Geochemical Analysis of Underground Hydrogen Storage in a Saline Carbonate Aquifer

Abstract Underground hydrogen storage (UHS) is an emerging technology storing excess hydrogen produced (H2) in the subsurface, including porous media reservoirs, to meet energy demand during peak season. Currently, majority of research is focused on sandstone saline aquifers and salt domes for hydrogen storage. Some challenges associated with carbonate reservoirs include complex reservoir rock mineralogy, high brine salinity (>100,000 ppm), high reservoir temperatures (40 to 150 °C), and extreme permeability/porosity heterogeneity, including natural fractures. This study is based on numerical simulation of a saline carbonate aquifer in the United States for H2 storage. A single porosity 3D compositional reservoir model of an actual carbonate formation was built. A four-component fluid system (H2, CH4, CO2, and H2O) was modeled using the Peng-Robinson Equation of State (PR EOS). The PVT model was calibrated based on published measured data. The reservoir was initialized as a saline aquifer, with a horizontal well to inject and produce H2. Ten cycles of H2 injection and production over five years were simulated. The working gas was estimated to be ∼10 Bscf. Advanced microscale models, such as aqueous diffusion and geochemical reactions, were modeled to understand their impact on produced H2 purity and recovery efficiency. For geochemical reactions, dolomite, calcite, and anhydrite were considered as primary reservoir minerals. The results indicated a high H2 recovery of 93%. Additionally, loss of H2 due to geochemical reactions was ∼0.6% over the period of 10 years. H2 was observed to be 100% pure. The average reservoir pressure stabilized between 3000-3500 psi throughout H2 injection and production cycles. In addition, geochemical reaction showed that precipitation of minerals was dominant as compared to dissolution, with an insignificant reduction in rock porosity that may be due to the alkaline nature of brine and no change in well productivity/injectivity was observed because of mineral precipitation. In conclusion, after simulating the feasibility of UHS in a saline carbonate aquifer, the results proved that it seems feasible to store large volumes of H2 with high recovery and purity in these reservoirs. Our future work includes additional minerals in the geochemical model, biological reactions, and grid resolution sensitivity in the simulations.