Unexpected fault reactivation during underground gas storage in depleted reservoirs: defining operational pressure bandwidths

Introduction Underground gas storage and emerging subsurface storage applications for CO 2 , H 2 , and N 2 increasingly rely on depleted, faulted reservoirs. In such settings, pressure recovery and cyclic operations may bring faults close to reactivation even when reservoir pressure remains below the initial value and within the range previously experienced during primary production. We investigate how stress redistribution caused by previous fault slip influences fault criticality during subsequent pressure recovery and storage operations. Methods We use a three-dimensional hydro-mechanical modeling framework with explicit frictional contact along faults. Pore-pressure histories are obtained from reservoir-scale flow simulations and introduced into a quasi-static geomechanical model that accounts for Coulomb friction and slip weakening. The approach is applied to a conceptual faulted reservoir representative of Upper Rotliegend storage sites in the Netherlands. A one-at-a-time screening analysis is used to identify the parameters controlling fault criticality, including compartment offset, fault dip, fault strength, reservoir stiffness, pressure imbalance between compartments, and the initial stress regime. Physically plausible combined scenarios are then defined for CH 4 , CO 2 , H 2 , and N 2 storage. Results The simulations show that slip during primary production can produce permanent stress redistribution. Subsequent pressure recovery or cycling can therefore reload the fault system along a different stress path, increasing fault criticality without exceeding the historical maximum pressure. The combined scenarios indicate that low fault friction, compartment offset, and previous activation during primary production are the most important factors contributing to renewed fault criticality during storage. Based on these results, pressure-bandwidth criteria are derived to reduce the likelihood of modeled fault reactivation in depleted, compartmentalized gas reservoirs. Discussion The results demonstrate that historical fault activation and associated stress redistribution should be considered when assessing the geomechanical integrity of depleted reservoirs used for gas or CO 2 , H 2 , and N 2 storage. Fault reactivation may become more likely during pressure recovery or cycling even when pressures remain below previously experienced levels. The proposed pressure-bandwidth criteria provide a practical screening framework for storage operations. Because the model is quasi-static, however, the criteria should be interpreted as bounds on fault reactivation potential rather than as direct predictions of seismic rupture or microseismic occurrence.