A three-dimensional thermo-poroelastic model for fracture response to injection/extraction in enhanced geothermal systems

Water injection in enhanced geothermal systems sets in motion coupled poro-thermo-chemo-mechanical processes that impact the reservoir dynamics and productivity. The variation of injectivity with time and the phenomenon of induced seismicity can be attributed to the interactions between these processes. In this paper, a three-dimensional transient numerical model is developed and used to simulate fluid injection into geothermal reservoirs. The approach couples fracture flow and heat transport to thermo-poroelastic deformation of the rock matrix via the displacement discontinuity (DD) method. The use of the boundary integral equations, for the pressure diffusion and heat conduction in the rock matrix, eliminates the need to discretize the infinite reservoir domain. The system of linear algebraic equations for the unknown displacement discontinuities, and fluid and heat sources are used in a finite element formulation for the fluid flow and heat transport in the fracture. This yields a system of equations which are solved to obtain the temperature, pressure, and aperture distributions within the fracture at every time step. In this way, the temporal variation of the fracture aperture and fluid pressure, caused by pressurization and thermo-poroelastic stresses, are determined. Numerical experiments using the model illustrate the feed-back between matrix dilation, shrinkage, and pressure in the fracture. It is observed that whereas the poroelastic effects dominate the early stage of injection pressure profile and the fracture aperture evolution, thermoelastic effects become dominant for large injection times.