Geochemical reactions and pore structure analysis of anhydrite/gypsum/halite bearing reservoirs relevant to subsurface hydrogen storage in salt caverns

Salt caverns stand as one of the most suitable options for safe storage and efficient recovery of subsurface hydrogen storage, due to their favorable geologic characteristics. The geochemical reactivity of hydrogen to salt minerals is a key factor that may affect the stability and integrity of salt caverns during UHS. Pure salt minerals are chemically inert in the presence of hydrogen, however, the presence of impurities in the salt cavern might affect its integrity. Evaluation of these impurities is lacking in the literature and requires adequate investigations. In this study, we perform a comprehensive experimental investigation on the reactivity of evaporate minerals (Anhydrite, Gypsum, and Halite) with hydrogen and assess the implications on physical and pore structure properties. After 90 days of hydrogen treatment under 1450 psi at 75C, the results indicate no notable changes in the mineralogy of halite and anhydrite. Gypsum displayed a complete transformation to Bassanite mineral when treated at 75C due to the dehydration process, however, when gypsum is treated with hydrogen at room temperature, no changes in mineral composition are reported. Thermogravimetric analysis shows that the tendency of mass reduction with temperature for all samples did not change, suggesting very minor changes in the pore structure. Scanning Electron Microscope (SEM) analysis indicates high stability of all samples in the presence of hydrogen. However, a few cracks and fractures appeared in between the gypsum sheets on some spots on the surface after the hydrogen treatment, which is most likely related to the dehydration process of gypsum with temperature or the depressurizing of the sample. Nitrogen adsorption/desorption isotherms results show that the amount of adsorbed nitrogen after hydrogen treatment is similar compared to the initial conditions, implying that no major alterations in pore geometry occurred, with no evidence of pore expansion or shrinkage. Consequently, negligible alterations are reported in specific surface area and total pore volume for all samples (not exceeding 5 %), with stable pore size distribution profiles. These observations imply that the geochemical reactivity of hydrogen and salt cavern minerals is very weak within 90 days of hydrogen treatment, demonstrating high stability and integrity of salt caverns during short hydrogen storage cycles, with no major alterations in pore structure analysis.