Comparison of Fatigue Hydraulic Fracturing of Granite Cores Subjected to Creep and Cyclic Injection

Earlier experiments have shown that cyclic hydraulic fracturing (CHF) systematically reduces the monotonic breakdown pressure (MBP). However, cyclic injection also causes a significantly longer injection time to failure as compared to the monotonic injection tests and complex fracture propagation that is hard to predict. In this study, a different injection scheme employing rock fatigue behavior, named creep injection, was tested on granite cylinders. The creep injection creates continuous pressurization under a constant borehole pressure (CBP) with a pre-defined maximum value below the MBP. Three different pressure ratios (CBP/MBP) of 0.85, 0.9 and 0.95 were tested. We found that both the CHF and hydraulic fracturing with creep injection can reduce the breakdown pressure by ca. 15 ~ 20% without confining pressure. Two mechanisms could explain the reduction: the influence of fluid infiltration within the theory of linear poroelasticity and stress corrosion within the subcritical crack growth theory. The lifetime of the granite cores subjected to creep injection is comparable with previous CHF experiments employing the same pressure ratio. In addition, the lifetime increases logarithmically when the ratio of CBP/MBP is decreased. This relationship has a high regression coefficient of R² = 0.97, and the lifetime can be well predicted using a stress corrosion index of 70. On the contrary, CHF shows a significantly larger variance in the lifetime with a regression coefficient of R² = 0.19 and, therefore, is hard to predict. Our results also point out that the injection scheme can modify hydraulic fracture patterns, in terms of fracture aperture, branching, and fracture propagation.