Enhancing performance of aquifer-based underground hydrogen storage via numerical simulation with cushion gas optimization

Hydrogen energy is preferred by several countries as a significant component of the energy transition aimed at reducing the release of greenhouse gases. Large-scale underground hydrogen storage (UHS) can be utilized to meet energy demands. Aquifers have garnered considerable attention as highly viable locations for hydrogen storage. However, there remains a lack of understanding regarding the role of cushion gas, which can significantly impact the hydrogen storage performance of aquifers. This study analyzes various factors, including the type of cushion gas, injection rate, and injection period. As a result, the potential for using mixed gas as a cushion gas is suggested. Through the above analyses, the following insights are obtained: (1) The introduction of cushion gas exerted a significant impact on the cumulative gas production volume, recovery efficiency, and gas molar fraction of hydrogen. (2) A moderate increase in the injection rate of cushion gas could enhance the cumulative gas production volume and recovery efficiency. (3) The impact of the injection period of the cushion gas on the hydrogen storage performance of the aquifer was minimal without considering the gas-aquifer interaction. (4) The gas-aquifer interaction was unfavorable for UHS. Exploring alternative methods to mitigate the gas-aquifer interaction is essential for enhancing the performance of UHS. (5) A gas mixture (CH₄: CO₂ = 1 : 1) could improve the recovery efficiency and gas mole fraction to a certain degree. These findings help to guide the selection of cushion gas type and the optimal operating strategy.