  Underground hydrogen storage (UHS) in geological formations is a promising
technology for large-scale hydrogen energy storage. Although lessons were
learned from similar studies, including geological carbon sequestration and
underground gas storage, the unique thermodynamic and physical properties of
hydrogen distinguish UHS from the other subsurface storage projects. We
developed a two-phase, three components reservoir simulator, which incorporated
essential physics based on the fully coupled multi-physics framework of the
Delft Advanced Reservoir Simulation (DARSim). Hydrogen rich fingers were
observed in the aqueous phase when CO2 was used as the cushion gas, because
dissolved CO2 increased brine density, leading to density-driven downward
convection which was favorable for hydrogen dissolution into the aqueous phase.
The highest purity of produced hydrogen was observed when CO2 was used as the
cushion gas, whereas using CH4 and N2 as the cushion gas was favorable for the
hydrogen production rate and mobility. This work is the first study that
utilizes an EoS based reservoir simulator to investigate hydrogen's flow
patterns and interactions with cushion gases in an underground storage system.
The developed reservoir simulation tool and research findings from this study
will be valuable to support decision making in practical UHS projects.
