Geochemical modeling of H2 storage in a sandstone reservoir: Sensitivity analysis of abiotic H2S and CO2 gas formation

: Hydrogen storage in geologic reservoirs is a promising technology for storing surplus renewable energy, but the H 2 gas quality may be compromised by fluid-rock-gas interactions. Using redox-decoupled, kinetically-controlled, abiotic 0D batch models, this study simulates the 30-year geochemical evolution of a sandstone reservoir (117 °C, 150 bar P total , 98 mol% H 2 ) and quantifies the temporal evolution of gas concentrations (H 2 , CO 2 , H 2 S). The models reveal two concurrent reaction regimes: initial perturbation of the pre-existing carbonate-fluid-gas equilibrium, causing CO 2 degassing (∼4.2 mol%) and siderite dissolution, and long-term pyrite reductive dissolution with secondary pyrrhotite precipitation. Leaching of Fe 2+ from Fe-chlorite enhances sulfide scavenging via pyrrhotite formation, stabilizing H 2 S (gas) at 26-28 ppm. Sensitivity analyses (40-140 °C, 5-250 bar P total , 0.001-2.0 vol% pyrite) demonstrate that the reservoir geochemistry is governed by a reaction feedback loop involving: i) sulfide release from reductive pyrite dissolution, ii) aqueous sulfide removal by pyrrhotite precipitation, and iii) secondary Fe 2+ -mobilization from ferrous minerals. An Fe-mineral buffer sequence (Fe-chlorite siderite ankerite) determines the geochemical impact on the reservoir. Siderite alteration notably increases dilution of the H 2 storage gas mixture by CO 2 . Under storage-relevant conditions ( T ≤ 120 °C, P total ≥ 50 bar), our batch models indicate H 2 S (gas) concentrations ranging from 0.5-80 ppm at H 2(gas) concentrations from 85-98 mol%. At temperatures ≤ 80 °C, the mineral reaction kinetics are slow enough to significantly reduce gas contamination, with abiotically-derived concentrations > 97 mol% H 2 . Our gas estimates are conditional, but stress the importance of non-sulfide Fe-minerals to prevent excessive H 2 S accumulation.