Coupled thermo-hydro-mechanical-damage modeling of cold-water injection in deep geothermal reservoirs

Rock damage significantly affects coupled thermo-hydro-mechanical (THM) behavior in deep geothermal exploitation through changing thermal and hydrological properties of rocks. For this, a thermo-hydro-mechanical-damage (THMD) coupled model was developed to describe the coupling between rock damage and mechanical, fluid flow and heat transfer fields. The model considers rock heterogeneity, and incorporates the Mohr-Coulomb failure criterion and the maximum tensile stress criterion to evaluate shear and tensile damage. This numerical modeling methodology was first verified against analytical solutions and experimental results, and was then used to simulate the THMD coupling behavior in deep geothermal exploitation. A coupled numerical model was set up to simulate the geothermal fluids extraction and re-injection process in a reservoir at 1 km depth over a 7-year period. Rock damage was found to accelerate the propagation of cold fronts away from the injection well, and have a distinct effect on the performance of geothermal exploitation. When the rock damage was considered, the field injectivity increases by 8.4 times, the range of cooled regions increases by 18.6 times, and the vertical deformation changes by 1.2 times after 7 years of geothermal operations, compared to the scenario where it was not considered. Parametric studies have suggested that thermal contraction dominates the rock damage evolution, and that thermal-induced rock damage only occurs at a sufficiently large temperature difference between fluids injected and the reservoir. This work underscores the importance of accurately accounting for the damage effect on reservoir response during fluid injection activities that cause significant cooling of reservoir rocks.