Experimental study on shale fault instability and reactivation induced by fluid injection

To investigate the causes of frequent seismic events in shale gas extraction areas of Sichuan basin, a series of experiments on fault instability and reactivation in Longmaxi shale induced by fluid injection were designed and conducted using the MTS 815 rock mechanics testing system. The study analyzed the processes and characteristics of fault instability and reactivation, explored the sensitivity of shale faults to fluid injection under different critical stress states, and examined the effects of normal stress and injection rate. Furthermore, the evolution of permeability before and after fault reactivation was revealed, and the mechanisms of fault activation and the causes of fluid overpressure were discussed. The results show that changes in critical stress state, normal stress, and injection rate significantly influence fault instability and reactivation. Fault sensitivity to fluid injection increases as the shear stress approaches its shear strength, enabling fault reactivation at relatively low fluid pressures. An increase in normal stress reduces the sensitivity of the fault to fluid injection, while a higher injection rate enhances it. Both factors lead to more intense fault reactivation. Additionally, the permeability of the fault increases significantly after reactivation, which can be attributed to the instantaneous shear dilation effect that enlarges the effective hydraulic aperture. Fluid overpressure is primarily caused by the heterogeneous distribution of fluid pressure along fault surface. When a continuous hydraulic pathway is formed along the fault length, the heterogeneity of fluid pressure across the fault width should be prioritized as the key mechanism triggering fluid overpressure. This study contributes to understanding the mechanisms of fluid injection-induced fault reactivation in shale gas extraction and provides theoretical guidance and scientific references for designing engineering measures to mitigate seismic risks.