Investigating rock alterations in underground hydrogen storage: A geochemical and geomechanical baseline study

Underground Hydrogen Storage (UHS) in saline aquifers and depleted gas reservoirs has the potential for longterm energy storage. However, numerous challenges and barriers exist that prevent this technology from becoming a widely available decarbonization solution. The UHS process comprises multiple cycles of injection and withdrawal. These cycles may elevate the risk of fracturing or seal integrity failure in certain reservoirs. Furthermore, it is crucial to assess the potential alteration of rocks caused by hydrogen exposure to understand its impact on the mechanical properties of the reservoir rock. In this study, we conducted static reactivity tests on cores of three different lithologies (sandstone, shale, limestone) at 3.45 MPa for a duration of 30 days for shale and limestone, and 90 days for sandstone. The rock samples were dry. The aim was to establish a baseline of how the geomechanical properties of these rocks could change when exposed to hydrogen. Our findings indicate that for dry rock samples, hydrogen exhibits low reactivity with the rock, resulting in minor alterations in petrophysical properties. Only shale cores showed significant petrophysical changes: a 50 % increase in porosity and a double increase in permeability. Minor compositional changes were observed in all the samples, which were determined through X-ray diffraction (XRD) analysis before and after hydrogen exposure. Additionally, geomechanical tests were conducted to assess changes in Young's modulus, yield point, and maximum stress. Both sandstones and shales exhibited similar Young's moduli before and after testing. However, the limestones showed a significant increase in their Young's moduli, around 80 %, indicating increased rigidity or stiffness. After exposure to hydrogen, no major mechanical changes were seen in sandstones, but small changes in the unconfined compressive strength (UCS) were observed in shales, around 17 %. Finally, the UCS in carbonate was almost the same, but the Young's modulus increased. The findings from this research provide baseline data that can be used to compare future experiments with hydrogen exposed to different rocks and conditions typical of the subsurface.