The coupling between gas flow and transport dynamics in the context of heterogeneous Underground Hydrogen Storage reservoirs

Understanding the flow and transport dynamics of hydrogen in heterogeneous porous media is crucial for advancing research on Underground Hydrogen Storage (UHS), a promising solution for the large-scale storage of renewable energy. Currently, there exist robust theoretical frameworks to predict transport properties (e.g., mixing and macrodispersion) of chemical components that are passively transported by flows, namely, where the hydrodynamic fluid properties (HP), density and viscosity, remain independent of the two primary variables (PV), pressure and mass fraction. However, gases feature a strong dependency of HP on PV that has been seldom taken into account to predict hydrogen transport dynamics in the context of UHS. Here, we investigate the impact on transport properties exerted by the non-linear relationship between PV and HP, both numerically and analytically in the context of heterogeneous UHS reservoirs. We simulate gas injection at different rates into gas- and water saturated reservoirs exhibiting a heterogeneous distribution of permeability. By considering test cases with different relationships between PV and HP, we observe and quantify the isolated effect of variability in each HP on the mixing and dispersion dynamics of the invading gas front.