Understanding the response of faults to the injection of high-pressure fluids is important for several subsurface applications, for example, geologic carbon sequestration or energy storage. Lab-based experiments suggest that fluid injection can activate fault slip and that this slip can lead to increased fluid transmission along low-permeability faults. Here we present in situ observations from a cross-borehole fluid-injection experiment in a low-permeability shale-bearing fault, which show fault displacement occurring before fluid-pressure build-up. Comparing these observations with numerical models with differing permeability evolution histories, we find that the observed variation in fluid pressure is best explained by a change in permeability only after the fault fails and slips beyond the pressurized area. Once fluid migration occurs along the fault as a result of slip-induced permeability increase, the fault experiences further opening due to a decrease in the effective normal stress. We suggest that decoupling of fault slip and opening, leading to a rapid increase in fluid pressurization following the initial fault slip, could be an efficient driver for fluid migration in low-permeability faults.

了解断层对高压流体注入的响应对于几种地下应用非常重要，例如地质碳封存或能量储存。基于实验室的实验表明，流体注入可以激活断层滑动，并且这种滑动可以导致沿着低渗透断层的流体传输增加。在这里，我们展示了在低渗透性页岩断层中进行的跨钻孔流体注入实验的现场观测结果，表明断层位移发生在流体压力形成之前。将这些观察结果与具有不同渗透率演变历史的数值模型进行比较，我们发现观察到的流体压力变化最好用渗透率变化来解释，只有在断层破裂并滑出受压区域后。一旦由于滑动引起的渗透率增加而沿断层发生流体运移，由于有效正应力的降低，断层会进一步张开。我们认为断层滑动和张开的解耦，导致初始断层滑动后流体加压的快速增加，可能是低渗透断层流体迁移的有效驱动因素。