The micromechanical properties of organic matter (OM) and organic pore structures in the over-mature stage are crucial for determining shale reservoir quality and assessing shale gas resource potential. However, there is still debate about the influence of micromechanical properties of OM on the micro-mesopore structures in over-mature shale. In this study, shale cores from the Niutitang Formation have been specifically chosen for OM isolation, adsorption testing, atomic force microscopy examination, and focused ion beam scanning electron microscopy (FIB-SEM) analysis to assess the micromechanical properties of OM and pore structures. The findings indicate that organic micropores and mesopores predominantly exhibit elliptical, circular, or irregular shapes. Organic pores mainly provide pore volume (PV) and specific surface area (SSA) of shale. In the over-mature stage, residual kerogen and pyrobitumen transition towards a graphite structure, increasing Young's modulus of OM. Additionally, as thermal maturity increases, the absence of a rigid mineral framework and pore fluid pressure results in the compaction of pores, leading to a decrease in PV and SSA. The organic micropores are more vulnerable to collapse and compaction because of the increased brittleness of OM. The organic micropores and mesopores gradually evolve from regular circular and elliptical shapes to irregular shapes during the over-mature stage. The research findings provide valuable insights into the micromechanical mechanism of pore evolution in over-mature marine shale within complex structural regions. © 2025. The Author(s).

Multistage hydraulic fracturing leads to prolonged interactions between shale reservoirs and slickwater fracturing fluids, resulting in changes to the pore structure and micromechanical properties of the shale. However, systematic studies of the impact of shale mineral composition on hydraulic fracturing remain limited. This research investigates the effects of fracturing fluids on reservoirs with different mineral compositions, focusing on the Wufeng-Longmaxi shale in northern Guizhou, China. Comprehensive analyses were conducted on core samples before and after immersion in fracturing fluid, utilizing various testing methods, including field emission scanning electron microscopy, adsorption experiments, mercury intrusion porosimetry, and atomic force microscopy to characterize the pore structure, surface morphology, and modulus values of the shale. Additionally, sulfur-carbon analysis, vitrinite reflectance testing, and X-ray diffraction were employed to assess the mineral composition and geochemical parameters of the shale. The results indicate that after reacting with the fracturing fluid, clay minerals exhibit swelling and dispersion, and carbonate minerals undergo dissolution, while quartz remains largely unchanged. Furthermore, the better the retention of the pore volume and specific surface area in the shale, the more rapidly the modulus values decrease. Clay-rich shales retain more pores compared with quartz-rich and organic-rich shales, facilitating shale gas migration. However, the modulus values of clay-rich shales significantly decrease compared to those of quartz-rich shales, which may undermine the effectiveness of proppants, resulting in fracture closure and reduced permeability. Therefore, maintaining the modulus values of clay-rich shales is crucial for sustainable extraction of shale gas. The addition of clay stabilizers to the fracturing fluid may help preserve the modulus values and porosity of the shale reservoir postfracturing.

The heterogeneity of shale pores, a crucial factor in the occurrence and migration of shale gas, exerts a significant impact on the pore connectivity. Pores exhibiting a range of structural attributes to the complexity of pore connectivity. A comprehensive study into the impact of pore heterogeneity on pore connectivity in shale is lacking. This study aims to explore the impact of pore heterogeneity on pore connectivity in shale and the primary controlling factors of pore connectivity. Shale pore connectivity, pore structures, mineral composition, and geochemical parameters were assessed using various tests, including spontaneous imbibition, adsorption, mercury intrusion capillary pressure (MICP), focus ion beam-scanning electron microscopy (FIB-SEM), sulfur and carbon analysis, vitrinite reflectivity, and X-ray diffraction (XRD). Pore heterogeneity was quantitatively analyzed using the multifractal dimensions method. Results show that the increased heterogeneity of pore structure and surface roughness facilitating the potential for increased connectivity. The presence of well-developed micro-mesopores within the organic matter, alongside numerous disconnected and independent pores, contributes to an increase in pore heterogeneity. Micro-mesopore structures have a more pronounced impact on pore heterogeneity than macropores. The heterogeneity of the pores is predominantly influenced by variations in the PV and SSA of micro-mesopores. Furthermore, the heightened heterogeneity of pore structure (D1) and surface roughness (D2) leads to a more complex pore structure with increased roughness, promoting the potential for enhanced connectivity through additional throats and facilitating the formation of secondary microfractures. Shale cores characterized by high total organic carbon (TOC) content, porosity, and clay mineral content are conducive to improved pore connectivity. Conversely, the increase of thermal evolution maturity of shale in the over-mature stage is not conducive to the increase of pore connectivity. Thermal evolution maturity, TOC content, and clay mineral are key factors that control pore heterogeneity, further affecting the connectivity of shale pores. High pore connectivity is conducive to spontaneous imbibition capacity and the formation of hydraulic microfractures, promoting the migration of shale gas. © 2024

The presence of random joints, cracks, and other defects significantly affects the meso-damage mechanism and macro-mechanical behavior of the rock. This study employed micro-CT scanning, digital image processing (DIP), and the rock failure process analysis system (RFPA3D) to reconstruct a genuine mesostructure, creating a three-dimensional (3D) numerical model of jointed sandstone. Under uniaxial stress, this model facilitated the meso-damage evolution process of prefabricated cracks in sandstone with varying dip angles. Additionally, the influence of jointed sandstone heterogeneity and prefabricated cracks with various dip angles on its failure mode and meso-damage mechanical properties were investigated. Utilizing the MATLAB platform, a 3D box dimension algorithm was developed to analyze the fractal characteristics of the mesodamage evolution in the sample. This algorithm enabled the quantitative characterization of the meso-damage evolution of sandstone. This study categorized three types of sandstone final failure modes: composite shear failure, shear failure along the joint surface, and tensile failure. Additionally, linear variations in the elastic modulus and compressive strength of the jointed sandstone were observed with increasing prefabricated fracture inclination, highlighting significant anisotropy. The presence of joints was found to induce and control the failure mode of sandstone. The meso-damage evolution process of sandstone was described in terms of the fractal dimension, indicating that more severe damage corresponded to a larger fractal dimension. This approach offers a novel statistical method for studying the progression of rock damage.The impact of crack angles on the sandstone failure mode and meso-damage properties was examined in 3D. Sandstone heterogeneity can initiate and control internal crack propagation. A custom box dimension algorithm was developed to quantify the meso-damage evolution in jointed sandstone by calculating the fracture volume.

Water saturation of shale reservoirs significantly influences the permeability and compressibility of propped fractures. This study focused on the Longmaxi Formation shale reservoir in northern Guizhou, China, where the permeability of water-saturated shale under varying gas and confining pressures was measured. A compressibility model for proppant embedment and compaction deformation was developed and validated against the experimental results. This study examined the compressibility of supported fractures considering water-rock interactions and elucidated the intrinsic relationship between compressibility and water saturation. The findings demonstrated a decreased trend in shale fracture permeability with increasing water saturation under identical conditions. Compared to dry shale, the permeability decreased by 1.2%-16.4% and 2.0%-17.8% at water saturation of 15% and 50%, respectively. The results of the model calculations demonstrate that fracture compressibility is contingent on the degree of variation of the fracture width. Prolonged water-rock interactions intensified the variation in the fracture width increasing the compressibility under the same stress conditions. As the water saturation increased from 0% to 50%, the fracture closure rate increased from 0.034 to 0.179 with the increase in effective stress. Increased water saturation also increases the sensitivity of the fracture compressibility to effective stress while decreasing the elastic modulus of the rock, thereby enhancing the proppant embedment depth and significantly increasing the fracture compressibility. This study provides critical insights into the dynamic evolution of fracture permeability during hydraulic fracturing and offers valuable implications for gas production forecasting.

Tectonic deformation has an evident impact on the pore structure of organic-rich marine shale. To study the impact of tectonic deformation on the pore structure of shale, cores were taken from different tectonic regions in the Niutitang Formation. The paleotectonic stress, pore structures, mineral composition, and geochemical parameters of the shale were determined by conducting the following tests: paleotectonic stress test, core thin-section observation, focus ion beam-scanning electron microscopy (FIB-SEM) observation, vitrinite reflectivity test, low-temperature nitrogen adsorption test, total organic carbon (TOC) test, X-ray diffraction (XRD) test, and nano-CT. The results showed that the shale in the deformed region mainly developed meso-macropores, and the pores were mainly slit-type and conical. The shale in the transition and stability regions primarily developed mesopores and micropores, which were primarily elliptical, slit-type, and columnar. The pore volumes of the shale in the deformed region were 9.54 cm3/kg and 13.9 cm3/kg higher than those in the transition and stability regions, respectively. The shale in the stability and transition regions had average microporous volumes 8.67% and 7.93% higher than that in the deformed region, respectively. The shale reservoir in the stability region developed more micropores and had a higher specific surface area and microporous volume. Furthermore, the organic matter had a greater impact on the micropores than on the meso-macropores. Thermal maturity had a greater impact on the microporous volume than on the macroporous volume. The shale pore connectivity of Well TX 1 was 86.48% and that of Well FC 1 was 22.33%. The pore connectivity of the shale in the stability region was higher than that of the shale in the deformed region. The shale samples from the deformed region had larger pore volumes and primarily developed macropores. The difference in the meso-macropores is significantly greater than the difference in the microporous volume. Hence, the geological tectonism had a greater impact on the meso-macropores than on the micropores in the shale cores. © 2023 Elsevier Ltd

A large number of natural cracks exist in shale reservoirs, and the presence of natural cracks weakens the integrity of shale, which is an important factor governing the effectiveness of shale gas extraction. In this paper, shales from the Lower Cambrian Niutitang Formation in northern Guizhou were scanned by electron microscopy, their microstructures were selected for digital image processing, and uniaxial compression numerical tests were conducted on shale models containing different natural crack dips using the rock fracture process system RFPA2D-DIP to study the effects of natural cracks on the mechanical properties and fracture patterns of shales at the microscopic scale. The study shows that the peak strength and elastic modulus of shale increase with increasing natural crack inclination angle. The fracture modes of shale at the microscopic scale can be roughly divided into four categories: similar to I-type fractures (0 degrees), oblique I-type fractures (15 degrees, 45 degrees, 60 degrees, 75 degrees), folded line fractures (30 degrees), and V-type fractures (90 degrees). Natural cracks within shale are found to have a significant effect on the distribution of stress. Acoustic emission can reflect the stress change and rupture process for shales containing natural cracks with different dip angles at the microscopic scale. The presence of natural cracks has a significant effect on the AE energy and fractal dimension. The magnitude of the AE energy increases with increasing stress level and reaches a maximum value at 90 degrees, while the value of the fractal dimension is found to zigzag upwards because the value of the fractal dimension is jointly influenced by both newborn cracks and native natural cracks.

Due to the heterogeneity of rock media, it is difficult to truly reflect its internal three-dimensional microstructure in physical tests or numerical simulation. In this study, CT scanning technology and numerical image processing technology are used, and the finite element software RFPA-3D is used to establish a three-dimensional non-uniform numerical model that can reflect the meso structure of rock mass. In order to study the fracture mechanism of shale with prefabricated fractures, seven groups of three-dimensional numerical models with prefabricated fractures from different angles were constructed, and Brazilian fracturing numerical simulation tests were carried out. The results show that method of reconstructing 3D numerical models by CT scanning is feasible and provides a viable method for in-depth study of the micromechanics of shale. Prefabricated fractures and quartz minerals have significant effects on the tensile strength of shale, and both will weaken the destructive strength of shale specimens. The damage modes of Brazilian disc specimens containing prefabricated fissures can be divided into four categories. The damage process is divided into budding, plateauing and surge periods by acoustic emission. The crack initiation angle of the prefabricated fissure tip increases with increasing fissure angle, and the MTS criterion can be used as a basis for judging prefabricated fissure initiation. The results of the study are important guidance for the fracture initiation mechanism and fracture expansion law of the fractured layer containing natural fractures in the hydraulic fracturing process.

为研究流固耦合作用下，单孔页岩水压致裂过程中孔洞裂隙扩展规律，利用RFPA2D-Flow数值软件，建立不同层理倾角、不同水压作用下的黔北地区牛蹄塘组单孔页岩进行单轴数值模型试验研究。研究结果表明页岩抗压强度以及弹性模量均表现出明显的各向异性，并通过声发射进一步揭示了页岩的破坏规律研究成果为黔北地区凤冈三区下寒武纪统黑色页岩在水力压裂下页岩气开采提供重要理论指导。

利用液态金属作为阴极分离、提取稀土元素有很多优点。以液态金属Zn为阴极，研究Pr（Ⅲ）离子在液态Zn阴极上还原的电化学机理。在LiCl-KCl-PrCl3熔盐中，分别采用循环伏安法、半积分法研究W电极和液态Zn电极上Pr（Ⅲ）的电化学还原过程。结果表明，在该实验温度下，只有一种富锌的PrxZny金属间化合物生成。通过循环伏安法和半微分法计算了LiCl-KCl熔盐中Pr（Ⅲ）的扩散系数。根据电化学机理研究，采用液态金属Zn为阴极恒电位电解提取稀土Pr。电感耦合等离子体发射光谱仪（ICP）结果表明，随着电解时间的增长，熔盐中Pr（Ⅲ）离子的浓度逐渐降低。电解2 h后，提取效率为45.38%,当电解时间达到40 h时，提取效率为99.48%。X射线衍射（XRD）和扫描电镜-能谱（SEM-EDS）点分析结果表明，恒电位电解2 h得到的沉积物为Zn11Pr3。