Modeling Impacts of Fe Activity and H₂ Partial Pressure on Hydrogen Storage in Shallow Subsurface Reservoirs

Advancing underground hydrogen storage (UHS) is essential for a sustainable, emission-free future, with its success highly contingent on the unique properties of each subsurface reservoir. To ensure optimal storage, detailed site assessments are required. One of the critical gaps in knowledge necessary for ensuring safe storage is geochemical redox reactions, especially those involving iron. These redox reactions are crucial as they influence hydrogen retention or loss in the subsurface environments. In this study, we have theoretically addressed hydrogen consumption via abiotic reduction of a Fe³⁺ oxide under different Fe²⁺ activities. Simulations indicate that in scenarios, where the initial hydrogen partial pressure is extremely low (around 10⁻⁵ bars), decreasing the activity of Fe²⁺ by a factor of 10 can lead to a marked decrease in the initial hydrogen pressure by a maximum factor of 1000 within a few years. Variations in Fe²⁺ activity can significantly influence abiotic hydrogen consumption only under very low hydrogen partial pressures. This is primarily due to enhanced dissolution of Fe³⁺ oxides. In comparison, in conditions where hydrogen partial pressure is higher (> 10⁻² bars), reduction of Fe³⁺ oxide can yield magnetite, resulting in a muted loss of hydrogen over time. The transition in the reduction behavior of Fe³⁺ oxide from a ‘dissolution-driven’ process to ‘magnetite crystallization,’ which also determines the fate of stored hydrogen, depends on initial hydrogen partial pressure. Our results demonstrate that low quantities of hydrogen can be maintained within typical storage cycles spanning less than a year, depending upon aqueous Fe content.