Geochemical modelling on the role of redox reactions during hydrogen underground storage in porous media

Abstract

Underground Hydrogen Storage (UHS) in porous media appears to be a promising means for large-scale hydrogen storage, underpinning the full-scale of hydrogen supply chain development. Hydrogen-brine-rock interactions play an important role in hydrogen conversion and contamination during hydrogen cycling process. While the redox reaction triggered by injected H2 and pre-existing O2 is unique in UHS compared to other types of gas subsurface storage, few research have been done to understand the role of redox reactions in hydrogen solubility, pH, and fewer works have looked beyond its process on hydrogen conversion and contamination, which may affect the stored H2 purity and storage efficiency. In this context, we examined the redox reactions on hydrogen-brine-minerals (e.g., calcite, siderite, quartz and pyrite) reactions as a function of dissolved oxygen concentration (from 5.5 to 5500 ppm), temperature, and pressure through geochemical modelling using geochemical solver PHREEQC.Our results showed that increasing dissolved oxygen concentration from 5.5 to 5500 ppm resulted in negligible impact on hydrogen solubility and pH for all tested minerals. As the sensitive minerals, siderite and calcite can react with H2 through the redox process, leading up to a certain hydrogen loss at the pressure of 20 MPa, respectively. Meanwhile, quartz and pyrite are insensitive minerals to hydrogen, causing less than 0.2% hydrogen loss at the same pressure condition. Our results indicate that the mineral oxidation due to the pre-existing O2 dissolved in formation brine played a negligible role in H2-brine-rock interactions. The results also showed that carbonate minerals such as siderite and calcite may act as electron acceptors, which triggered hydrogen dissociation and thus formed a strong reduction environment based on PHREEQC geochemical database. This process likely causes measurable hydrogen loss associated with abiotic geochemical reactions for the lifetime of the underground hydrogen storage operation. Taken together, we suggest that clean standstone reservoirs will signifcantly reduce the hydrogen conversion and contamination during underground hydrogen storage from abiotic geochemical perspective.