Modelling Injection Induced Seismicity on rate and state friction faults with the Quasi-Dynamic Earthquake Simulator QDYN

Abstract

Fluid injection, as a result of geothermal projects, CO2 storage or waste-water disposal, is known to trigger seismicity. These injection induced earthquakes are a problem since they locally increase the risk on public safety and damage to infrastructure. Mitigating the risk of injection induced earthquakes has proven to be difficult, since diffusive fluid flow in the subsurface continuous after shut-in of the injection well, which can lead to the triggering of large seismic events even after termination of the project.

We developed a model to get a better insight in the role of pore pressure changes and fluid flow on fracture reactivation and seismicity as a result of fluid injection. Therefore, we implemented fluid flow functions into the quasi-dynamic earthquake simulator QDYN: a boundary element software that simulates earthquake cycles on rate and state governed faults. We created 0D spring slider models and 2D planar fault models inspired by the geothermal site of Soultz SousFor^ets, where a major fracture zone intersects the granitic reservoir rock. The spring slider model was used to perform a sensitivity analysis on the rate and state constitutive parameters (a-b), the characteristic slip distance and the effective normal stress. The effective normal stress perturbations were applied for a short period during the interseismic cycle at different times from instability with different magnitudes. The same stress perturbations are applied to the 2D fault models, including the additional model that simulated injection followed by pressure diffusion. The models were used to evaluate the impact of the time and magnitude of injection on the occurrence, magnitude and nucleation of slip events.

The model proved to be a useful tool to study injection induced seismicity on stable and unstable RSF faults, however for accurate modelling knowledge about the site-specific fault conditions such as material properties, maturity of the fault zone and stress state is required. Fluid injection early in the seismic cycle results in the occurrence of a slip event earlier than originally expected. On the contrary, injection close to instability can reduce the magnitude of the slip event, to even aseismic slip rates. Injection followed by pressure diffusion affects the stress state of a fault in such a way that the hypocenter of the slip event can shift to the rim of the stimulated zone, which may have significant effects on the intensity of subsequent ground motion.