Fully coupled conductive–convective thermo-poroelasticity modeling of pore–fracture systems in geological reservoirs

In deep geothermal reservoirs, primary pores and hydraulic fractures coexist in hot dry rock and have different properties in terms of compressibility, permeability, and heat conductivity. During geothermal energy utilization, fluid mass leakage occurs between the pore space and fractures. Recent studies have shown that pore fluid and fracture fluid have completely different values of flow velocity, fluid pressure, and temperature, which would significantly affect reservoir stability and utilization efficiency. In this study, new constitutive equations based on local thermal non-equilibrium (LTNE) theory are derived for a pore–fracture system in saturated porous media incorporating heat convection and fluid leakage. A fully coupled conductive–convective thermo-poroelastic model is then proposed to describe the thermo–hydro–mechanical response of a saturated dual-porosity formation, incorporating thermo-osmosis effect and local temperature differences between different phases. By adopting generalized plane strain theory and the fully implicit method, a novel finite element procedure based on the proposed model is developed to simulate the spatiotemporal geomechanical response of a dual-porosity formation due to cold water injection. Numerical results show that fluid pressure and temperature variations in pore and fracture spaces are distinctly different in regions near wellbores, which will change the distributions of effective stresses and could not be predicted by thermo-poroelastic models of single-porosity materials. Finally, sensitivity analysis is undertaken to determine how fluid leakage, heat transfer coefficients, and convection impact the response of the formation. The developed model can be applied to simulate and perform stability analyses of geothermal reservoirs with pore–fracture systems.