Using Molecular Dynamics Simulation to Estimate Storage Capacity in Depleted Gas Reservoirs for Underground Hydrogen Storage

Abstract The Underground Hydrogen Storage (UHS) project, a significant potential solution, not only offers clean fuel but also holds the promise of replacing traditional fossil fuels, thereby significantly reducing CO2 emissions. Subsurface geologic formations, particularly depleted gas reservoirs, have been identified as crucial geological targets for injecting and storing H2 into underground formations for CCS and UHS projects. Accurate storage capacity assessment requires estimating the amount of H2 that can be safely stored in underground formations is essential. Because H2 injection in depleted gas reservoirs can escape through caprock as the breakthrough pressure of injected gas is above the capillary entry pressure, which is described as a function of contact angles (θ) and interfacial tensions (γ) and effectively capillary (or pore) radius. While experimental measurement of γ and θ can be challenging and inaccurate under reservoir conditions, especially in high pressure, high temperature or the presence of toxic gas (H2S) or flammable gas (CH4 or H2), Molecular dynamics (MD) simulations have been conducted to accurately determine the γ and θ under extreme conditions without safety concerns. This study presents an approach of Molecular Dynamics (MD) simulations to predict interfacial tension and contact angle and investigate the effects of an H2-CH4 mixture to assess gas column height. The study indicates that the difference between MD and experimental outcomes is less than 5%. Furthermore, in the case of shale as caprock, the gas column height in depleted reservoirs reduces with increasing H2 concentration in an H2-CH4 mixture. It offers a solution to quickly evaluate the impacts of risk and uncertainty of key parameters (such as interfacial tension, contact angle or density difference) in assessing H2 column heights in depleted gas reservoirs. The advancements made in this study significantly contribute to the de-risking and safety of large-scale UHS projects, thereby instilling confidence in the successful decarbonization of the energy supply.