Thermal coupling may control mechanical stability of geothermal reservoirs during cold water injection

Hydraulic stimulation and geothermal reservoir operation may compromise the rock mechanical stability and trigger microseismic events. The mechanisms leading to this induced seismicity are still not completely understood. It is clear that injection causes an overpressure that reduces the effective stress, bringing the system closer to failure conditions. However, rock instability may not result only from hydraulic effects, but also from thermal effects. In fact, hydro-mechanical (i.e., isothermal) models often fail to reproduce field observations because the injection of cold water into a hot reservoir induces thermal stresses due to rock contraction. Thus, rock instability is likely to result from the superposition of hydraulic and thermal effects. Here, we perform coupled thermo-hydro-mechanical and hydro-mechanical simulations to investigate the effects of cold water injection in a fracture zone-intact rock system. Results show that thermal effects induce a significant perturbation on the stress in the intact rock affected by the temperature drop. This perturbation is likely to trigger induced seismicity in the surroundings of critically oriented fractures near the injection well. Hydro-mechanical simulations show that the behavior depends on the orientation of the faults and on the initial stress tensor. In the direction of the fractures, where the strains are more constrained, total stress increases with increasing pressure; thus, deviatoric stress increases or decreases depending on the initial stress state. The comparison between hydraulic and thermal effects shows that, when the largest confining stress acts perpendicular to the fractures, thermoelastic effects dominate and could trigger induced seismicity.