Underground hydrogen storage in naturally fractured reservoirs: Matrix scale modeling for cushion gas selection

Hydrogen is a key zero-carbon fuel for the future, but seasonal fluctuations in production and energy demand require effective storage solutions. Underground hydrogen storage (UHS) in naturally fractured reservoirs (NFRs), such as depleted hydrocarbon reservoirs, is a promising approach due to their proven ability to keep fluids securely. This study simulates UHS using a single matrix block (SMB) model to explore hydrogen storage in gas and water-invaded parts of an NFR, with and without the use of cushion gases (CO2, CH4, N2). Cushion gases provide pressure support during hydrogen withdrawal and prevent direct contact between hydrogen and reservoir fluids. The results reveal that 150 m3 and 65 m3 of hydrogen can be stored in a gas and water-invaded SMB, respectively, without cushion gas. The use of CO2 as a cushion gas in water invaded zone resulted in a three-fold increase in hydrogen storage capacity. In contrast, in gas invaded zone, hydrogen storage capacity decreased by 12-18%, depending on the type of cushion gas used. Cushion gases also reduce hydrogen dissolution in liquids, which contributes to hydrogen loss. CH4 was the most effective in reducing hydrogen loss, lowering it from 11.61 m3 to 8.64 m3 in gas-invaded SMB and from 23.73 m3 to 18.68 m3 in water-invaded SMB. These findings indicate that while cushion gases may be optional in high-pressure gas-dominated formations, they are essential for efficient UHS in aquifers and liquid-saturated reservoirs.