What makes a suitable underground hydrogen storage site in porous media?

Underground Hydrogen Storage (UHS) in porous media, such as saline aquifers and depleted oil and gas reservoirs, are promising options for storing renewable energy, enabling large-scale energy storage, and balancing supply-demand fluctuations in a decarbonised energy system. The availability and extent of these underground formations may provide long term TWh-scale capacity compared to other storage technologies such as surface infrastructure or salt caverns. The suitability of a storage site for UHS encompasses a variety geological, economy, environmental, legal, regulatory, and social aspects. In this paper, we focus on the subsurface elements and postulate what ‘UHS site suitability’ could be by framing it through the integration of three deeply interrelated aspects of the UHS operation: safety, efficiency, and cost. Here we explore the relationships and trade-offs to determine how safety, efficiency, and cost affects UHS site suitability. Factors influencing the safety, efficiency, and cost of an UHS operation include potential leakage pathways and the cyclical hydrogen injection and production are discussed. By doing so, the ideal conditions for UHS site in porous media are proposed and unsuitable options in terms of safety, efficiency, and cost are highlighted. Lastly, different considerations when selecting suitable UHS site in aquifers and depleted oil and gas reservoirs such as fluid composition and cushion gas selection are identified and discussed. The outcome of this paper can be used to guide UHS site selection and optimisation, providing a framework for evaluating the suitability of UHS site in porous media.