Shear-tensile transition and shear strength evolution of shale fault planes with varying undulation angles: Implications for fault slip and casing deformation during hydraulic fracturing

Shear-induced casing deformation has become a critical geomechanical challenge during hydraulic fracturing of deep shale gas reservoirs, yet the shear behavior of shale fault planes with different geometries remains insufficiently quantified. In this study, a series of direct shear experiments were conducted on shale structural planes with systematically varied undulation angles under medium-to-high normal stress conditions. The results reveal a distinct transition from shear-dominated to tensile-dominated asperity failure, which governs displacement behavior, dilatancy evolution, and the non-monotonic variation of shear strength with undulation angle. Based on the identified failure-mode transition, a shear strength model incorporating a failure-mode influence coefficient was proposed and shows good agreement with experimental results. The proposed framework provides a practical tool for evaluating fault slip potential and mitigating casing deformation risks during hydraulic fracturing in structurally complex shale reservoirs.