A three-dimensional thermo-hydro-mechanical coupled model for enhanced geothermal systems (EGS) embedded with discrete fracture networks

Abstract

Fractures play an important role in the geothermal energy recovery from enhanced geothermal systems (EGS) involving interacted multi-physical fields. A strongly coupled thermo-hydro-mechanical (THM) model is proposed to simulate the process of the long-term geothermal production in three-dimensional geothermal reservoirs containing arbitrary discrete fracture networks (DFNs). By introducing a strong discontinuity concept into the DFN model, the aperture variation of each fracture induced by the fluid pressure, external stresses and thermal expansion in the period of production can be captured. Non-isothermal fluid flow in DFN and the local thermal non-equilibrium (LTNE) between fluid and rock matrix are formulated and coupled with the fracture deformation model. Verification is carried out against an analytical solution, followed by a sensitivity and convergence analysis concerning time step and mesh size. This approach is then applied to Habanero EGS project in Australia to evaluate the geothermal productivity and efficiency for a period of 20 years with different injection and production pressures. The results demonstrate that the proposed model is robust and effective to simulate the THM coupled process in 3D fractured reservoirs.