A comparative analysis of gas mixing during the underground hydrogen storage in a conventional and fractured reservoir

Abstract

Climate change and political tensions are reducing the time for the energy transition. Large-scale underground hydrogen storage (UHS) can be crucial in future energy systems for decarbonizing the world's economy as well as ensuring energy security. There are challenges, however, that prevent it from being used on large scale as an energy carrier. The high cost of hydrogen cushion gas is one of them. It is not economical that a portion of hydrogen remains in a reservoir as a cushion gas during UHS until the end of a storage operation. Consequently, inert gases such as nitrogen can be used as cushion gas. Hydrogen's heating value gets reduced, however, due to gas mixing. Since gas mixing behavior differs in reservoirs with different mechanisms, hydrogen heating value was employed to examine the gas mixing behavior during UHS in conventional and fractured reservoir models (dual-porosity) for the first time in this study. The mixing behavior of hydrogen gas with replaced cushion gas will first be examined in the absence and presence of diffusion using a conventional gas reservoir model. Then some reservoir and operational parameters will be investigated in order to determine how they affect the gas mixing behavior, including reservoir porosity, permeability, rock compressibility, and cushion gas type. Next, a basic UHS scenario with dual-porosity will be examined to determine how hydrogen gas and cushion gas mix in the presence and absence of diffusion and the effects of specific parameters on gas mixing behavior in the fractured reservoir (NFR) model, including matrix-fracture transmissibility and cushion gas type. According to the results, in the conventional reservoir model, considering diffusion during early hydrogen production results in a decrease in hydrogen heating value. While in the second stage of hydrogen production, diffusion increases the hydrogen heating value, resulting from the sweeping effect. Dual-porosity models illustrate the opposite behavior. As well, porosity reduction resulted in higher hydrogen heating values during early production but reversed during late production. Unlike the effect of rock compressibility on heating value of hydrogen, a lower hydrogen heating value was obtained during hydrogen production as the reservoir permeability increased. Also, carbon dioxide cushion gas showed reversed effect on gas mixing during UHS in the conventional compared to the NFR case.