A Fully-Coupled Fully-Implicit Flow and Geomechanics Model: Application for Enhanced Geothermal Reservoir Simulations

Abstract

In recent years, enhanced geothermal system (EGS) has taken the center stage in the international community in search of future renewable energy resources. However, there is the growing public concern on EGS-induced earthquakes. Many commercial EGS fields (Basel in Switzerland, Landau in Germany, Berlin in El Salvador, etc.) have reported increasing seismic activities once the production and injection started. As a result, earthquake risk assessment is vital for EGS developments. Fluid production and cold-water injection will alter pressure and temperature state in geothermal reservoirs. Consequently, this combined effect causes rock deformation and failure, creating underground seismic activities. Moreover, the deformation leads to the change in hydraulic properties, such as porosity and permeability, which affects fluid flow and heat transfer. To capture such physical processes, coupled effects need to be considered for the analysis of fluid flow, heat transfer, and mechanic responses. This paper presents a numerical model of a fully coupled, fully implicit flow-geomechanics model for fluid and heat flow in porous media. The simulated stress and strain can be used to perform shear slip analysis. The developed simulator is built on TOUGH2 (Pruess et al, 1999), a well-established simulator for geo-hydrological-thermal analysis with multiphase, multi-component fluid and heat flow. We employ the staggered grid system, where the flow related primary variables (p, T, S) are located at the center of simulation grid blocks and the geomechanics related variables (ux, uy, uz) are located at the edge. The numerical scheme is successfully verified against the analytical solution of Mandel and Cryer problem for transversely isotropic poroelastic media (Abousleiman et al., 1996) and against the published numerical results of a field-application geothermal reservoir simulation (Rutqvist et al., 2008). In addition, we present an application example for a 5-spot EGS model.