Influence of Anisotropic Texture on Fracture Toughness of Shale Rocks under Cyclic Loading during Drilling

Abstract

Shale rocks exhibit strong mechanical anisotropy due to their layered texture, which significantly influences fracture behavior under the cyclic loading conditions encountered during drilling. This study investigates the effect of anisotropic texture, characterized by bedding orientation angle β, on the Mode I fracture toughness degradation of shale subjected to cyclic loading. Notched semi-circular bend specimens were prepared at five bedding angles (β = 0°, 30°, 45°, 60°, and 90°) and tested under sinusoidal cyclic loading at 5 Hz, simulating bit-rock interaction and drill-string vibrations. Static baseline fracture toughness showed pronounced anisotropy, decreasing from 1.68 MPa·m 1/2 at β = 0° to 0.71 MPa·m 1/2 at β = 90°. Under cyclic loading, toughness degradation was markedly accelerated in bedding-parallel and oblique orientations, reaching up to 61% reduction after 10 4 cycles at β = 90°, compared to only 23% at β = 0°. Crack paths transitioned from transgranular crossing to inter-laminar delamination with increasing β, accompanied by higher acoustic emission activity and stiffness loss. These findings reveal that weak bedding planes act as preferential paths for fatigue damage accumulation. The results provide new insights for optimizing drilling parameters and improving wellbore stability predictions in anisotropic shale formations.