A Laboratory Investigation of Friction and Frictional Stability of Epidote Gouge Under Shear-Reactivation and Concurrent Fluid-Flow

Epidote is a common and abundant metamorphic mineral in geothermal systems driven by hydrothermal–rock interaction. Precipitation of epidote on natural fracture/fault surfaces potentially alters frictional resistance and stability in geothermal reservoirs and thus the potential to spawn earthquakes. Elevated pressures due to fluid-injection during hydraulic stimulation alter effective stresses and impact shear strength and the evolution of gouge porosity in a complex manner. We report concurrent fracture reactivation-flow experiments on simulated epidote gouge to illuminate key processes. Experiments are at a room temperature and normal stresses of 1–4 MPa, fluid flow rates of 3–7 ml/min and shear-reactivation velocities of 1–50 μm/s to define anticipated changes in fault frictional strength and stability under varied stresses and shearing and fluid flow rates. Friction coefficients of the epidote gouge are ~ 0.49 and insensitive to elevated normal stress or fluid flow rate but slightly impacted by shearing velocity. Increasing the normal stresses (~ 4 MPa) and fluid flow rates induce stick–slip sequences indicative of enhanced frictional instability at higher shearing velocities. Solely increasing shearing velocity at constant normal stress and fluid flow rate also destabilizes the fault—suggesting that all these parameters impact instability although frictional strength is largely unaffected. The transition to instability is indexed to a critical stress and change in porosity driven by shear-induced dilation. Our results have implications in understanding the evolution of frictional strength and stability of metamorphic mineral- (epidote-) filled faults in geothermal reservoirs and the earthquake hazard they pose.