Search Results (24)

The mechanism of low- to middle-rank coal seam gas accumulation in the Baode block on the eastern edge of the Ordos Basin is well understood. However, exploration efforts in the Shizuishan area on the western edge started later, and the current understanding of enrichment and accumulation rules is unclear. It is important to systematically study enrichment and accumulation, which guide the precise exploration and development of coal seam gas resources in the western wing of the basin. The coal seam collected from the Shizuishan area of Ningxia was taken as the target. Based on drilling, logging, seismic, and CBM (coalbed methane) test data, geological conditions were studied, and factors and reservoir formation modes of CBM enrichment were summarized. The results are as follows. The principal coal-bearing seams in the study area are coal seams No. 2 and No. 3 of the Shanxi Formation and No. 5 and No. 6 of the Taiyuan Formation, with thicknesses exceeding 10 m in the southwest and generally stable thickness across the region, providing favorable conditions for CBM enrichment. Spatial variations in burial depth show stability in the east and south, but notable fluctuations are observed near fault F1 in the west and north. These burial depth patterns are closely linked to coal rank, which increases with depth. Although the southeastern region exhibits a lower coal rank than the northwest, its variation is minimal, reflecting a more uniform thermal evolution. Lithologically, the roof of coal seam No. 6 is mainly composed of dense sandstone in the central and southern areas, indicating a strong sealing capacity conducive to gas preservation. This study employs a system that fuses multi-source geological data for analysis, integrating multi-dimensional data such as drilling, logging, seismic, and CBM testing data. It systematically reveals the gas control mechanism of “tectonic–sedimentary–fluid” trinity coupling in low-gentle slope structural belts, providing a new research paradigm for coalbed methane exploration in complex structural areas. It creatively proposes a three-type CBM accumulation model that includes the following: ① a steep flank tectonic fault escape type (tectonics-dominated); ② an axial tectonic hydrodynamic sealing type (water–tectonics composite); and ③ a gentle flank lithology–hydrodynamic sealing type (lithology–water synergy). This classification system breaks through the traditional binary framework, systematically explaining the spatiotemporal matching relationships of the accumulated elements in different structural positions and establishing quantitative criteria for target area selection. It systematically reveals the key controlling roles of low-gentle slope structural belts and slope belts in coalbed methane enrichment, innovatively proposing a new gentle slope accumulation model defined as “slope control storage, low-structure gas reservoir”. These integrated results highlight the mutual control of structural, thermal, and lithological factors on CBM enrichment and provide critical guidance for future exploration in the Ningxia Autonomous Region. Full article

To investigate the effects of impact and static compaction methods on the mechanical properties and crack evolution of rock-like materials with varying particle sizes. Uniaxial compression tests combined with Digital Image Correlation (DIC) technology were conducted on specimens of two aeolian sand gradations (0.075–0.18 mm and 0.22–0.5 mm) and one quartz sand gradation (0.22–0.5 mm). The study focused on elastic modulus, peak strength, stress-strain behavior, failure modes, surface deformation fields, crack propagation paths, and strain evolution at characteristic points under both compaction methods. Finally, the microstructure of specimens was analyzed and compared with natural rock analogs. Key results include: (1) At an identical density of 1.82 g/cm³, static-compacted specimens of fine-grained aeolian sand (0.075–0.18 mm) exhibited higher elastic modulus and peak strength compared to impact-compacted counterparts, whereas inverse trends were observed for coarse-grained aeolian sand (0.22–0.5 mm) and quartz sand specimens; (2) Under equivalent compaction energy (254.8 J), the hierarchy of mechanical performance was: quartz sand > coarse-grained aeolian sand > fine-grained aeolian sand; (3) Static-compacted specimens predominantly failed through tensile splitting, while impact-compacted specimens exhibited shear-dominated failure modes; (4) DIC full-field strain mapping revealed rapid propagation of primary cracks along pre-existing weak planes in static-compacted specimens, forming through-going tensile fractures. In contrast, impact-compacted specimens developed fractal strain field structures with coordinated evolution of shear bands and secondary tensile cracks; (5) Microstructural comparisons showed that static-compacted fine-grained aeolian sand specimens exhibited root-like structures with high porosity, resembling weakly consolidated sedimentary rocks. Impact-compacted coarse-grained aeolian sand specimens displayed stepped structures with dense packing, analogous to strongly cemented sandstones. Full article

Fractal dimension analysis provides a quantitative approach to characterizing the heterogeneity of pore structures in reservoirs. In this study, casting thin sections, scanning electron microscopy (SEM), and high-pressure mercury intrusion porosimetry (MIP) were integrated with fractal theory to determine the fractal dimensions of different pore types and investigate the controlling factors of reservoir pore structure heterogeneity. This study identifies three primary pore types—residual intergranular, dissolution, and intergranular pores—and classifies the reservoir into three distinct types based on their mercury intrusion curves and pore-throat radius distributions. The fractal analysis of pore structures reveals three segments corresponding to macropores, mesopores, and transition pores, with average fractal dimensions of 2.28, 3.67, and 2.43, respectively. Furthermore, the overall fractal dimensions for Type I, II, and III reservoirs are 2.69, 2.72, and 2.92, indicating an increasing trend in heterogeneity from Type I to Type III. The fractal dimension shows a negative correlation with reservoir properties, median pore-throat radius, maximum mercury saturation, and the content of quartz and feldspar, while it is positively correlated with displacement pressure. No significant correlation is observed with clay mineral content. These findings offer valuable insights into the heterogeneity of reservoir pore structures and provide a basis for evaluating reservoir quality. Full article

Low-permeability sandstone reservoirs have low permeability, but due to their high porosity and difficulty in development, the development difficulty is relatively high. They can fully tap into the high potential of oil and gas resources in low-permeability sandstone reservoirs and occupy an important position in the global energy supply The study area belongs to low-permeability dense sandstone reservoir, and the destination layer has complex lithology, strong physical inhomogeneity, and complicated pore–permeability relationship, so the conventional core pore–permeability regression method and NMR SDR method do not satisfy the requirements of fine evaluation in terms of the accuracy of permeability calculation. According to the principle of resistivity measurement by electromagnetic waves with Logging While Drilling (LWD), this paper analyzes the reasons for the magnitude of resistivity divergence with Logging While Drilling at different exploration depths. There is a “low invasion phenomenon” during the drilling process of the drill bit. The higher the permeability of the formation, the more severe the “low invasion phenomenon”, and the greater the magnitude of resistivity divergence. In this paper, through the conventional log curve response characteristics and correlation analysis, the P40H/P16H parameter were selected to characterize the magnitude of resistivity divergence, and a fine evaluation model of the reservoir based on the P40H/P16H parameter was established in the study area by relying on the theory of the flow unit, and was applied to the prediction of permeability of new wells. The application results show that the calculated permeability is in good agreement with the results of core analysis, which provides a theoretical basis for the fine evaluation of low-permeability tight reservoirs. Full article

With the accelerating global transition to clean energy, underground hydrogen storage (UHS) has gained significant attention as a flexible and renewable energy storage technology. Ontario, Canada, as a pioneer in energy transition, offers substantial underground storage potential, with its geological conditions of salt, limestone, and sandstone providing diverse options for hydrogen storage. However, the hydrogen transport characteristics of different rock media significantly affect the feasibility and safety of energy storage projects, warranting in-depth research. This study simulates the hydrogen flow and transport characteristics in typical energy storage digital rock core models (salt rock, limestone, and sandstone) from Ontario using the improved quartet structure generation set (I-QSGS) and the lattice Boltzmann method (LBM). The study systematically investigates the distribution of flow velocity fields, directional characteristics, and permeability differences, covering the impact of hydraulic changes on storage capacity and the mesoscopic flow behavior of hydrogen in porous media. The results show that salt rock, due to its dense structure, has the lowest permeability and airtightness, with extremely low hydrogen transport velocity that is minimally affected by pressure differences. The microfracture structure of limestone provides uneven transport pathways, exhibiting moderate permeability and fracture-dominated transport characteristics. Sandstone, with its higher porosity and good connectivity, has a significantly higher transport rate compared to the other two media, showing local high-velocity preferential flow paths. Directional analysis reveals that salt rock and sandstone exhibit significant anisotropy, while limestone’s transport characteristics are more uniform. Based on these findings, salt rock, with its superior sealing ability, demonstrates the best hydrogen storage performance, while limestone and sandstone also exhibit potential for storage under specific conditions, though further optimization and validation are required. This study provides a theoretical basis for site selection and operational parameter optimization for underground hydrogen storage in Ontario and offers valuable insights for energy storage projects in similar geological settings globally. Full article

Tight oil reservoirs are considered important exploration targets in lacustrine basins. High-quality reservoir prediction is difficult as the reservoirs have complex distributions of depositional facies and diagenesis processes. Previous research has found that the diagenesis process of tight oil sandstones varies greatly in different depositional facies. However, diagenesis variation in different depositional facies is still poorly studied, especially in distributary channels of shallow water delta deposits in lacustrine basins. Based on the description of core samples, the observation of rock slices, the interpretation of well logging data, and the analysis of porosity and permeability data, the differences in the lithofacies types, diagenesis processes, and pore structures of different distributary channels have been clarified. Ultimately, a model of diagenesis and reservoir heterogeneity distribution in the shallow-water delta of Chang 8 Member of the Yanchang Formation in the Caijiamiao area of the Ordos Basin has been established. This research indicates that the main distributary channels in the study area are dominated by massive bedding sandstone lithofacies, while the secondary distributary channels are primarily characterized by cross-bedding sandstone lithofacies. There are significant differences in the compaction, dissolution, and cementation of authigenic chlorite and carbonate among different parts of the distributary channels. Plastic mineral components, such as clay and mica, are abundant in sheet sands, and are more influenced by mechanical and chemical compaction. Influenced by the infiltration of meteoric water and hydrocarbon generation, dissolution pores are relatively well-developed in the underwater distributary channel reservoirs. A large amount of carbonate cementation, such as calcite and siderite, is found within the sandstone at the interface between sand and mud. The occurrence of authigenic chlorite exhibits a clear sedimentary microfacies zonation, but there is little difference in the kaolinite and siliceous cementation among different microfacies reservoirs. Finally, a model of diagenetic differences and reservoir quality distribution within dense sand bodies has been established. This model suggests that high-quality reservoirs are primarily developed in the middle of distributary channels, providing a theoretical basis for the further fine exploration and development of oil and gas in the study area. Full article

The sandstone reservoir in the Lule River group of Mahai East District is strong, densely dense, and the expansion of clay material is very easy to block the pores. Therefore, the new pressure cracks and cationic cluster stabilizers with double-season ammonium salt clay stabilizers are used. Experimental analysis is performed under the circumstances of the perimeter. Use nuclear magnetic real-time testing different fracture liquids to drive the pore structure structure changes before and after the sandstone, combined with differential theory to calculate the fractal dimensions of sandstone. As a result, under the condition of water full, as the perimeter pressure increases, the rocks are increased, and the rocks are increased, and the rocks are increased. The matrix occurs under the common action of the fence and the fracture fluid to prevent the inflation of clay minerals under the combined action of the fence and the fracture fluid, which causes a significant reduction in large holes and increased micropores and medium holes. Exploring different fracture liquids to reservoirs is of great significance to the development of dense sandstone reservoirs. Full article

In order to study the hydration absorption behaviors and characteristics of sandstone in Mogao Grottoes in China, the pressure-less hydration absorption experiment on the outcrop porous sandstone of Mogao Grottoes was carried out by using the self-developed real-time monitoring experimental system. The hydration absorption was measured and the curve of hydration absorption with time was drawn. At the same time, the digital image correlation method (DIC) was used to measure the full-field deformation, and the speckle pattern of the sample was analyzed using Match ID, and the displacement field and strain field of the sandstone sample at different hydration absorption moments were computed. Moreover, the sparse area and dense area of sandstone are used as regions of interest (ROI) for DIC analysis. According to the test results, it is concluded that the hydration absorption of sandstone increases rapidly in the initial stage, and gradually tends to be stable with the change of time. This corresponds well with the deformation characteristics of sandstone analyzed using DIC. In the initial stage, the deformation of sandstone increases rapidly. With the change in time, the deformation of sandstone samples gradually slows down. When the hydration adsorption reaches saturation, the sandstone continues to deform for a period of time before stopping hydration absorption. The results of the mercury injection test and the XRD test show that the porosity of the sparse area is larger than that of the dense area and the particle content of the dense area is lower. When the sandstone is saturated with water, the liquid is immersed in the pores between the solid particles, which makes the sparse area more prone to stress concentration, and the deformation in the sparse area is larger. Therefore, when analyzing the hydration absorption deformation of sandstone, the porosity should be considered. Full article

In order to investigate freeze–thawed red sandstone failure processes under cyclic loading and unloading conditions, real-time acoustic emission (AE) and scanning electron microscopy (SEM) techniques were used to reveal the fracture process of the saturated red sandstone after cyclic loading and unloading tests using uniaxial compression. The results show that the stress–strain curves of the freeze–thawed sandstones show signs of hysteresis and exhibit a two-stage evolution of “sparse → dense”. In the cyclic loading and unloading process, the modulus of elasticity in the loading process is always larger than that in the unloading process, while the Poisson’s ratio is the opposite, and the radial irreversible strain and cumulative irreversible strain are larger than those in the axial direction. As the number of freeze–thaw cycles increases, the rock specimens need more cycles of loading and unloading to make the crack volume compressive strain $\Delta {\epsilon }_{cv}^{+}$ reach the maximum value and tend to stabilize, while the crack volume extensional strain $\Delta {\epsilon }_{cv}^{-}$ tends to decrease gradually. This study also shows that the growth phase of the cyclic loading and unloading process has more ringing counts and a shorter duration, while the slow degradation phase has more ringing counts with loading and less with unloading. In addition, the F-T cycle gradually changes the internal microcracks of the red sandstone from shear damage, which is dominated by shear cracks, to tensile damage, which is dominated by tensile cracks. This study’s findings contribute to our knowledge of the mechanical characteristics and sandstone’s degradation process following F-T treatment, and also serve as a guide for engineering stability analyses conducted in the presence of multiphysical field coupling. Full article

The foundations of bridges and other tall buildings are often subjected to cyclic loads. Therefore, it is essential to investigate the mechanical properties of rock–concrete composite foundations under cyclic loads. In this paper, uniaxial cyclic loading and unloading tests were conducted on rock–concrete composite specimens using the TFD-2000 microcomputer servo-controlled rock triaxial testing machine. The stress–strain curves, elastic modulus variation, and energy dissipation were analyzed. The results showed that the stress–strain curves of composite specimens under uniaxial cyclic loading and unloading conditions formed hysteresis loops. The hysteresis loop exhibited a sparse–dense–sparse pattern under the upper stress of 27.44 MPa, which was 90% of the uniaxial strength. The elastic modulus, as well as the dissipated energy, decreased rapidly in the first few cycles and then gradually decreased at a constant rate, with the upper stress increasing to 27.44 MPa. Both the elastic modulus and the dissipated energy exhibited an accelerated stage before specimen failure. The primary failure mode of the composite specimen was split failure from concrete to sandstone. A damage variable was derived to better reflect the laws governing the damage evolution of the composite under cyclic loads. Full article

The unconventional tight oil and gas resources in the Xinzhao East belt of the Hangjinqi Prospect area in the Ordos Basin of China are abundant. However, the reservoir’s internal storage space is complex, and the microscopic pore throat structural features are not well recognized, which has led to some trouble in the deployment of oil and gas exploration. To reveal the microscopic characteristics of the dense sandstone gas layer in the first member of the Lower Stone Box Formation of the D-well Zone in the Xinzhao East belt of the Hangjinqi Prospect area, a three-dimensional space digital core was built, and the stored set spatial data were extracted, based on rock sheet and coring data and X-CT scanning technology. Quartz grain size was segmented and analyzed based on an adaptive approach. The microscopic characteristics of the gas layer in the studied section and the factors influencing its development were studied, combining the use of a field emission scanning electron microscope, helium porosimeter, and gas permeability meter. We found that in the studied section, the porosity is relatively high, the pore throat size is large, and the pore permeability correlation is good. The reservoir space, which consists of intergranular pores, intragranular pores, and microcracks at the grain edges in the study area, is characterized by a complex distribution pattern. Within the gas layer, isolated pores are connected by microcracks to form a network of reservoir spaces, which increases the pore throat size, enhances the connectivity of the pore throat, and makes the microscopic characteristics of the reservoir space better. The first member of the Lower Stone Box Formation could be an advantageous reservoir. Hole–throat connectivity is poor because of the gas layer having underdeveloped primary pores, the blockage of pores by unstable minerals (kaolinite, etc.), and poorly connected pore throats based on insoluble mud cementation. The high content of quartz brittle minerals and the development of natural microcracks within the gas formation are favorable conditions for fracking development. The quartz grain size within the gas layer is positively correlated with the pore throat size, which suggests that the quartz grain size somewhat influences the microscopic characteristics of the reservoir space. This comprehensive study shows that the methodology of the study is more advantageous than traditional methods in the fine and three-dimensional spatial characterization of the microstructure of dense sandstone reservoirs. The research results of this paper have certain guiding significance for further reservoir evaluation and advantageous reservoir prediction in the Hangjinqi Prospect area in the Ordos Basin. We also provide the basis for the subsequent efficient development of the gas reservoir. Full article

Reservoir heterogeneity is an important factor in oil and gas exploration and development. It has guiding significance for favourable target optimization because it helps clarify the formation and development characteristics of laminae: thin, alternating layers of sediment deposited in a repeating pattern in Cretaceous sandstone reservoirs. Reservoir heterogeneity is higher when laminae are densely distributed. For example, laminae have a significant influence on reservoir properties in the Kelasu structural belt in the Bozi–Dabei area, Tarim Basin, China, where oil and gas wells have very low productivity. Hence, this study attempts to develop a classification scheme based on laminae to identify how they influence reservoir properties. Based on an analysis of cores, thin section and logging imaging data, laminae in this area can be classified into three types: magnetite-enriched, iron-stained argillaceous-enriched, and grain-size change. Results show that magnetite-enriched and iron-stained argillaceous-enriched laminae are well-developed in the BZ1 and DB10 well areas in northern Bozi. They have much lower porosity compared to a non-laminae reservoir and their lateral permeability is greater than vertical permeability. Grain-size change laminae are well-developed in the southern Bozi region. For the laminated reservoirs, increasing the length of the perforation interval and the perforation density using sand or acid fracturing is an effective method for communicating with the vertical reservoir layers, improving permeability, and increasing single-well production. Full article

Industrial oil and gas eruptions underwater have been found in the pyroclastic rocks of the Huoshiling Formation in the continental lacustrine basin of the Changling fault depression, Songliao Basin. This paper investigates the reservoir space characteristics, physical characteristics, and pore structure differences of subaqueous pyroclastic reservoirs in the Huoshiling Formation, and the causes of physical property differences of different types of reservoirs and their formation and evolution processes are analyzed. (1) The content of volcanic glass in tuff is higher, the reservoir space is dominated by devitrification pores and dissolution pores, and the coarser the grain size, the more favorable the physical properties, with larger pore sizes and higher porosities. The content of clay minerals in sedimentary tuff is high, the pores between clay minerals are the main pores, and the physical properties of sedimentary tuff are poor. The content of soluble components such as feldspar, debris, and laumontite is high in tuffaceous sandstone, which is dominated by dissolution pores. (2) Primary pores are not developed in the pyroclastic reservoirs in the study area, and the reservoirs are relatively dense, with an average porosity of 2.43% and an average permeability of 0.076 mD. The coarse-grained tuff has the highest porosity, followed by tuffaceous sandstone and fine-grained tuff, and the sedimentary tuff has the least favorable physical properties. (3) Devitrification was an important cause of the high-porosity and ultralow permeability of tuff reservoirs. Two oil and gas charges in the middle diagenetic stage led to the organic acid dissolution of rocks. In addition, fractures can provide migration channels for organic acids and deep hydrothermal fluids, leading to late dissolution, and can connect various scattered dissolution pores to improve the effectiveness of the reservoir space. (4) Coarse-grained tuff reservoirs that developed in the proximal facies are favorable targets for hydrocarbon exploration. Full article

In this study, a set of laboratory experiments was carried out to study the parameters of hydraulic fractures induced in the dry and mineral-oil-saturated rocks and compare them with the geomechanical characteristics of tested samples. We chose sandstone and thermally treated granite as the materials for research. There are very few known studies related to the mechanical and acoustic properties of oil-saturated rocks, and even fewer studies describing, in detail, the parameters of hydraulic fractures generated in oil-saturated rocks. The hydraulic fracture parameters were determined using a set of independent sensors installed to measure the axial deformation of the sample (which is directly related to the aperture of created hydraulic fracture), fluid pressure, fluid volume injected into hydraulic fracture, and localization of acoustic emission (AE) events, generated during the propagation of hydraulic fractures. Our study focuses on the investigation of the influence of rock properties, altered by mineral oil saturation and thermal treatment, on such parameters of hydraulic fracturing as breakdown pressure (BP), fracture aperture, and the resulting roughness of the hydraulic fracture surface. In addition, we studied the influence of injected fluid viscosity on the parameters of created hydraulic fractures. It was revealed that the saturation state caused a reduction in the values of mechanical parameters such as Young’s modulus, compressive strength, and cohesion, and had a similar reducing influence on the breakdown pressure. The values of HF parameters, such as fracture width and the volume of fracturing agent injected into the HF, are higher in the tests for both saturated sandstone and saturated TT granite. However, we found out that thermal treatment of granite samples led to a much more significant reduction in the values of mechanical and acoustic parameters than the mineral-oil saturation procedure because it created a dense network of thermally induced cracks. The results obtained in our laboratory studies can be taken into account in the modeling of hydraulic fracturing in the field. Full article

Tight sandstone reservoirs are of interest due to their potentially favorable prospects for hydrocarbon exploration. A better understanding of tight sandstone outcrop reservoir characteristics and their influencing factors is thus needed. By laboratory observation, thin section analysis, and experimental analysis, the current work carried out a detailed investigation of densely sampled tight sandstone outcrops of the Shanxi Formation in the Liujiang River Basin, paving the way for further research on rock types, reservoir spatial distribution, physical properties, and their key controlling factors. The application of the Pressure Pulse Attenuation Method made it possible to determine the porosity and permeability, as well as the analysis of debris composition and filling content. The findings indicate that the main rock type of the tight sandstone outcrop reservoirs in the Shanxi Formation in the Liujiang River Basin is lithic quartz sandstone, some of which contains fine sand-bearing argillaceous siltstone, giving them very low porosity (average porosity of 4.34%) and low permeability (average permeability of 0.023 mD) reservoirs. Secondary pores—mostly dissolved pores among and in grains—are widely developed in the target region. In addition, diagenesis primarily includes mechanical compaction, cementation, and dissolution. The main controlling factors of tight sandstone reservoirs in the target region are sedimentation, diagenesis, and tectonics, whereby sedimentation affects reservoir physical properties that become better as the clast size increases, reservoir properties are negatively impacted by compaction and cementation, and reservoir properties are somewhat improved due to dissolution and the impact of tectonism. In addition, the tilt of the crust will produce faults during the tectonic action, generating reservoir cracks that improve the reservoir’s physical properties. This study tends to be helpful in the prediction of high-quality reservoirs in the Permian Shanxi Formation in North China and can also be used for analogy of high-quality reservoirs in similar areas with complete outcrops. Full article