A reactive thermo-poroelastic analysis of water injection into an enhanced geothermal reservoir

• A thermo-poro-chemo-mechanical model is developed to study injection and production in EGS.
• Undersaturated-cold water injection dissolves silica from rock matrix, increasing the fracture aperture.
• Natural fracture stiffness heterogeneity develops a non-uniform flow path within the fracture.
• Higher joint normal stiffness areas show lower aperture increases by poro-thermoelastic processes, but a higher aperture expansion by dissolution.

Coupled thermo-poro-chemo-mechanical processes in geothermal systems impact the reservoir response during injection and production procedures by affecting fracture permeability. A three-dimensional numerical model is presented to analyze these processes during fluid injection into geothermal reservoirs. The solid mechanics aspect of the problem is computed using the displacement discontinuity boundary element method (BEM) while transport processes within the facture are modeled using the finite element method (FEM). The FEM and BEM formulations are integrated to set up a system of equations for unknown temperature, pressure, concentration, and fracture aperture. The fluid diffusion, heat conduction and solute diffusion in the reservoir are treated using BEM so that the need of infinite reservoir domain discretization is eliminated. The numerical model is used to analyze the fracture response to non-isothermal reactive flow in EGS. Numerical examples of SiO2 undersaturated-cold water injection into the geothermal reservoir show that silica dissolves from the rock matrix, increasing the fracture aperture. The zone of silica dissolution spreads into the fracture with continuous fluid injection. At large injection times, thermoelastic stress has a greater impact on fracture aperture compared to poroelastic stress. Simulations that consider natural fracture stiffness heterogeneity show the development of a non-uniform flow path within the crack, with lower rock matrix cooling and thus enhanced silica reactivity in the high stiffness regions. As a result, areas of higher joint normal stiffness show lower aperture increases in response to the thermo-poroelastic processes, but a higher aperture expansion due to silica dissolution. Depending on the injectate saturation state with respect to quartz, silica is added or removed from the rock matrix. This process is likely to impact the rock matrix properties and its mechanical response to stress perturbations associated with fluid circulation.