Numerical studies of hydrogen buoyant flow in storage aquifers

Understanding buoyancy-driven flow is essential for ensuring the safety, reliability, and efficiency of underground hydrogen storage in saline aquifers. In this study, we develop a three-dimensional (3D) numerical model of an aquifer for hydrogen storage, with an isothermal system at 323.15 K, to investigate the behavior of hydrogen buoyant flow. We perform a series of sensitivity simulations to examine the influence of storage formation heterogeneity, including Dykstra-Parsons coefficient, autocorrelation length, and permeability anisotropy, as well as fluid-rock interaction parameters, such as capillary pressure and relative permeability, on storage efficiency. We evaluate the efficiency of geologic storage containment by quantifying a defined metric "escape ratio," which is the mass fraction of hydrogen in a presumed escape region over the total mass of hydrogen. Our results indicate that hydrogen escape ratios and migration characteristics are primarily influenced by formation heterogeneities. The escape ratio decreases as Dykstra-Parsons coefficient increases and permeability anisotropy ratio decreases. The specific hydrogen escape path is controlled by spatial structures (i.e., autocorrelation length). Inclusion of capillary heterogeneity results in a decrease in the escape ratio, and the escape region exhibits a smoother appearance in the absence of capillary heterogeneity trapping as compared to when it is modeled. As for fluid-rock interactions, the hydrogen escape ratio is highly susceptible to the exponent of BrooksCorey relative permeability function. Nevertheless, the magnitude of the escape ratio variation is predominantly determined by formation heterogeneity.