A critical review of cushion gas in underground hydrogen storage: Thermophysical properties, interfacial interactions, and numerical perspectives

Increased greenhouse gas emissions from fossil fuel consumption have significantly contributed to global warming, promoting the transition toward renewable energy sources. Underground hydrogen storage (UHS) represents a large-scale energy storage system, aiming to ensure a consistent supply by storing hydrogen generated from surplus energy. In the practice of UHS, cushion gas is generally injected into the formation to retain the reservoir pressure for efficient hydrogen withdrawal. This paper reviews the impact of cushion gas on the performance of UHS from both experimental and numerical simulation perspectives. The thermophysical (e.g., density, viscosity, compressibility, and solubility) and petrophysical (interfacial tension, wettability, and relative permeability) properties of different cushion gases, as well as the mixing and diffusion behavior of different cushion gases, were compared. The corresponding impact of different cushion gases on plume migration and trapping potential is then discussed. Furthermore, this review critically analyzes and explains the impact of various factors on the performance of UHS, including the type of cushion gas, the composition of cushion gas mixtures, the volume of injected cushion gas, and the effects of bio-methanation processes. The corresponding analysis specifically focuses on key performance indicators, including H 2 recovery factor, formation pressure, brine production, and H 2 outflow purity. Thus, this review provides a comprehensive analysis of the role of cushion gas in UHS and provides an understanding of the effective management and optimization of cushion gas injection in field-scale UHS operations.