Superposition approach to understand triggering mechanisms of post injection induced seismicity

Induced microseismicity is a not-so-rare consequence of fluid injection into deep geological formations. Most seismicity occurs during injection, likely caused by overpressure. In some occasions, though, felt earthquakes of higher magnitude than those occurring during injection are triggered after stopping injection. The cause of post injection seismicity remains unclear. Thus, advancing its understanding is crucial to minimize its occurrence. Here, we study a case of hydraulic stimulation of deep geothermal systems to analyze the mechanisms that may induce or trigger co- and post-injection seismicity. Apart from the direct impact of fluid pressure increase, we acknowledge thermal effects due to cooling and stress redistribution due to fault slip. We analyze the effect of these three processes both separately and superposed. We find that preferential flow through conductive fractures or fault zones provokes pressure and temperature perturbations that result in not only heterogeneous variation of the stress field, but also highly anisotropic variations of the local stress tensor. Anisotropic variations cause stress tensor rotations that tend to stabilize some fractures, but destabilize others. Stress redistribution becomes especially acute after (local) failure because shear slip causes a significant variation of the stress field that enlarges the range of critical fracture orientations. The different response times of mechanical, hydraulic and thermal effects lead to further variations on the superposed stress field. Specifically, post-injection seismicity may occur on unfavorably oriented faults that were originally stable. During injection, such faults become destabilized by thermal and shear slip stress changes, but are held static by the superposition of pressure forcing. However, they become unstable and fail when the pressure forcing dissipates shortly after injection stops abruptly, which suggests that the magnitude of post injection seisms can be lowered by reducing flow rate slowly.