为揭示四川盆地页岩气开采区地震事件频发诱因, 采用MTS815岩石力学系统设计和开展了流体注入诱发龙马溪页岩断层失稳活化试验, 分析了页岩断层失稳活化的过程与特征, 探讨了不同临界应力状态下页岩断层对流体注入的敏感性及法向应力、注水速率的影响, 揭示了断层失稳活化前后的渗透率演变规律, 讨论了流体注入激活断层机制及超压现象的触发成因。研究结果表明: 临界应力状态、法向应力和注水速率变化会对断层的失稳活化产生影响。剪切应力越接近抗剪强度, 断层对流体注入的敏感性越强, 较低的流体压力即可诱发断层失稳活化; 增大法向应力会降低断层对流体注入的敏感性, 提高注水速率会增强其敏感性, 两者的上升均会导致断层失稳活化更加剧烈。此外, 断层失稳活化后的渗透率显著增加, 这与瞬时剪切膨胀效应引起的有效水力孔径增大有关。流体超压现象主要源于断层面上流体压力的异质性分布, 当沿断层长度方向已形成完整水力通道时, 应优先考虑断层宽度方向上流体压力的异质性分布对流体超压现象的触发机制。本研究有助于揭示页岩气开采中流体注入诱发断层失稳活化机制, 并为降低地震风险的工程措施设计提供理论参考和科学依据。

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.