Stress perturbations and fault reactivation during cold fluid Injection - impact of hydraulic anisotropy

Cold fluid injection into a hot subsurface reservoir alters the in situ temperature, pore pressure, and stress fields through multiple interacting physical mechanisms. Poroelastic stress changes arise from pressure diffusion, whereas thermomechanical stresses are driven by reservoir cooling and associated thermal contraction. In this study, we investigate how hydraulic anisotropy in the reservoir controls the spatio-temporal evolution of these stress perturbations and related failure potential. We present results from fully coupled thermo-hydro-mechanical (THM) simulations using a three-dimensional reservoir-scale generic model, considering different injection scenarios, including single injection wells and doublets, as well as isotropic and anisotropic hydraulic properties. In general, both temperature and pore pressure variations affect the radial and tangential stress components relative to the injection site in distinct ways, even under isotropic material conditions. This distinction is critical for evaluating slip tendency and calculating Coulomb failure stress changes (ΔCFS) for the faults in the vicinity of the injection well. For anisotropic reservoir conditions, we compare the temporal evolution of pore pressure and temperature during single-well injection against isotropic reference cases and assess the implications for ΔCFS. For 20 years of continuous injection and permeability anisotropy factor of 10, the temperature front propagates approximately 20 times faster along the high-permeability direction. While the rate of pressure diffusion scales with the permeability component in the direction of propagation, the resulting pressure magnitude is governed by permeability components in the perpendicular directions. Similarly, thermally induced stresses evolve more rapidly in high-permeability directions and more slowly in low-permeability directions, as well as producing different magnitude changes in radial and tangential stress components. The modeled ΔCFS indicates that although fault orientation influences the calculated stress changes, the dominant control arises from directional fluid flow associated with hydraulic anisotropy. In conclusion, hydraulic anisotropy exerts a first-order control on the spatial and temporal distribution of pressure and temperature perturbations, leading to pronounced directional variations in induced stress fields and the corresponding Coulomb failure stress evolution in the vicinity of geothermal boreholes. These results provide a basis for optimized drill site selection and well orientation strategies aimed at minimizing fault reactivation and reducing the risk of injection-induced seismicity.