Modeling the effects of in-situ conditions on induced seismicity

Abstract

Fluid injection into deep geological reservoirs may induce seismicity. For example, wastewater injection in the US mid-continent has recently called the public's attention by causing several M>5 earthquakes. It is proposed that the injection leads to a pressurization of the reservoir and adjacent fault zones that can be reactivated if sufficiently high pressures are reached. However, to date it is not fully understood how exactly different in-situ pressure conditions affect the timing and size of the induced earthquakes. In this study, we therefore investigate the effect of varying initial pore pressure conditions on fault reactivation focusing on the timing and the size of induced earthquakes. Our results suggest that an initially overpressurized reservoir may lead to an earlier fault reactivation than a reservoir with hydro-static initial conditions, but the size of the induced earthquake does not differ significantly. Compartmentalization also leads to a similar size of the induced events, but delays the time of reactivation. We further test how the injection rate affects the induced earthquakes, and we find that a reduction in injection rate substantially delays the earthquake, but leads to a larger magnitude.