Seismic activities have been reported by the large-scale fluid injection in shale reservoirs both during hydraulic fracturing operations and wastewater disposal processes. Fluid overpressure has been regarded as the primary cause for the injection-induced seismicity since the fluid lubricates the fault and decreases the effective normal stress applied to the pre-existing faults. However, how fractures/faults slip after the activation remains unclear. The rate-and-state friction law has been widely used to describe the fracture stability during slip. Hence, we performed a series of velocity-stepping slip experiments under various combinations of fluid pressure and normal stress states with shale samples, which aims to investigate the role of fluid pressure on the rate-dependent parameter (a–b) and critical slip distance (D_c) evolution. We observed the frictional stability transits from velocity strengthening to velocity weakening with the increase of fluid pressure in shale samples. Moreover, the critical slip distance increases dramatically due to the fluid pressure increases, which is the result of the fluid oscillation phenomenon. Through the calculation of critical fracture rheologic stiffness of shale samples under fluid pressure, the results indicated that a higher possibility for fluid injection-induced seismicity with the increase of fluid pressure. Our experimental observations suggest that the fluid pressure can change the frictional stability characteristics of shale fractures and favor the potential seismic slip, which could be a possible mechanism for the fluid injection-induced seismicity, especially in unconventional shale reservoirs.

据报道，在水力压裂作业和废水处理过程中，向页岩储层中进行大规模的流体注入都引起了地震活动。流体超压已被认为是注入引起地震作用的主要原因，因为流体润滑了断层并降低了施加于先前断层的有效法向应力。但是，尚不清楚激活后骨折/断层如何滑动。速率和状态摩擦定律已被广泛用于描述滑移过程中的断裂稳定性。因此，我们在页岩样品的流体压力和法向应力状态的各种组合下进行了一系列速度步进滑动实验，旨在研究流体压力对速率相关参数的作用（a – b）和临界滑移距离（D _c） 进化。我们观察到随着页岩样品中流体压力的增加，摩擦稳定性从速度增强过渡到速度减弱。此外，由于流体压力增加，临界滑动距离急剧增加，这是流体振荡现象的结果。通过计算流体压力下页岩样品的临界断裂流变刚度，结果表明随着流体压力的增加，流体注入引起的地震活动的可能性更高。我们的实验观察表明，流体压力可以改变页岩裂缝的摩擦稳定性特征，并有利于潜在的地震滑动，这可能是流体注入引起的地震活动的可能机制，尤其是在非常规页岩油藏中。