hide
Free keywords:
Cyclic hydraulic fracturing · Creep injection · Time-delayed failure · Acoustic emission monitoring · Fluid
infiltration · Fracture pattern
Abstract:
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.
