Combined Effects of Thermal Perturbation and In-situ Stress on Heat Transfer in Fractured Geothermal Reservoirs

We develop a new fully coupled thermo-hydro-mechanical (THM) model to investigate the combined effects of thermal perturbation and in-situ stress on heat transfer in two-dimensional fractured rocks. We quantitatively analyze the influence of geomechanical boundary constraints and initial reservoir temperature on the evolutionary behavior of fracture aperture, fluid flow and heat transfer, and further identify the underlying mechanisms dominating the coupled THM processes. The results reveal that, apart from enhancing normal opening of fractures, the transient cooling effect of thermal front may trigger shear dilations under the anisotropic in-situ stress condition. It is found that the applied in-situ stress tends to impose a strong impact on the spatial and temporal variations of fracture apertures and flow rates, and eventually affect heat transfer. The enhancement of reservoir transmissivity during transient cooling tends to be significantly overestimated if the in-situ stress effect is not incorporated, which may lead to unrealistic predictions of heat extraction performance. Our study also provides physical insights into a fundamental thermo-poroelastic behavior of fractured rocks, where fracture aperture evolution during heat extraction tends to be simultaneously governed by two mechanisms: (1) thermal expansion-induced local aperture enlargement and (2) thermal propagation-induced remote aperture variation (can either increase or decrease). The results from our study have important implications for optimizing heat extraction efficiency and managing seismic hazards during fluid injections in geothermal reservoirs.