Friction and stability of granite faults in the Gonghe geothermal reservoir and implications for injection-induced seismicity

The Gonghe Basin in northwest China has significant potential for the recovery of deep geothermal fluids. However, a large number of earthquakes were observed during stimulation by hydraulic fracturing with the maximum magnitude reaching ∼ ML 2. To understand the mechanisms of deep fault stability and the clusters of earthquakes, we recovered seven granite cores from the Gonghe geothermal reservoir and powdered them to simulate fault gouges. XRD results show that the granites are dominated by quartz and feldspar and show a high chlorite content. Triaxial shear experiments were conducted on the seven gouges to explore the frictional and stability properties at conditions typifying the depths of 2450–3600 m in the Gonghe geothermal site. Results show that the friction coefficients of the tested gouges are high and close to 0.70. At hydrothermal conditions, all gouges show slight velocity-weakening to velocity-neutral behavior, which is indicative of potentially unstable fault slip. Our results have important implications for understanding the fault stability behavior and induced seismicity during hydraulic fracturing in granite geothermal reservoirs.