Mineral stability and porosity dynamics in Halite- and Kainite-bearing rocks after hydrogen batch reaction test: a case study of Realmonte mine, Sicily (Italy)

Giusy Anzelmo , Manuela Rossi, Abner Colella, Katriona Edlmann , Andrew Kilpatrick, Rosanna Maniscalco, Alessandra G. Pellegrino, Rosanna Rizzi, Giovanni Mario Cella, Luca Visconti, Claudia Di Benedetto, Maria Portarapillo, Almerinda di Benedetto, DaVID Iacopini

Underground hydrogen storage in salt caverns is among the most mature geological storage options, yet the behaviour of potash-salt lithologies under hydrogen exposure remains entirely uncharacterised. This study presents a multi-technique characterisation, performed on selected representative samples, of Messinian halite and kainite-bearing evaporites from the Realmonte mine (Sicily) subjected to static hydrogen exposure (10 MPa H2, 120 h) and to cyclic loading under constant hydrogen pressure (0.789 MPa) with confining stress cycled between 3.8 and 19.2 MPa, at room temperature. Samples were analysed before and after H2 exposure using X-ray powder diffraction, whole-rock and trace-element geochemistry, optical and scanning electron microscopy, laboratory and synchrotron micro-CT, mercury intrusion porosimetry and PHREEQC thermodynamic modelling. No mineralogical reactions or neoformation of phases were detected in either halite or kainite, but localised microstructural reorganisation, including dissolution-reprecipitation of halite and kainite, brine migration, and pore-size redistribution, is observed in post-exposure samples. These results extend to a hydrated sulphate-chloride double salt the geochemical-stability evidence previously documented for Zechstein, Lotsberg, and Eocene halites. Micro-CT three-dimensional imaging reveals that pore architecture is governed by mineralogical heterogeneity: pure halite hosts a connected grain-boundary fracture network (φ_CT = 2.13%) and the kainite-halite assemblage hosts a nanometre-scale grain-boundary pore system (φ_CT = 0.106%; MIP modal diameter 5.5 nm). Post-exposure MIP data show systematic pore-size redistribution driven by brine migration and grain-boundary rearrangement without significant changes in bulk porosity. Cyclic hydrogen injection tests on an impure halite-kainite sample yield permeabilities of K = 3.48 x 10-21 m2 (seasonal cycling) and K = 2.46 x 10-20 m2 (monthly cycling) within or slightly above the range reported for intact rock salt and three to four orders of magnitude below tightness thresholds proposed for gas-storage caverns. These results provide the first experimental evidence that hydrogen acts as a physical stress agent rather than a chemical reactant, suggesting a similar behaviour to pure halite evaporites.