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The permeability of shale reservoirs is an important parameter in evaluating the feasibility of shale gas commercial exploitation. The influence of structural anisotropy, bedding planes, and effective stress is of great significance to the permeability of shale reservoirs; thus, a further study on the permeability of shale rock containing bedding planes and fractures is necessary. In this paper, to investigate the gas conductivity of shale rock in different bedding directions and fracture surfaces, permeability tests were conducted on the intact specimens with different bedding inclinations and specimens containing fractures of Longmaxi shale. The pulse decay method is adopted in the determination of the intact shale specimen, and the steady-state method is adopted to measure the permeability of fractured shale specimen. Combined with experiments and theoretical analysis, the anisotropic characteristics of the permeability of intact shale specimens and the permeability of fractured specimens under the applying of effective stress are studied. The permeability of the two kinds of shale specimens decreased exponentially with the increase in effective stress. The main controlling factors on permeability were studied, besides, the flow characteristic of fluid inside the rock with bedding plane or sandwich structures were described. Furthermore, a new model was proposed to describe the anisotropy of rock permeability characteristics. As for shale specimen contains fracture, the function between its equivalent permeability and fracture permeability was derived, as well as the function between equivalent permeability and effective stress, and the function between fracture permeability and effective stress.

At present, shale gas plays a significant role in hydrocarbon reservoirs. Hydraulic fracturing is generally employed in the exploration and exploitation of shale gas. Economic and efficient hydraulic fracturing, known as volume fracturing, is largely associated with formation characteristics, including anisotropy and brittleness in rocks. Further research on the mechanical properties of rocks, particularly the anisotropy and brittleness behavior of shale, using hydraulic fracturing would be of practical significance. In this study, shale specimens were collected from an outcrop of the lower Silurian Longmaxi formation at Sichuan Basin in southwestern China, which is the most significant exploration area for unconventional gas in China. To better understand the size, type, and shape of brittle minerals, the matrix type (and rock texture), mineral composition, and microstructure of the shale matrix were tested through X-ray diffraction analysis and scanning electron microscopy. Furthermore, the anisotropic behavior of shale specimens, including strength, deformation, and failure behaviors, was tested and analyzed under conventional triaxial compression. In addition, the brittleness characteristics of shale specimens at different bedding inclinations under different confining pressures were analyzed based on the stress–strain curve characteristic and energy balance. Different brittleness indices, including a new one proposed in this study, were used to evaluate the brittleness of shale. The impacts of anisotropy and confining pressure on brittleness were discussed in detail. When compared with other brittleness indices, the proposed brittleness index demonstrates improved effectiveness and reflects the impact of confining pressure on brittleness significantly well. The relationship between brittleness and the failure mode was revealed using the new brittleness index, and the decreasing order of brittleness was concluded as follows: tensile splitting along bedding plane mode > tensile splitting through bedding plane mode > shear along bedding plane mode > shear through bedding plane mode.

Bedding planes or layers can have a serious effect on the mechanical behaviour of shale rock. Fractures occurring along bedding planes form a fracture network in shale hydraulic fracturing; thus, the tensile strength and fracture mode are important for hydraulic fracturing design. In this research, shale disk specimens are prepared and analysed under Brazilian test conditions. During the test, a 3D digital image correlation system (DIC) is employed to capture surface deformation, and an AE sensor and strain observation system are, respectively, used to record the AE event and central strain of the shale disk during the failure process. A new inherently anisotropic model is established with a banded-particle model and smooth joint model using the particle flow code (PFC2D). The laboratory test result shows that the Brazilian tensile strength (BTS) value decreased gradually along with bedding inclination, which corresponds to a trend of the decrease of the strength over the entire interval, but it is a rather systematic decrease, approximating a linear variation. By considering the specimen after failure, three types of fracture patterns are observed: arc fracture (AF) through outside the central part, central fracture along the loading direction (LA) and mixed fracture patterns of the two. The DIC and central strain observation systems confirm that the split fractures do not always propagate along the diametrical loading direction, which means that the traditional isotropic elastic theory no longer works for layered shale. The micro-level failure behaviour and mechanism are analysed by PFC simulation. The PFC simulation reveals that the rock matrix tensile fracture and shear fracture along bedding plane are the main fracture pattern of shale disk specimens under the Brazilian test. The bedding inclination and interlayer bonding force play a very important role in the anisotropic behaviour of the shale.

To investigate the role of fissure angle and heat treatment temperature on the mechanical properties and deformation failure behavior, uniaxial compression tests were carried out on granite specimens containing a single fissure. Using stress–strain curves, the peak strength, peak strain, and elastic modulus of the one-fissured granite specimens were analyzed in detail. The mechanical parameters are closely related to the fissure angle and the high temperature. As the fissure angle increases from 0° to 90°, the peak strength and elastic modulus first decrease and then increase, while the peak strain increases slowly. However, the peak strength and elastic modulus first increase and then decrease, while the peak strain first decreases and then increases with increasing treatment temperature. During the experiments, the crack evolution process and acoustic emission (AE) counts were obtained using real-time photography and the AE monitoring technique. In the granite specimens containing a pre-existing fissure, large AE counts are clearly observed before the peak strength, which indicates crack initiation and propagation. The accumulated AE count first increases slowly, but is followed by a sharp increase, with increasing deformation. The AE events in the one-fissured specimen also depend on the heat treatment temperature. As the temperature increases, the rate of increase of the accumulated AE count curve is reduced. Finally, using a digital image correlation method, the full fields of surface deformation were obtained for the entire testing process. In addition, the local strain around the pre-existing fissure was measured using strain gauges. The full strain field and local strain concentration are discussed to describe the fracture mechanism of brittle granite.