Temperature and fluid pressurization effects on frictional stability of shale faults reactivated by hydraulic fracturing in the Changning Block, Southwest China

A shale fault reactivated during multi-stage hydraulic fracturing in the Changning block in the Sichuan Basin, southwest China accompanied a cluster of small earthquakes with the largest reaching M_L ~0.8. We illuminate the underlying mechanisms of fault reactivation through measurements of frictional properties on simulated fault gouge under hydrothermal conditions. Velocity-stepping experiments were performed at a confining pressure of 60 MPa, temperatures from 30 to 300 ℃, pore fluid pressures from 10 to 55 MPa and shear velocities between 0.122 and 1.22 μm/s. Results show that the gouge is frictionally strong with coefficient of friction of 0.6-0.7 across all experimental conditions. At observed in-situ pore fluid pressure (30 MPa), the slip stability response is characterized by velocity strengthening at temperatures of 30-200 ℃ and velocity weakening at temperatures of 250-300 ℃. Increasing the pore fluid pressure can increase values of (a – b) at temperatures ≥ 200 ℃, narrowing the temperature range where velocity weakening occurs. At the in-situ temperature (90℃), the simulated gouge shows only velocity strengthening behavior and aseismic slip at elevated pore fluid pressures, contrary to the observed seismicity. We postulate that the aseismic slip at elevated pore fluid pressures may trigger seismicity by activating adjacent earthquake-prone faults.