Experimental study of fatigue load effect on the hydraulic fracturing behavior of granite

Rock masses around fluid injection projects are usually subject to complex stress states, including hydraulic pressure, in situ stresses, and fatigue loads. Thus, a series of hydraulic fracturing experiments were performed on granite to simulate such stress states using the true triaxial dynamic testing system. Computed tomographic (CT) imaging was performed to identify the fatigue effects on hydraulic fractures. The results indicate that the increasing amplitudes of cyclic load applied on the minimum principal stress direction will change fracture initiation pressure and generate nonplanar and narrow fractures. When the disturbance direction was changed to the intermediate principal stress, the higher amplitude corresponds to the lower breakdown pressure and the shorter pressurize duration and leads to wider fractures. With the increasing disturbance frequency, complex fatigue cracks were generated, which might weaken the rock strength. The present experiments can enhance the understanding of the hazard of fatigue loads on hydraulic fracturing.