Enhanced geothermal system (EGS) has the potential to offer a large amount of clean energy by extracting stored thermal energy from the subsurface. The effectiveness of heat extraction is dependent not only on the permeability of fractured rocks but also on the stability of preexisting and induced fractures. A better understanding of fracture slip in granite during fluid injection is critical to optimize the strategies of hydraulic stimulation. We experimentally investigated the shear behaviors of a sawcut fracture, a gouge-filled fracture, and a natural fracture in Bukit Timah granite in response to fluid injection under a constant normal stress and a constant critical shear stress, respectively. In the most cases, the pore pressure at the injection-induced failure exceeds that predicted by the Mohr-Coulomb failure criterion. This is attributed to the nonuniform distribution of fluid over the fracture plane, which is associated with lower permeability of fracture and host rocks and higher injection rate. The shear behaviors of sawcut and natural fractures show a complex combination of creep and stick before injection failure, which is presumably dependent on the state of asperity contacts. The gouge-filled fracture always creeps preceding the injection-induced failure, because the stiffness of testing system overweighs the critical rheologic stiffness of fracture. For these three fractures, the slip rate at injection failure increases with higher injection rate, releasing more strain energy. The slip rate induced by fluid injection in this study falls within the slip rate range of slow slip events observed in natural faults.
Harvesting heat trapped in igneous rocks offers us an affordable and sustainable solution to reduce our dependence on fossil fuels. Because of the extremely low permeability of igneous rocks, fluid injection has been used to create and/or activate fractures, enhancing the permeability of the host rocks. Besides the permeability evolution of rock fractures, the effectiveness of heat extraction is also dependent on the frictional stability of preexisting and induced fractures, because frictional instabilities of fractures result in seismic events. For example, fluid injection-induced seismicity in an enhanced geothermal system (EGS) in Basel, Switzerland led to the closure of the project (Majer et al. 2007), so did the California project in the United States also aiming at extracting underground geothermal energy (Elsworth et al. 2016). Therefore, a better understanding of the frictional stability evolution of fractures in igneous rocks is of critical importance to optimize the hydraulic stimulation strategy for EGS.
The fracture is induced to slip when the shear stress acting on the fracture exceeds the shear strength of it according to the Mohr-Coulomb failure criterion (Zoback 2010) and the effective stress law (Terzaghi 1923). When a fracture starts to slip, the slip can either be seismic or aseismic, depending on the friction rate parameter and relative magnitude of critical rheologic stiffness of the fracture and the stiffness of elastic surroundings (Marone 1998). Specifically, when the friction rate parameter is positive, the fracture is intrinsically rate strengthening and can always slip stably (aseismic), while a fracture with a negative friction rate parameter is conditionally rate weakening, tending to slip unstably (seismic) if the critical rheologic stiffness of the fracture is larger than the stiffness of elastic surroundings.
