The effect of gas solubility on the selection of cushion gas for underground hydrogen storage in aquifers

Underground hydrogen storage (UHS) is gaining worldwide attention as an efficient solution for energy supplydemand imbalance. However, deploying UHS at full scale encounters several challenges. One of the primary challenges is selecting a suitable cushion gas to optimize hydrogen storage and withdrawal efficiency. In the existing literature cushion gas dissolution in the resident reservoir fluid is not considered as one of the technical selection criteria and we aim to show in this paper that it is critically important as it affects hydrogen recovery, pressure maintenance and hydrogen purity. With a focus on hydrogen storage in aquifers, we investigated cushion gas solubility in water. Three potential cushion gases-carbon dioxide, methane, and nitrogen-were assessed through large-scale compositional simulations using a conceptual aquifer model, considering temperature and pressure conditions corresponding to the reservoir depth. Gas solubility analysis involved examining changes in reservoir pressure, water production, injected cushion gas volume, and gas saturation changes during the cyclic process. Two distinct scenarios, with and without gas solubility, were considered to understand the effect of solubility on produced hydrogen purity and efficiency. Results indicate that increasing cushion gas solubility decreases final hydrogen recovery. For the case of carbon dioxide, due to its high solubility in the reservoir water, hydrogen purity is reduced to 30 % and hydrogen recovery factor falls by 18 % compared to using methane and nitrogen. Despite this, carbon dioxide dissolution can favorably reduce cumulative water production and mitigate severe reservoir pressure drops during production. Therefore, the dissolution phenomenon should be a key consideration in the screening of cushion gases and subsequently in the design and operation of UHS. Understanding the relationship between cushion gas solubility and hydrogen production efficiency contributes to enhancing the overall performance, and the long-term success of UHS projects.