| Abstract |
Several case studies of injection-triggered seismicity have suggested that the largest seismic events often occur in crystalline basement rock. In geothermal settings, the target reservoir is typically located in this type of rock. In wastewater disposal settings, crystalline basement rock is often considered a hydraulic barrier beneath the brine aquifer targeted for injection. Because earthquake magnitude is directly proportional to the surface area of the fault available for slip, it may not be surprising that the most significant earthquake activity is associated with the relatively large faults that can develop at greater depths in basement rock. In this paper, we present a preliminary investigation of the physical mechanisms related to fluid injection in the subsurface that can affect slip on basement faults. We used a numerical model that coupled fluid flow in discrete faults and surrounding matrix rock, geomechanics, friction evolution, and permeability evolution to simulate earthquake behavior for a fault that had a direct hydraulic connection with an injection well. We were primarily interested in understanding the behavior following shut-in of the injection well. The numerical results indicated that it is very important to consider fluid leakoff from the fault zone into the surrounding matrix rock in order to more fully understand post shut-in seismic response. For the particular fault configuration modeled in this study, it was observed that significant post shut-in seismicity occurred for matrix permeabilities less than 10 μd. This type of post shut-in seismicity was attributed to pressure redistribution in the fault. Conversely, for matrix permeabilities greater than or equal to 10 μd, no significant post shut-in seismicity was observed. In the latter cases, fluid leakoff from the fault zone into the surrounding rock encouraged pressure dissipation within the fault, which overcame pressure redistribution effects within the fault zone. These results suggest that if significant post shut-in seismic activity is observed at a field site, then it is important to consider the contrast in permeability between the fault zone and matrix rock in order to determine whether pressure redistribution or some other mechanism (for example, thermoelasticity) is the dominant mechanism contributing to seismicity. |