Microbial H2S generation in large-scale aquifer gas storage: Multiphase reactive transport insights for potential biomethane-natural gas blend

Underground storage of renewable gases in deep aquifers is a promising pathway for large-scale, low-carbon energy storage. A key question is the generation of hydrogen sulfide (H2S) from microbial and geochemical processes, which may compromise gas quality and affect wells and reservoir integrity. In this study, we develop a coupled multiphase reactive transport model with the HYTEC code to establish a reference case based on natural gas storage in a deep sandstone aquifer. Microbial sulfate reduction is described using a modified Monod-type approach that incorporates water availability, calibrated against experimental data. Simulations of long-term seasonal injection and withdrawal cycles reveal heterogeneous H2S distributions: H2S accumulates in the deeper parts of the reservoir, constrained by the gas-aquifer interface. Maximum H2S concentration reached at the end of withdrawal initially rises but eventually plateaus as dissolved sulfate becomes depleted in the gas-dominated zone. Mineralogical composition exerts a strong control: gypsum sustains microbial activity and drives high H2S levels, while iron-bearing minerals promote abiotic trapping through sulfide precipitation. Acid gases, particularly co-injected CO2, lower pH and enhance H2S exsolution, whereas carbonate dissolution provides pH buffering. A sensitivity analysis shows that de- pressurization effects are negligible at our modeled reservoir depth (1100 m), but become critical in shallower systems where pressure drops below 50 bar. This baseline gas storage case provides a mechanistic understanding of microbial H2S generation and its controlling factors. A forthcoming companion study will extend this framework to biomethane injection, allowing evaluation of compositional effects and the specific challenges of renewable gas storage.