A review of underground hydrogen storage in depleted gas reservoirs: Insights into various rock-fluid interaction mechanisms and their impact on the process integrity

This study reviews rock-fluid interaction during the underground hydrogen process in depleted gas reservoirs. According to the review, although over 700 publications can be found in the literature related to rock-fluid interaction related to other deep-earth applications, only few studies have paid attention to underground hydrogen storage in depleted gas reservoirs. Rock-fluid interactions happen in various ways during underground hydrogen storage in depleted gas reservoirs, mainly: 1) geochemical interactions, called abiotic interactions, and 2) microbial activities, called biotic reactions. All these interactions are greatly affected by some physicochemical processes that occur in reservoirs during hydrogen storage, including changes in wettability, interfacial tension, diffusion, adsorption, and solubility. Hydrogen interacts with rock minerals during abiotic reactions, resulting in mineral dissolution and precipitation and, preferably, occurs in the reservoir’s reactive zone. Most abiotic reactions happen only under extreme pressures and temperatures during hydrogen storage. Biotic reactions have been identified as the major reason for the loss of hydrogen during the storage process because these happen under typical reservoir conditions (below 130 °C temperature and 35 MPa pressure) and create other issues. For example, the sulphate-reducing reaction creates an acidic environment and sulphide precipitation near the wellbore, causing material corrosion and permeability reduction. Changes in wettability, interfacial tension, adsorption, diffusivity, and solubility can significantly impact rock-fluid interactions during hydrogen storage. Hydrogen wettability in reservoir rocks is much less than in other gases; therefore, a gas like nitrogen can recover trapped hydrogen during recovery. Hydrogen's absorption/desorption characteristics are important in reservoirs with high surface areas, such as coal, and significantly change with pressure and the type of cementation material. Hydrogen solubility in brine is much less than in other gases, and in contrast, hydrogen is highly buoyant and diffusive compared to other gases. However, diffusion is critical only in the first stage of hydrogen storage, and the total diffusive loss of hydrogen in a reservoir is <2%. Rock-fluid interactions significantly affect the safety of the storage process, including caprock and wellbore integrity. According to the conducted comprehensive review study, some areas need further research for an effective underground hydrogen storage process. In particular, the combined effect of biotic and abiotic interactions on mineralogical, hydrological and mechanical properties of reservoir rocks, their impacts on wellbore and caprock integrities, hydrogen loss caused by fingering under heterogeneous reservoir conditions, the recovery possibility of residually trapped hydrogen using other gases, and rock-fluid interactions in complex reservoir rocks such as coal.