Subsurface Hydrogen Storage in Limestone Rocks: Evaluation of Geochemical Reactions and Gas Generation Potential

Underground hydrogen storage (UHS) in carbonate reservoirs is a suitable solution for safe storage and efficient hydrogen recovery during the cycling process. The uncertainties associated with potential geochemical reactions between hydrogen, rock, and brine may impact the long-term containment of produced hydrogen in carbonate formations. Despite the current interest in studying hydrogen-rock reactions, only limited work is available in the literature. In this study, we experimentally evaluate the reactivity of carbonate rocks to hydrogen and address the potential of gas generation induced by geochemical reactions. Limestone samples are treated with hydrogen under 1500 psi and 75 degrees C temperature for a duration between 6 and 13 months using simple reaction cells. Scanning electron microscopy (SEM) analysis is performed to examine the potential dissolution/precipitation reactions induced by hydrogen. In contrast, gas chromatography (GC analyzer) analysis and inductively coupled plasma optical emission spectroscopy (ICP-OES) are conducted to detect gas generation and ion precipitation. The experimental results indicate no significant impact of hydrogen treatment on surface morphology and pore structure even after 6 months of treatment, suggesting that abiotic reactions in carbonate rocks are unlikely to occur during the first stages of UHS. Furthermore, in the presence of brine, there are no apparent indications of geochemical reactions occurring between hydrogen and calcite, and no traces of any other gases are detected after 13 months of treatment. Besides, the solutions' pH remains almost unchanged, with a minor increase in calcium (Ca2+) ions in the solution, which is attributed to the presence of water, not hydrogen reactions. The results of this work promisingly support the utilization of carbonate reservoirs for long-term hydrogen storage.