What can fluid-injection field experiments tell us about fault stability?

Abstract:

Fluid injections are ubiquitous in the exploitation of geoenergy resources but can cause nearby faults to slip. This reactivation is often attributed to the reduced frictional resistance accompanying an increase in fault zone fluid pressure. Once a fault has been reactivated, however, it is still unclear what controls whether the ensuing slip is seismic or aseismic and restrained to the fluid- pressurized zone or not. In this study, we investigate how injecting fluids into a well-instrumented natural fault can help us answer these questions without inducing a large-scale earthquake. We first show, using a fully-dynamic rate-and-state friction model, that the slip observed during the pressurization stage of one such field experiment is consistent with a range of models that would have led to different slip stability if injection had continued. We then restrict the range of possible models by considering the slip response during the depressurization stage of the experiment. Unlike higher-friction models, simulations with low quasi-static and dynamic friction overpredict the fault slip observed during depressurization. Our modelling reveals how optimally designed depressurization and spatially distributed monitoring could resolve fault friction and stability while suppressing earthquake nucleation at the injection site.