The effect of fluid pressure on frictional stability transition from velocity strengthening to velocity weakening and critical slip distance evolution in shale reservoirs

Seismic activities have been reported by the large-scale fluid injection in shale reservoirs both during hydraulic fracturing operations and wastewater disposal processes. Fluid overpressure has been regarded as the primary cause for the injection-induced seismicity since the fluid lubricates the fault and decreases the effective normal stress applied to the pre-existing faults. However, how fractures/faults slip after the activation remains unclear. The rate-and-state friction law has been widely used to describe the fracture stability during slip. Hence, we performed a series of velocity-stepping slip experiments under various combinations of fluid pressure and normal stress states with shale samples, which aims to investigate the role of fluid pressure on the rate-dependent parameter (a–b) and critical slip distance (Dc) evolution. We observed the frictional stability transits from velocity strengthening to velocity weakening with the increase of fluid pressure in shale samples. Moreover, the critical slip distance increases dramatically due to the fluid pressure increases, which is the result of the fluid oscillation phenomenon. Through the calculation of critical fracture rheologic stiffness of shale samples under fluid pressure, the results indicated that a higher possibility for fluid injection-induced seismicity with the increase of fluid pressure. Our experimental observations suggest that the fluid pressure can change the frictional stability characteristics of shale fractures and favor the potential seismic slip, which could be a possible mechanism for the fluid injection-induced seismicity, especially in unconventional shale reservoirs.