Dynamic Hydro-geomechanical Simulation of Earthquakes Induced by Fluid Injections in Geothermal Reservoirs

Abstract

This paper describes development of a coupled hydro-geomechanical method to simulate earthquakes induced by subsurface fluid injection using two available open-source codes. First, the TOUGH2 multiphase flow code is used to model fluid flow under high injection pressure through fractured porous rock to calculate time-dependent changes in the fluid pressure field. Second, at each time step through the period of injection the perturbed pressure field is passed to the geomechnical code PyLith to model nucleation of slip due to reduction in effective normal stress on faults and fractures under constant-rate tectonic shear loading, and subsequent dynamic earthquake rupture and elastic wave propagation. The evolution of fault slip and earthquake nucleation and arrest are modeled using a rate-and-state fault friction law. Resulting coseismic dynamic and static stress changes are redistributed throughout the model domain, and changes in porosity and permeability are passed back to TOUGH2 to be used to calculate the pressure field for the next time step. Evolution of the stress and fluid pressure fields over the injection time period enables simulation sequences of induced events. We demonstrate initial application of the coupled methodology to simple synthetic enhanced geothermal stimulation scenarios incorporating realistic injection pressure and rate histories and a few earthquake source faults.