Thermal stressing is likely to reactivate distant faults in hot sedimentary aquifers

The widespread development of geothermal energy is deemed to accelerate the transition to a low-carbon future. Hot Sedimentary Aquifers (HSA) provide cost-effective and non-intermittent geothermal resources. However, HSA development has reportedly been associated with seismic events, harming the public perception of exploiting these resources. This work digs deeper into thermo-hydro-mechanical (THM) mechanisms raised by water circulation in a HSA and their control on fault reactivation. We numerically simulate the problem by a 2D plane-strain model. The model consists of a porous and permeable hot aquifer sandwiched between the tight seal and base rocks and laterally bounded by two normal faults, representative of extensional tectonic environments. The horizontal injection-production well pairs are spaced 500 m apart at the middle of the aquifer, and the faults are located on each side of the doublet at a distance of 1 km. We consider two scenarios: low-permeability faults, mimicking a compartmentalized reservoir, and high-permeability faults, across which fluid flow takes place with further ease. We show that the fault permeability governs the hydraulic response of the reservoir. While the pore pressure slightly increases around the injector and decreases around the producer for the case of high-permeability faults, the compartmentalized reservoir experiences a global pore pressure decline. The latter is supported by the fact that the injected cold water is denser than the extracted hot water and occupies less space in the pore system. As soon as the thermal breakthrough occurs, which is after 12 years in the current setting, a more uniform temperature distribution across the doublet is established and the pressure begins to increase in the vicinity of the injector. Provided the high permeability of the reservoir rock, pore pressure and poroelastic stress perturbations impose rapid but minor effects on the fault stability. On the contrary, the cooling front formed around the injector lags much behind the pore pressure front toward the fault. The reservoir cooling contracts the rock and triggers stress reductions. Thermal stresses are transmitted much ahead of the cooled region and destabilize the fault located on the injection side. The fault begins to slip after 18 and 21 years of circulation for the high- and low-permeability scenarios, respectively. The reservoir pressure decrease in the latter case, attenuating the fault slip tendency, feeds into the observed difference in reactivation timings. Although thermal stresses initiate the slip, the static stress transfer jointly contributes to rupture nucleation along the fault. Interestingly, the slip-induced shear stress release, tied to a slip weakening frictional behavior, slows down elastic energy build-up on the other fault closer to the production well and impedes its reactivation. Our findings on the prolonged but dominant role of thermal stresses on the reactivation of distant faults have direct implications for safe and long-term production from geothermal systems.