Failure characterization of Longmaxi shale under direct shear mode loadings

Insight into the shear failure of shale is significant for the analysis of hydraulic fracturing and wellbore stability in a shale gas reservoir. In this work, a series of direct shear tests is conducted on cubic Longmaxi shale samples with various bedding orientations, i.e., α = 0°, 30°, 60° and 90°, where the bedding orientation α is defined as the angle between the bedding and shearing direction. The anisotropies in the shear stress-displacement relation, failure pattern and shear strength are then analyzed and highlighted. Stress fluctuation is widely observed near the peak shear stress for most samples with α = 30° and 60°, and the shear displacement at the peak shear stress reaches its maximum and minimum values at α = 60° and α = 0°, respectively, for a given normal stress. The failure surface is formed by shear sliding along a weak bedding plane if α = 0°, resulting in a smooth failure surface with a much lower fractal dimension value; when α = 30°, 60° and 90°, some inclined cracks form across the failure surface during shearing, resulting in much rougher fracture surfaces. The shear strength and cohesion reach their minimum values at α = 0°, while the smallest internal friction angle is observed at α = 90°. Finally, by considering the anisotropies induced by the material microstructure and stress state, a new anisotropic shear failure criterion is presented and found to achieve good agreement with the test results. With the new criterion, the bedding angle corresponding to the maximum shear strength is found to vary with the applied normal stress.