Understanding hydrogen reactivity with anhydrite in underground hydrogen storage

With increasing global demand for a sustainable and efficient energy source, hydrogen has established its potential with natural presence in geological rocks, high combustion efficiency and lower carbon foot print. But ensuring safe storage and availability of such inflammable and low-density energy source has proven problematic and has acquired the attention from the researchers. Underground Hydrogen Storage (UHS) in geological formations offers a promising solution for a safe, large-scale and long-term renewable energy integration. These natural storage sites can offer cheap, strong and readily available alternatives for addressing renewable energy intermittency and can easily overcome seasonal supply constraints. However, the reactivity of hydrogen with reactive minerals present in these mineral rich geological formations requires careful attention. This study investigated the geochemical reactions between hydrogen and a common storage/seal rock forming mineral anhydrite (CaSO₄) in presence of calcite (CaCO₃), under stimulated UHS conditions (120-500 °C, 500 psi) to evaluate possible temperature and mineral dependent hazards. Batch reactor experiments with brine and variable calcite concentrations (1%, 5%, & 50%) reveal critical thresholds. The results showed that there is no toxic H₂S produced at 120 °C even with calcite buffering suggesting low reactivity in shallow reservoirs. At 250 °C and 500 °C, H₂S constituted from ~ 18% to ~ 45% of the observed gas phase respectively, suggesting activation of thermochemical sulfate reduction (TSR) or abiotic sulfate reduction (ASR) at higher temperatures. Prescence of high calcite content (50%) at 120 °C triggered anhydrite to gypsum hydration (40% conversion), reducing reservoir integrity. X-ray diffraction results show no intermediate phases (bassanite) present in the residues after experiments suggesting rapid gypsum crystallization. These findings highlight that the geological formations with anhydrite mineral as a major constituent are viable for UHS at low temperatures (< 120 °C), while high temperature sites (> 200 °C) can risk H₂S contamination, H₂ loss and reservoir integrity.