Effects of coal-rock interface inclination and stress difference on hydraulic fracture propagation

Coal-rock interface inclination and stress difference are key geometric and mechanical factors, respectively, that affect hydraulic fracture propagation. Investigating their effects and coupling helps elucidate mechanisms of hydraulic fracture propagation across complex coal-rock interfaces. Previous studies have mainly focused on single-factor effects on hydraulic fracture propagation. To further elucidate how interface inclination and stress difference jointly regulate hydraulic fracture propagation in coal-rock composites, hydraulic fracturing tests were conducted on composite samples with different interface inclinations using a true triaxial hydraulic fracturing test system, and fracture propagation under the coupled control of coal-rock interface inclination and horizontal stress difference was quantitatively characterized. Results showed that ① at a small inclination (15°), fractures readily penetrated the interface directly and exhibited simple propagation morphologies. An interface with a moderate inclination (30°) was most likely to induce fracture branching and deflection, producing a complex fracture network. Although an interface with a large inclination (45°) could still be penetrated by fractures, it substantially increased the risk of interfacial shear slip. ② Under a low stress difference, fracture propagation paths were random, fracture morphologies were simple, and interface-induced disturbance was weak. A complex trifurcated fracture network was more likely to form under a moderate stress difference. Under a high stress difference, fractures propagated stably and strictly along the direction of the maximum horizontal principal stress, effectively suppressing the weak-interface effect and directly penetrating the interface. ③ With increasing horizontal stress difference, fracture initiation pressure first decreased and then increased. Interface inclination had no significant effect on fracture initiation pressure, whereas propagation pressure increased with interface inclination.