Experimental Study on Mechanical Behavior and Brittleness Characteristics of Longmaxi Formation Shale in Changning, Sichuan Basin, China

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.