Fluid injection in a frictional weakening fracture

Understanding the mechanism of nucleation of dynamic rupture is an important issue in seismology since it is the key factor in determining the seismic potential of pre-existing faults under long-term loadings. Furthermore, the activation of Mode II crack by means of fluid injection in a fracture is one of the mechanisms that enhance the permeability in deep geothermal reservoirs (Enhanced Geothermal Systems) whose efficacy rely on the shear-induced dilation. Since we are interested in modelling a fracture network under hydraulic stimulation, a deep understanding of the shear crack propagation induced by fluid injection is needed. Locally elevated pore pressure associated with fluid injection leads to a reduction of the fault frictional strength (product of the local normal effective stress and the slip-weakening friction coefficient) which may eventually falls below the background shear stress. As a result, a shear crack will start to propagate with an initially moderate velocity (quasi-static) as it is induced by fluid pressure diffusion. As slip accumulate, the quasi-static crack growth may become unstable due to the slip-weakening nature of friction, resulting in the nucleation of a dynamic rupture (micro-seismicity) until residual frictional strength is reached. Theoretical and numerical models now exist to simulate shear fault/fracture under fluid injection for both quasi-static (QS) and quasi-dynamic (QD) crack growth. However, a comparison between the QS and QD approaches has not been done. Therefore the question that arises is how the inertial effects affect the dynamic instability and the rupture run-out distance.