Numerical Analysis of Hydrogen Fingering in Underground Hydrogen Storage

Underground hydrogen storage has gained interest in recent years due to the enormous demand for clean energy. Hydrogen is more diffusive than air, with a smaller density and lower viscosity. These unique properties introduce distinctive hydrodynamic phenomena in hydrogen storage, one of which is fingering. Fingering could induce the fluid trapped in small clusters of pores, leading to a dramatic decrease in hydrogen saturation and a lower recovery rate. In this study, numerical simulations are performed at the microscopic scale to understand the evolution of hydrogen saturation and the impacts of injection and withdrawal cycles. Two sets of micromodels with different porosity (0.362 and 0.426) and minimum sizes of pore throats (0.362 mm and 0.181 mm) are developed in the numerical model. A parameter analysis is then conducted to understand the influence of injection velocity (in the range of 10-2 m/s to 10-5 m/s) and porous structure on the fingering pattern, followed by an image analysis to capture the evolution of the fingering pattern. Viscous fingering, capillary fingering, and crossover fingering are observed and identified under different boundary conditions. The fractal dimension, specific area, mean angle, and entropy of fingers are proposed as geometric descriptors to characterize the shape of the fingering pattern. When porosity increases from 0.362 to 0.426, the saturation of hydrogen increases by 26.2%. Narrower pore throats elevate capillary resistance, which hinders fluid invasion. These results underscore the importance of pore structures and the interaction between viscous and capillary forces for hydrogen recovery efficiency. This work illuminates the influence of the pore structures and the fluid properties on the immiscible displacement of hydrogen and can be further extended to optimize the injection strategy of hydrogen in underground hydrogen storage.