Hydrogen storage in depleted gas reservoirs with carbon dioxide as a cushion gas: Exploring lateral and vertical cushion gas separation strategies

Cushion and working gas management plays a critical role during underground H2 storage, as injection strategies can directly influence gas mixing and overall storage performance. Cushion gas requirements and selection strongly influences the economics of a storage operation and can constitute a large initial investment, especially when expensive H2 is used. More economical alternatives, such as CO2 or in-situ CH4, can lower capital expenditures, with CO2 additionally supporting carbon capture and storage integration. However, mixing between these alternative cushion gases and H2 can degrade withdrawn H2 purity and increase operation expenses to drive downstream gas separation. This study evaluates vertical and lateral separation strategies of cushion gas and working gas in laterally extensive reservoirs, such as those found in the Southern North Sea. A conventional vertical separation strategy reduced H2 mixing with in-situ CH4 but increased H2–CO2 mixing. Lateral separation mitigated this mixing by injecting CO2 cushion gas and H2 working gas to either side of the reservoir, limiting the contact surface area of both gases, and therefore mixing; however, mixing between in-situ CH4 and H2 was exacerbated. Well placement also influenced mixing, where a central configuration reduced CH4 mixing in vertical cases but had little effect on CO2 mixing. Overall, this study provides insight into the benefits and trade-offs of various cushion gas strategies that can be considered in planning future underground hydrogen storage operations.