under formation conditions, some scholars carried out pressure experiments and found that both porosity and permeability of shale decreased gradually with the increase of effective pressure, and proposed exponential function, binomial, and other functional models to describe the change of porosity and permeability with pressure during pressurization [15-22]. However, for individual shale samples, the differences in porosity and permeability between different samples under the same pressure conditions are large [17-19], and the reasons for these differences are still unclear and need to be further explored. In this study, the Es3x-Es4s shales from the Jiyang depression were selected. Through petrology, mineralogy, porosity and permeability experiments under effective pressure, and FE-SEM observation, the physical characteristics of the shale reservoirs under effective pressure and the influence of rock fabric on porosity and permeability were discussed to provide a basis for the prediction and evaluation of the shale oil desserts in the Paleocene shale of the Jiyang depression.The Jiyang depression is located in the southeast of the Bohai Bay Basin, China, with four sub-depressions from south to north, Dongying, Huimin, Zhanhua, and Chezhen (Figure 1(a)) [23]. It has made great breakthroughs in multiple layers and types of shale oil in Dongying, Zhanhua, and other sags during the past ten years of exploration, showing a good exploration prospect of shale oil in Jiyang depression [24-26]. Currently, Es4s, Es3x, and Es1 are the main exploration intervals for shale oil in Jiyang depression (Figure 1(b)), characterized by high carbonate content, with an average content of more than 50% [23, 27, 28]. In addition, the laminae are well developed with alternating light laminae and dark laminae [23, 29, 30].As the main exploration intervals for shale oil in Jiyang depression, there are many industrial oil flow wells and sampling wells in Es4s-Es3x. The samples in this study were collected from Well BYP5, F201, and FYP1 (Figure 1(a)), and the basic information is shown in Table 1.Before experiments, the samples were treated in different ways as illustrated in Figure 2. First, a plug sample (diameter of 25 mm and length of 60 mm) was drilled parallel to the bedding from the core sample. Then, a 2 cm high micro plug was cut from one side of the plug, which was used for thin section and scanning electron microscopy observations. The remaining plug sample was used for the measurement of porosity and permeability under effective pressure. After completing the measurement of porosity and permeability, the plug sample was crushed for TOC and mineral composition analysis.A Zeiss microscope was used to observe the thin sections of rocks with a thickness of 30 μm to clarify the development characteristics and mineral distribution of the laminae. A high-resolution field emission scanning electron microscope (Zeiss Sigma 500) was used to observe the morphology, size, and distribution of pores. The surface of the samples (10 × 10 μm) was polished by hand grinding and argon ion polishing (Leica EM 3X) to achieve better image quality.Vitrinite reflectance (Ro%) measurements were undertaken using a Zeiss microscope equipped with a J&M MSP200 microphotometer. The total organic carbon (TOC) contents were determined using a Vario MICRO cube elemental analyzer. Before testing, the shale samples were ground to less than 200 mesh and reacted with dilute HCL for 72 hours to remove carbonate minerals. The X-ray diffraction (XRD) analysis was conducted using a Dmax IIIa diffractometer with a Cu x-ray tube (40 kV, 30 mA). The separation and XRD tests of clay minerals were carried out according to the Chinese petroleum industry standard (SY/T 5163–2018). The above experiments were carried out in the Key Laboratory of Surficial Geochemistry (KLSG), Ministry of Education, Nanjing University.The porosity and permeability under effective pressure conditions were measured by PoroPerm-200 automatic porosity-permeability instrument. The realization of overpressure conditions mainly relies on a core holding system, which uses gas as the pressurized medium to mimic the pressurization of hydraulic fracturing, thus realizing the online testing of porosity permeability under overpressure conditions. During the experiment, high-pressure air was used for the pressurized gas source and high-purity helium was used for the test. To avoid the influence of slip effect on the permeability of shale samples, the driving pressure was kept constant during the pressurization process. The effective pressure points set in this experiment were kept constant during the pressurization process. The effective pressure points set in this experiment were 2.41, 4.48, 6.55, 8.62, 10.69, 12.07, 20, 30, and 50 MPa, respectively. The measurements were kept at each pressure point for 30 minutes, and then the porosity and permeability under the corresponding pressures were measured after stabilization. Before the overpressure experiments, the shale samples need to be extracted (72 hours) with dichloromethane and methanol (93:7) to remove the residual oil in the samples.The shale is characterized by laminations in the Jiyang depression [29-31]. According to the thickness of the lamination, the shale samples were divided into laminated shale (<2 mm), bedded shale (2 mm to 2 cm), and massive shale (>2 cm) by combing core and thin section observations. Except for massive shales, the laminations of laminated shale and bedded shale are well-developed, showing as alternating light lamination and dark lamination (Figure 3).In addition, there is a significant difference in the mineral composition between light lamination and dark lamination. The main mineral of the light lamination is calcite, with small amounts of quartz, feldspar, and dolomite floating between mud crystalline calcite and sparry calcite; the mineral composition of dark lamination is more complex, and is dominated by clay minerals, also contains some calcite, dolomite, quartz, feldspar, and organic matter (OM). The difference in the mineral composition of lamination results in strong heterogeneity in the distribution of minerals in laminated shales and bedded shales. In contrast, massive shales lack laminae and thus have a relatively homogeneous distribution of minerals.The mean vitrinite reflectance (Ro%), TOC content, and mineral composition of shale samples are presented in Table 2. The shale samples in this study have lower thermal maturity (0.78% Ro to 1.12% Ro) and moderate TOC content (2.85 to 6.26 wt%, avg. 3.51 wt%). The minerals include calcite (7 to 69 wt%, avg. 37 wt%), clay minerals (4 to 30 wt%, avg. 22%), quartz (10 to 34 wt%, avg. 19 wt%), feldspar (4 to 22 wt%, avg. 11 wt%), dolomite (3 to 22 wt%, avg. 10 wt%) and a small amount of pyrite (1 to 3 wt%, average <2 wt%). Among clay minerals, illite/smectite mixed layer (20 to 66 wt%, avg. 51 wt%) is dominated, followed by illite (25 to 72 wt%, avg. 46 wt%), with a small amount of kaolinite and chlorite.Based on scanning electron microscopy observations and previous studies [32, 33], four types of pores are found in the Jiyang depression: interparticle (InterP) pores, intraparticle (IntraP) pores, fracture pores, and a few OM pores.Fracture pores, a major pore type in the Es3x-Es4s shales, mainly include interlayer fractures, tectonic fractures, and irregular fractures. The interlayer fractures are mostly developed at the interface of the laminae in laminated shales and bedded shales. The main body of the interlayer fractures is nearly horizontal and relatively stable in extension (Figure 4(a–c)). Tectonic fractures usually form an angle with the direction of the laminae and span multiple layers. Irregular fractures are mostly related to shrinkage by hydrocarbon generation and the dehydration of clay minerals [34], and usually extend around rigid particles (Figure 4(d)), with poor continuity.IntraP pores occur inside mineral particles/crystals and are controlled by mineral types, including the pores in clay aggregates (Figure 4(e–f)), dissolution pores in calcite and dolomite (Figure 4(e–f)), pores in feldspar cleavages, and pores in pyrite aggregates (Figure 4(g)). IntraP pores are mostly isolated and have poor connectivity. InterP pores occur between mineral particles/crystals or at the edges of mineral particles/crystals and mainly include the pores between calcite granules (Figure 4(i–j)), between clay minerals (Figure 4(h)), and between calcite and dolomite, quartz, feldspar (Figure 4(k)) in the research area. Compared with IntraP pores, the InterP pores generally have good connectivity. Most of the InterP pores are wedge-shaped (5–10 μm) or slit-shaped in calcite laminae of laminated shales and bedded shales (Figure 4(i–j)), which are related to calcite recrystallization and have reformed by late dissolution [35]. The shape of InterP pores is irregular in massive shales and argillaceous laminae of laminated shales and bedded shales, with diameters ranging from 500 nm to 2 μm. OM pores are not developed in this study, and only a small amount of elongated slit-shaped OM pores were observed inside or at the edges of the OM (Figure 4(l)).The porosity and permeability of shale samples at different effective pressures are shown in online Supplementary Material Tables S1 and S2. It can be seen that the porosity and permeability of shale samples under effective pressure are significantly lower than the initial porosity and initial permeability. During the pressurization process, the porosity and permeability of the samples decreased with the increase of the effective pressure, and the corresponding porosity loss and permeability loss continued to increase (Figure 5). When the effective pressure reaches the maximum value (50 MPa) of this experiment, the porosity and permeability of the samples are the lowest, and the corresponding loss rate also reaches the maximum value. At this time, the porosity loss rate of shale samples was 9.48%–54.32%, and the permeability loss rate was 95.18% to 99.5%. Compared with porosity, the permeability loss of shale samples is generally greater.From the viewpoint of the whole pressurization process, the decrease in porosity and permeability shows an obvious stage with increasing effective pressure. This phasing is specifically manifested in the permeability as follows: at lower pressure, the permeability decreases rapidly, and the permeability loss rate increases rapidly; at medium pressure, the decrease in permeability slows down, and the increase in the permeability loss rate slows down; and at high pressure, the decrease in permeability is very slow, and the increase in the permeability loss rate is also very slow. Thus, based on the differential decrease in porosity and permeability during pressurization process, the pressurization process can be divided into three stages: low-pressure stage (<8 MPa), medium-pressure stage (8–15 MPa), and high-pressure stage (>15 MPa).Within each stage, the decrease in porosity and permeability with effective pressure can be fitted with a linear function (Figure 6), which exhibits a higher fitting degree than overall fitting. The absolute values of the slopes of the porosity-effective pressure and permeability-effective pressure fitting curves are defined as the porosity reduction rate and permeability reduction rate under effective pressure, respectively. According to the fitting results (Figure 6), the porosity reduction rate (%/MPa) of shale samples is 0.0093–0.2620 at the low-pressure stage, which is 0.0027–0.0562 at the medium-pressure stage, and 0.0045–0.0564 at the high-pressure stage. In other words, the porosity reduction rate is the highest at the low-pressure stage, and lowest at the medium-pressure stage, with a difference ranging 2–15 times. The reduction of permeability with effective pressure exhibits similar phased characteristics (Figure 6). The permeability reduction rate (mD/MPa) of shale samples is 0.0002–16.1060 at the low-pressure stage, which is 0.000003–1.9931 at the medium-pressure stage, and 0.0000004–0.0912 at the high-pressure stage. Thus, the permeability reduction rate is the highest in the low-pressure stage, and lowest in the high-pressure stage, with a difference of two orders of magnitude. In conclusion, with the increase of effective pressure, the reduction of porosity goes through three stages of rapid-slow-rapid changes in sequence, and the reduction of permeability goes through three stages of rapid–slow–basically stable changes in sequence.Previous studies have shown that the permeability of laminated shales and bedded shales is generally higher than that of massive shales [36, 37]. The shale samples in this study exhibit the same characteristics: the permeability (mD) of the laminated samples ranges from 0.0156 to 5.1990, and that of the bedded samples ranges from 0.0026 to 110.7375, which is much higher than that of the massive sample (0.0014 mD). This may be related to the differential development of fractures in different lithofacies. As mentioned in section 3.3, the interface of laminae in laminated shales and bedded shales is prone to the development of interlayer fractures, while only a small number of small-scale irregular fractures are present in massive shales. Fractures can greatly improve the permeability of shale reservoirs [38], thus the permeability of laminated shales and bedded shales is generally higher than that of massive shales.The permeability of laminated shales and bedded shales remains higher than that of massive shales under effective pressure, while the permeability loss of massive shale isgreater, especially under lower effective pressure (online Supplementary Material Tables S3 and S4). When the effective pressure is 2.41 MPa, the permeability of the massive shale sample decreases by 85.17%, compared to a decrease of 2.79%–4.48% for the laminated samples and 1.42% ~ 7.50% for the bedded samples. As the effective pressure increases, the difference in permeability loss rate gradually decreases, and the permeability loss rate of laminated shales and bedded shales is always lower than that of massive shales. When the effective pressure reaches 4.48 MPa, the permeability of the massive shale decreases by 91.31%, compared to a decrease of 52.75% ~ 69.92% for the laminated samples and 32.82%–73.48% for the bedded samples. When the effective pressure reaches 20 MPa, the permeability loss of shale samples is more than 96%.The reduction of permeability can be attributed to the following factors: (1) compression and closure of fractures, (2) the reduction of pores’ size and closure of matrix pores, and (3) loss of connectivity between pores [15, 16, 39]. For laminated shales and bedded shales containing a large number of interlayer fractures, the reduction of permeability is mainly attributed to the closure of fractures, followed by the compression and closure of matrix pores. For massive shales dominated by matrix pores, the reduction of permeability is mainly related to matrix pores (Figure 7). Massive shales have small pore size and poor connectivity, so the reduction of pores’ size and closure of pores lead to higher permeability reduction at lower pressure. The fractures in laminated shales and bedded shales are also compressed but not completely closed, so the permeability reduction is relatively low. In addition, the relatively low permeability reduction of laminated shales and bedded shales may also be related to uneven compression. The distribution of pores is relatively uniform in massive shales, while it varies significantly due to mineral composition in laminated shales and bedded shale. Argillaceous laminae are rich in clay minerals which are plastic, and have strong compressibility. However, the compressibility of slit-shaped and wedge-shaped intergranular pores in calcite laminae is weaker than pores in argillaceous laminae due to the support of rigid calcite particles. This is also the main reason why the porosity of laminated samples and bedded samples is generally higher than that of massive samples in this study.As mentioned in 3.3, mineral composition affects the type and development of pores [40, 41]. In addition, researches have shown that mineral composition also affects the compressive capacity of rocks to some extent [1, 42]. Felsic minerals have high hardness and strong anticompression ability, clay minerals have strong plasticity and are easy to be compressed under pressure, while carbonate minerals are in between [1, 42]. Therefore, the correlation coefficient method was adopted to analyze the contribution of individual mineral components to the porosity-permeability reduction at effective pressure in this study.As shown in Figure 8, there is a positive correlation between carbonate minerals content and porosity-permeability reduction rate. This positive correlation indicates that carbonate-related pores have compressibility to some extent. The correlation coefficient (R2) between carbonate minerals content and average porosity reduction rate is only 0.07, which is basically not correlated, while the R2 between carbonate minerals content and stage porosity reduction rate is 0.58, 0.35, and 0.29, respectively, indicating a good correlation (Figure 8). Permeability also shows similar characteristics (Figure 8). In other words, the correlation between carbonate minerals content and stage permeability reduction rates (R2=0.69, 0.88, 0.82) is higher than that between carbonate minerals content and average permeability reduction rate (R2=0.62). This indicates that the influence of carbonate minerals on porosity-permeability reduction rate has obvious stage characteristics under effective pressure.In addition, the correlation between carbonate minerals and permeability reduction rate is significantly better than that of porosity. This implies that the contribution of carbonate minerals to the permeability reduction rate is greater than that to porosity reduction rate. This may be related to the pore structure of shales. In terms of pore structure, porosity is related to the total volume of all pores, while permeability depends on the connectivity between pores. As mentioned in 3.3, the shales develop carbonate-related pores, clay-related pores, and felsic-related pores in this study. Intergranular pores between carbonate minerals and felsic minerals have good connectivity, while clay-related pores have poor connectivity. Therefore, carbonate minerals have a greater influence on permeability, which is consistent with Su et al [43].There is a weak negative correlation between clay minerals and porosity-permeability reduction rate (Figure 9). Obviously, this is inconsistent with the conventional understanding of the compressibility of clay minerals [42, 44]. Further analysis shows that two samples (F201-2 and FYP1-6) with low clay minerals content (<10%) interfere with the changing characteristics of the whole curve. After removing the two samples with low clay minerals content, there is a positive correlation between the clay minerals content and porosity-permeability reduction rate, which shows that the porosity-permeability reduction rate increases with the increase of clay minerals content (Figure 9). Consistent with carbonate minerals, the correlation between the content of clay minerals and stage porosity-permeability reduction rate is better than the average porosity-permeability reduction rate.In contrast to carbonate minerals, the correlation between clay minerals content and permeability reduction rate is lower than its correlation with porosity reduction rate. This means that the contribution of clay minerals content to porosity reduction rate is higher than its contribution to permeability reduction rate. This is consistent with the pore development characteristics of shales. In this study, the shale samples are widely developed with clay mineral-related pores, which are numerous but poorly connected to each other (Figure 7).Under effective pressure conditions, there is a negative correlation between felsic minerals content and porosity-permeability reduction rate to a certain extent (Figure 10), which is consistent with the conventional understanding of the strong anticompression ability of felsic minerals. As shown in Figure 10, the correlation between felsic minerals content and stage porosity-permeability reduction rate is significantly better than that between felsic minerals content and average porosity-permeability reduction rate, which is similar to carbonate minerals and clay minerals. In addition, the contribution of felsic minerals content to the stage permeability reduction rate (R2>0.87) is higher than to the stage porosity reduction rate (R2=0.34), which is consistent with carbonate minerals. As described in 3.2 and 3.3, felsic minerals are low in content and dispersed in carbonate minerals and clay minerals. As a result, felsic minerals-related pores have a low contribution to the total porosity. Compared with other minerals, felsic minerals have a stronger anticompression ability at effective pressure, thus greatly protecting the connectivity of the pore-fracture system. Therefore, the permeability reduction rate shows a trend of increasing with the increase of felsic minerals content.In summary, carbonate minerals, clay minerals, and felsic minerals all affect the reduction of porosity and permeability at effective pressure to varying degrees. Carbonate minerals content and clay minerals content are positively correlated with porosity-permeability reduction rate, while felsic minerals content is negatively correlated with porosity-permeability reduction rate. Consistent with the phased changes in porosity-permeability reduction rate, this correlation is also presented with phased characteristics (Figures 8 and 9, Figure 10). In other words, the contribution of minerals to the porosity-permeability reduction rate is different in different pressure stages. Overall, porosity reduction rate is mainly related to clay minerals content (R2=0.74, 0.51, 0.52), followed by carbonate minerals content (R2=0.58, 0.35, 0.29); permeability reduction rate is mainly affected by carbonate minerals content (R2=0.69, 0.88, 0.82), followed by clay minerals content (R2=0.27, 0.46, 0.30).Previous studies have shown that the TOC content (%) has significant effects on the development of pores in organic-rich shales [32, 45-47]. Compared to minerals, OM has stronger ductility and is easier to compress [50]. In this study, there is no significant correlation between TOC content and porosity-permeability reduction rate (Figure 11). This may be attributed to the lower maturity of the shale samples.A large number of OM pores are developed in medium-high maturity shales, and thus the higher the TOC content, the more developed the OM pores and the larger the total pore volume [47]. In contrast, the maturity of the shale samples in this study is relatively low, with vitrinite reflectance ranging from 0.82% to 1.12%. The lower maturity corresponds to undeveloped OM pores. As described in 3.3, the pore types of shales in the study area are dominated by fractures and inorganic mineral pores, and OM pores are not developed, thus making the effect of TOC content on porosity permeability under effective pressure insignificant.Under the effective pressure influence, the porosity and permeability of shale decrease with the increase of effective pressure, and the reduction rate is characterized by an obvious stage change. According to the stage changes of porosity-permeability reduction rate, the pressurization process can be divided into three pressure stages: low pressure (<8 MPa), medium pressure (8–15 MPa) and high pressure (>15 MPa). Porosity reduction rate is highest in the low-pressure stage and lowest in the medium-pressure stage, with a difference of 2–15 times. Permeability reduction rate is highest at low-pressure stage and lowest at high-pressure stage, with a difference of 2 orders of magnitude.Under the effective pressure influence, the rock fabric affects porosity and permeability by influencing mineral distribution, pore development, and differential compaction. The laminae and interlayer fractures are poorly developed, and the distribution of minerals and pores is relatively homogeneous in massive shales. The presence of laminae is accompanied by well-developed interlayer fractures and the differential distribution of minerals and pores in laminated shale and layered shale. the minerals are dominated by calcite in calcite laminae where the pores mainly occur between calcite crystals and at the edges of calcite crystals, with better connectivity; the minerals are dominated by clay minerals in argillaceous laminae where the pores are mainly in clay minerals, with poorer connectivity. The compressibility of clay minerals is stronger than calcite. As a result, the clay mineral-related pores in the argillaceous laminae are more likely to be damaged and lose connectivity under effective pressure conditions, while the calcite-related pores in calcite laminae are less compressed and retain a certain permeability. In addition, interlayer fractures have not been completely closed at lower pressure, thus still able to maintain a certain connectivity. Therefore, compared with massive shale, laminated shale, and bedded shale generally have lower permeability loss and relatively higher permeability under effective pressure.During pressurization process, the porosity and permeability reduction rates are negatively correlated with felsic minerals content and positively correlated with carbonate minerals content and clay minerals content. Porosity reduction rate is mainly related to clay minerals content (R2=0.74, 0.51, 0.52), followed by carbonate minerals content (R2=0.58, 0.35, 0.29). Permeability reduction rate is primarily related to carbonate minerals content (R2=0.69, 0.88, 0.82), followed by clay minerals content (R2=0.27, 0.46, 0.30). The TOC content has no significant effect on the porosity and permeability of the shale in the study area due to lower maturity.All relevant data are within the manuscript and supplementary materials.The authors declared that they have no conflicts of interest to this work.This study was financially supported by the National Natural Science Foundation of China (Grant No.42072152).Table S1: Measured porosity of shale samples at different pressures.Table S2: Measured permeability of shale samples at different pressures.Table S3: Porosity loss rate calculated based on the values of measured porosity and pressure.Table S4: Permeability loss rate calculated based on the values of measured permeability and pressure.Table S5: Porosity-permeability reduction rate at different pressure stages based on the results of segmentation fitting.
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来源期刊
Lithosphere
Lithosphere GEOCHEMISTRY & GEOPHYSICS-GEOLOGY
CiteScore
3.80
自引率
16.70%
发文量
284
审稿时长
>12 weeks
期刊介绍: The open access journal will have an expanded scope covering research in all areas of earth, planetary, and environmental sciences, providing a unique publishing choice for authors in the geoscience community.
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