Underground hydrogen storage: application of geochemical modelling in a case study in the Molasse Basin, Upper Austria

Hydrogen storage in depleted gas fields is a promising option for the large-scale storage of excess renewable energy. In the framework of the hydrogen storage assessment for the Underground Sun Storage project, we conduct a multi-step geochemical modelling approach to study fluid-rock interactions by means of equilibrium and kinetic batch simulations. With the equilibrium approach, we estimate the long-term consequences of hydrogen storage, whereas kinetic models are used to investigate the interactions between hydrogen and the formation on the time scales of typical storage cycles. The kinetic approach suggests that reactions of hydrogen with minerals become only relevant over timescales much longer than the considered storage cycles. The final kinetic model considers both mineral reactions and hydrogen dissolution to be kinetically controlled. Interactions among hydrogen and aqueous-phase components seem to be dominant within the storage-relevant time span. Additionally, sensitivity analyses of hydrogen dissolution kinetics, which we consider to be the controlling parameter of the overall reaction system, were performed. Reliable data on the kinetic rates of mineral dissolution and precipitation reactions, specifically in the presence of hydrogen, are scarce and often not representative of the studied conditions. These uncertainties in the kinetic rates for minerals such as pyrite and pyrrhotite were investigated and are discussed in the present work. The proposed geochemical workflow provides valuable insight into controlling mechanisms and risk evaluation of hydrogen storage projects and may serve as a guideline for future investigations.