MODELING GAS-BRINE-MINERAL REACTIONS DURING UNDERGROUND HYDROGEN STORAGE

Hydrogen (H₂) is a potentially carbon-neutral and versatile energy carrier. Underground hydrogen storage (UHS) offers a solution for large-scale energy storage, tackling the issues of location dependence, fluctuation, and forecasting inherent in renewable energy sources. To increase renewable energy storage capacity from GWh to TWh, sites with large capacity and easy accessibility, such as depleted natural gas reservoirs, need to be investigated. Due to its highly reactive nature, hydrogen can drive redox reactions involving electron transfer between itself (as the reductant) and both subsurface minerals and aqueous species (as the oxidants). These reactions can result in gaseous hydrogen loss and alter the storage capacity and injectivity of the reservoir through mineral dissolution and precipitation, requiring models that consider three-phase (i.e., gas-brine-mineral) reaction networks. To model reactive transport phenomena linked to H₂(g) loss under diverse geological conditions, we first conducted time-dependent batch simulations of H₂(g) loss in the subsurface using PHREEQC. The static and kinetic equilibrium reaction results were then integrated into a multiphase reactive transport model, PFLOTRAN, providing a complete simulation of H₂(g) loss under realistic injection and storage conditions. The resulting suite of model simulations reflects an abiotic reaction network among H₂(g) and common subsurface minerals in the H₂-H₂O-CO₂-SiO₂-Ca-Al-Fe-S system. The formation of toxic gases from the reactions, such as hydrogen sulfide (H₂S(g)), was also examined. The results indicate that the reduction of Fe(III)-bearing minerals, such as hematite (Fe₂O₃) and goethite (FeOOH), is the primary control on subsurface H₂(g) loss. Quartz-rich sandstone reservoirs could be favorable for hydrogen storage due to their small reactivity with H₂(g). In addition, hydrogen dissolution in brine has a small impact on overall storage efficiency. Finally, we performed local and global sensitivity analyses to identify the key parameters influencing subsurface H₂(g) loss and H₂S(g) formation during storage. This study integrates reactive transport modeling with sensitivity analyses to enhance the safe and efficient implementation of large-scale underground hydrogen storage and provide critical insights into calculating the overall round-trip efficiency of hydrogen from generation to utilization. The findings from these sensitivity analyses highlight critical areas for future experimental studies.