From Legacy Gas Fields to Hydrogen Storage: 3D Seismic-Driven Geological Modelling and Dynamic Simulation in the Northern Upper Rhine Graben

The Upper Rhine Graben (URG) has been a key area for subsurface energy activities for decades, with a focus on hydrocarbons during the mid- to late 1900s and a gradual shift toward energy transition applications such as geothermal energy and, potentially in the future, underground hydrogen storage (UHS). This study focuses on the northern URG, where legacy hydrocarbon fields and saline aquifers provide suitable subsurface structures and infrastructure for assessing the feasibility of UHS, although data coverage varies and remains limited in some areas. Some of these were converted into underground gas storage (UGS) in past and are still in use, providing operational experience. The aim is to use data from well-understood sites to forecast UHS scenarios and assess the suitability for locations with similar geological settings. Two UGS sites, Stockstadt and Hähnlein were used as analogue fields due to their comprehensive production and storage datasets to investigate their potential for future UHS. A structural model was developed using well and seismic data, followed by dynamic simulation to investigate flow behaviour under shallow reservoir conditions (depth of 300–500 m and temperature of ~29 °C) in a porous reservoir. The model was calibrated through history matching of both earlier production and UGS phases to capture aquifer dimensions and their role in pressure support and recovery, which is particularly important in this region due to the presence of an active aquifer. Based on this validated model, two hypothetical UHS scenarios were simulated using different working gas (WG) compositions: a low-H2 case (5% H2 and 95% CH4) and a pure-H2 case, while the remaining natural gas in the reservoir was considered as cushion gas (CG). The simulation results highlight the high mobility and low viscosity of hydrogen, as the pure-H2 case shows sharper production peaks, faster decline, and increased water production compared to the low-H2 case. In contrast, the CH4-rich WG provides a more stable flow behaviour and smoother production response. Over successive cycles, the hydrogen fraction in the produced gas increases, indicating the gradual establishment of the WG zone. Based on the analogue, the potential of the nearby old gas fields was assessed using map-based estimation with Monte Carlo simulation, which indicated promising conditions for UHS development.