Laboratory Study on Injection-Induced Fault Activation and Slip Behavior on Fractures with Specified Roughness in Sandstone

It is widely acknowledged in current research that underground fluid injection activities may induce earthquakes, which has become a significant factor in constraining the exploitation of underlying resources. To better understand the effects of fault properties on injection-induced fault activation and shear slip behavior, sandstone samples containing single fractures with different roughness were prepared, and fluid was injected into the critically stressed samples under constant normal stress. The results show that the critical injection pressure, as well as the fluid overpressure ratio, decreases with increasing roughness. Rough fractures present an obvious dilatancy followed by shear slip mode, while dilatancy and shear slip occur almost simultaneously for smooth fractures. All fractures undergo varying degrees of abrasion failure, but shearing-off failure appears only on the roughest fractures where the damage is severe, and the failure types are contributed to the change in permeability. Further investigation reveals that the fluid overpressure ratio depends on the fluid diffusion rate which determines the homogeneity of the fluid pressure distribution over the fracture at the given injection rate. Since the peak slip rate is in the range of slow slip events in natural faults with apparent slip velocity variation and negligible stress drop, the injection-induced instability exhibits quasi-dynamic sliding and is insignificantly correlated with roughness. In terms of energy release, rough fractures accumulate more elastic energy in the critical state, so more seismic moment is released during the injection-induced slip process.