Phase‐Field Modeling of Borehole‐Pressure‐Induced Fracture Propagation in Coal‐Rock Composites Considering Compressive‐Shear Failure and Interface Effects

ABSTRACT Pressure‐induced fracture propagation in coal‐rock composites is strongly governed by in‐situ stress, material heterogeneity, and interlayer interfaces. This study develops a dual phase‐field model that incorporates tensile fracture, compressive‐shear failure, and coal‐rock interface diffusion. The model combines the Mohr–Coulomb criterion with a triple‐shear‐energy criterion and is validated against uniaxial compression tests on coal‐rock specimens containing pre‐existing flaws. Parametric simulations are performed to examine the effects of in‐situ stress difference, nominal borehole‐pressure loading, interface strength ratio, and its coupling with the coal‐rock strength ratio. Increasing stress difference suppresses branching and promotes a dominant crack confined within the coal seam. Stronger borehole‐wall loading enlarges the fracture initiation zone and improves fracture connectivity. Interface strength controls crack deflection and penetration, while its effect depends nonlinearly on the coal‐rock strength ratio. These results clarify fracture‐interface interaction mechanisms and provide mechanistic insights relevant to hydraulic‐fracturing design in heterogeneous coal‐rock reservoirs.