Performance of different cushion gases in underground hydrogen storage: Pressure behavior, hydrogen loss, and flow behavior modeling

Hydrogen storage in subsurface geological formations is a promising technique to address the energy supply-demand gap in a sustainable manner. In underground hydrogen storage (UHS), cushion gas is typically employed prior to the cyclic hydrogen injection and withdrawal to maintain formation pressure. While the impact of cushion gas has recently been investigated on UHS, there is still an ongoing debate on the relative performance of different types of cushion gases utilized. In addition, current studies lack quantitative analyses of gas trapping and the consideration of the flow behavior differences between cushion gas and hydrogen. In the present study, we conducted numerical simulations to investigate the performance of carbon dioxide, methane, nitrogen, and hydrogen as cushion gases during UHS in saline aquifers. The residual and dissolution trapping of both cushion gases and hydrogen were quantified. The injection of cushion gas significantly increases the hydrogen recovery factor, while H₂ itself as cushion gas demonstrated the maximum efficiency (i.e., H₂ recovery = 70.85%). The trapping analysis suggests that the residual and dissolution trapping can retain a significant fraction of cushion gas and hydrogen (i.e., up to 23.37% for residual trapping and 4.6% for dissolution trapping) within the formation. In addition, the flow behavior differences between CH₄, N_2, CO₂, and H₂ during the cushion gas injection phase result in different gas saturation distributions after the cushion gas injection compared to the case without flow differences. These flow behavior differences lead to increases in both ultimate H₂ recovery factor and brine production. In addition, CO₂ as the cushion gas requires the largest volume to achieve a comparable H₂ recovery factor across all cushion gas cases.