Carbon dioxide as cushion gas for large-scale underground hydrogen storage: Mechanisms and implications

The unique properties between hydrogen and water, such as high interfacial tension and capillary force, pose a significant risk of water intrusion in large-scale Underground Hydrogen Storage (UHS). With the utilization of the Pore Network Modeling (PNM) and reservoir numerical simulation methods, this study investigates the mechanisms and implications of using CO2 as cushion gas in UHS to promote efficient injection and production, with high recovery factors and low volumes of water production. Under a realistic geological condition of the Yakela gas reservoir, serving as a geological reference, the PNM indicates a considerable decrease in capillary force, and an appreciable increase in relative permeability after injection of CO2 cushion gas in UHS. Furthermore, a reservoir numerical mechanistic model shows that the CO2 cushion gas established a special protection zone at the leading edge of the hydrogen storage zone, characterized by low interfacial tension, high viscosity, and density. In a realistic model application in the Yakela gas reservoir, the use of 30 % CO2 as cushion gas leads to a significant reduction in water cut, dropping from 28 % to 8 %. Additionally, the period of stable production is extended to 5.5 months, leaving only 5.4 % of the hydrogen unrecovered following five cycles. This study highlights the significant potential of CO2 as a cushion gas in UHS, which not only enhances the recovery factor but also provides a valuable technical reference for cushion gas selection in real-world UHS projects.