Laboratory Simulation of Fracture Slip and Induced Seismicity by Fluid Injection

Shear slip of faults/fractures caused by pressurized fluid injection has been considered as an important permeability enhancement and seismicity triggering mechanism for many years. However, the underlying physics governing fault slip and induced seismicity still remains poorly understood. In this study, laboratory shearing experiments are conducted on a rough granite fracture under relevant reservoir stress conditions with concurrent acoustic emission (AE) monitoring to intestate the spatio-temporal evolution of induced AE /microseismic events in response to fracture slip by injection. The results show a sequence of aseismic-seismic-aseismic fracture motion, which correlates well with the changes of slip velocity, shear stress drop, fluid flow, and friction coefficient. It noticed that the aseismic “creep” (a deformation rate of ~10-8 m/s) is accompanied with friction strengthening, while a dynamic slip stage is dominated by friction weakening. In the seismic slip phase, early and late stage quasi-static slips with moderate slip velocity (~10-7 m/s) were observed before and after the dynamic slip (~10-4 m/s), respectively. This indicates that faults/fractures may slip in different modes (from creep, slow quasi-static slip, to fast dynamic slip) during fluid injection, and the large magnitude of seismicity tends to occur within the dynamic slip phase. The experiment showed significant flow rate/permeability enhancement due to the dilatant fracture slip as indicated by the increase of fracture normal dilation/displacement and the decrease of effective normal stress.