Study of the Macro-Mesoscopic Shear Anisotropic Mechanical Behavior of Reservoir Shale

Shear failure is pivotal in fracture evolution and stimulated reservoir volume (SRV) during hydraulic fracturing, particularly in bedded shale formations. However, the limited availability of coupled macro- and mesoscale experimental data on the shear behavior of reservoir shale constrains a comprehensive understanding of its anisotropic shear mechanical properties across scales. This study systematically investigates shear anisotropy at both macro- and mesoscales in shale with varying bedding orientations under different normal stress conditions. The key findings are summarized as follows: (1) At lower normal stresses, the anisotropy of peak shear strength was more pronounced, whereas the anisotropy of residual shear strength was relatively weak. As the normal stress increased, the anisotropic effects of bedding on peak and residual shear strengths exhibited opposite trends. The former exhibited a fluctuating decline, whereas the latter showed a progressive increase. (2) The internal friction angle of shale bedding planes was higher than that of the matrix, whereas cohesion exhibited the opposite trend. The internal friction angle corresponding to the peak shear strength reached its maximum at a bedding angle of 45°, while cohesion peaked at a bedding angle of 60°. (3) At lower normal stresses, the cumulative acoustic emission (AE) ringing count curves for shale shear failure followed an “S”-shaped pattern for bedded and matrix shear, differing from the piecewise linear pattern observed in bedded-matrix coupled shear. As the normal stress increased, the bedding-induced effects on macro- and mesoscale shear behavior evolved from non-uniformity to uniformity, reflecting a transition of anisotropy from uncoordinated to coordinated characteristics.