Zechstein Halite Behavior under Hydrogen Exposure: Implications for Underground Hydrogen Storage in Salt Caverns

This pilot study investigates potential abiotic geochemical alterations associated with underground hydrogen storage (UHS) in salt caverns under conditions representative of the Zechstein formation in the Norwegian North Sea. Rock salt samples (predominantly NaCl) from the Permian Zechstein deposits were subjected to controlled experiments designed to simulate realistic storage conditions (70 ± 1 °C, 250 ± 2 bar) with both hydrogen (H₂) and argon (Ar) as reference gases. All experiments were conducted in the presence of water to account for potential brine interactions. Pre- and postexposure, the samples were analyzed using the following methods: X-ray diffraction (XRD) for bulk mineralogy and scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) for surface morphology and elemental composition. Brine and gas samples were taken at the end of the experiment and analyzed with inductively coupled plasma mass spectrometry (ICP-MS) for brine chemistry analysis and gas chromatography (GC) for gas purity assessment. The XRD results demonstrated stability of the rock salt matrix, in the analyzed samples, which consist of approximately 94% halite and 6% polyhalite (K2Ca2Mg(SO4)4·2H2O), showing no detectable phase transformations or significant alterations in crystallographic structure following exposure. However, SEM analysis revealed the formation of needlelike calcium sulfate minerals (CaSO4·xH2O) on the surfaces following H₂ exposure and more distinct well-defined minerals after Ar exposure. We attribute the formation of the calcium sulfate phase (Ca-sulfate) primarily to rapid depressurization and cooling effects, which facilitated the decomposition of polyhalite as a Ca and S source, rather than specific gas-mineral interactions. In experiments involving NaCl buffered brine, ICP-MS analyses showed that the replacement of polyhalite by Ca-sulfate within the halite (NaCl) rock matrix was accompanied by systematic changes in dissolved Ca, Mg, K, and S concentrations. Despite these surface mineralogical changes, GC analysis confirmed preservation of hydrogen purity (>99.995%). This suggests that the observed mineral changes did not impact gas quality. The study concludes that mineralogical changes can occur in halite under UHS conditions in the presence of brine, which however do not compromise H₂ purity.