The Frictional Restrengthening and Permeability Evolution of Slipping Shale Fractures During Seismic Cycles

The fluid injection-induced seismicity has drawn widespread concern due to the dramatic rise in seismicity rate worldwide, especially recent events associated with hydraulic fracturing operations during shale gas development. The frictional restrengthening is a prerequisite for the seismic cycles and the rate-and-state friction law is commonly used to describe the frictional behaviour of fractures and faults. However, the permeability evolution of faults/fractures during the seismic cycles remain finitely understood. In this study, we perform a series of slide-hold-slide experiments with concurrently permeability measurement to explore the frictional restrengthening, and permeability response to seismic cycles with Longmaxi shale fractures. The results indicate that even though the Longmaxi shale fractures exhibit a lower frictional healing rate than granite fractures, they still have the potential for seismic activities with a relatively lower seismic moment or low-rate creep. Similar to the in-situ observations, the Longmaxi shale fracture permeability gradually decays during the whole seismic cycle. The permeability response due to the reactivation and repose is complicated, which is largely controlled by the fracture slip history and matching conditions. Fracture permeability enhancement due to reactivation results from the shear dilation, mineral particle mobilisation, and the destruction and breaching of the fracture sealing. In contrast, permeability decay mainly results from asperity degradation. These observations highlight that the small-scale fracture surface properties may largely affect the permeability recovery and decay during seismic cycles, which provides a deeper understanding of fracture frictional behaviours and mitigating seismic risks in shale reservoirs.