Experimental Study of H₂-Brine-Shale Geochemical Interactions and Their Impact on Caprock Integrity in Underground Hydrogen Storage

Underground hydrogen storage (UHS) in depleted hydrocarbon reservoirs presents a promising solution for safe, large-scale hydrogen storage. While shale caprocks are considered effective sealing layers for UHS reservoirs, the long-term effects of hydrogen, shale, and brine interactions on caprock integrity remain poorly understood, which may impact the viability of such storage solutions. In this study, we experimentally evaluate the reactivity of shale rocks to hydrogen and examine whether geochemical interactions change the geomechanical properties of the shale, which has direct implications for caprock integrity. Shale samples from two organic-rich formations (Wolfcamp and Kreyenhagen shale formations) with high calcite content were exposed to hydrogen in the presence of 3.5 wt.% sodium chloride (NaCl) brine at 1750 psi and 120°C for 30 days in a batch reactor. We analyzed the shale using scanning electron microscopy (SEM), X-ray fluorescence (XRF), quantitative X-ray diffraction (XRD), and ultrasonic wave velocity measurements before and after hydrogen exposure to assess changes in surface morphology, microstructure, mineral composition, and dynamic elastic properties. Additionally, we compared the behavior of the hydrogen-rock-brine system to an argon-rock-brine system. The results indicate no significant impact of hydrogen on surface morphology or microstructure, suggesting limited reactivity of hydrogen with calcite-rich shale rock under the tested conditions. Furthermore, there were no apparent indications of dissolution or precipitation reactions between hydrogen and shale, nor evidence of secondary mineral formation after treatment. The measured velocities and the derived elastic properties remained almost unchanged following reaction. Comparisons with an argon-rock-brine system confirm that observed changes were due to rock-brine disequilibrium rather than reactions with hydrogen. These results support the viability of calcite-rich and organic-rich shale as a caprock in UHS, demonstrating promising seal integrity for safe hydrogen containment.