Abstract
<title>Abstract</title> <p>High-temperature reservoir thermal energy storage (HT-RTES) can support district heating networks and, if paired with large-scale variable energy production, could also serve as a backbone for the electric grid. However, the limited operational experience to date means that current understanding is largely based on numerical assessments. Here, we conducted a risk assessment of fault reactivation and induced seismicity for the planned HT-RTES project DeepStor in the depleted Leopoldshafen oil field (Upper Rhine Graben, Germany). Using a coupled thermo-hydraulic numerical model and semi-analytical stress calculations with MACRIS (Mechanical Analysis of Complex Reservoirs for Induced Seismicity), we found that injection-induced stress perturbations cause only minor fault destabilization in the vicinity of the HT-RTES doublet. Two complementary approaches were applied to evaluate fault-failure hazard. First, an unstressed fault was analyzed using the slip-tendency distribution along the fault plane. Second, a nearly critically stressed fault was considered, with emphasis on changes in Coulomb stress on the fault. Depending on the friction angle of the fault, critical slip tendencies were be reached in 10% of the simulations for a friction angle of 30° and in up to 54% for a friction angle of 20° indicating that elevated risk is mainly associated with unfavorable reservoir properties and operational conditions. In both approaches, stress-field orientation and stress gradients exerted a strong control on potential fault failure. Overall, however, the results indicate that the risk of fault reactivation and induced seismicity in the studied HT-RTES system is low.</p>