Modeling of fluid-induced seismicity during injection and after shut-in

Abstract

We develop a fully coupled hydro-mechanical model to simulate fault slip due to fluid injection. We consider the interaction between a hydraulic fracture and pre-existing faults as well as the fluid exchange between the fracture/fault and the porous matrix. In order to consider a pressure diffusion mechanism, we set a relatively high permeability around the stimulated path. Our parametric study shows that a couple of factors affect the fault activation and its slip behavior such as fault properties, friction properties and injection scenario. We observe that pore pressure diffusion induces poroelastic stress change, which are able to produce shut-in events with a time and space lag. This mechanism also affects the slip behavior during injection in particular when the surrounding permeability is high (e.g., up to 1e-13 m2/s), and provides a new insight into understanding the occurrence of stronger seismic events after shut-in compared to the injection phase. In addition, we show that small perturbations may trigger large seismic fault slip which highlights the key role of the initial fault stress state. The results have profound implications for deep fluid injection related engineering as well as for soft cyclic injection strategies aiming to mitigate the risk of large earthquakes.