Assessment of Reservoir Performance for Alternative Cushion Gases for Underground Hydrogen Storage

Abstract

Abstract Underground Hydrogen Storage (UHS) is an emerging area of interest in the space of a future hydrogen economy to match fluctuating supply and demand. The European Commission highlights the development of large-scale hydrogen storage facilities in the upcoming years in the Hydrogen Strategy for a Climate-Neutral Europe (European Commission, 2020). Storing large volumes of hydrogen for extended periods will require safe, reliable, and economically feasible underground storage in addition to pipeline and surface storage, which will fall far short of providing the storage volumes that are expected to be required in a scenario of large-scale hydrogen implementation. Underground hydrogen storage capability builds on existing oil, gas and carbon dioxide storage, including subsurface and wells expertise, to provide a safe and cost-effective solution that ensures continuity of hydrogen customer supply. Building upon an initial reservoir engineering modelling study that was done to assess the viability of UHS in depleted gas fields (Huisman, et al., 2023), the analysis in this report assesses the technical viability, from a reservoir engineering perspective, of totally or partially replacing the hydrogen in the cushion gas by alternative gases such as methane, nitrogen, or carbon dioxide. This analysis aims to evaluate the options of total or partial replacement with a cheaper gas to reduce the hydrogen storage cost. The impact of the different alternatives was evaluated by: the resulting back produced gas compositions. the amount of tail gas to be reinjected/recirculated. The tail gas is formed when contaminants are separated at surface to comply with hydrogen export specifications. To understand if the performance of the cushion gas option could be improved, some other variables were also assessed such as an alternative cushion gas injection strategy using a more distant well to reduce gas mixing and back produced gas contamination, and the impact of less stringent export gas quality specifications. It was observed that even though less stringent specifications slightly reduce tail gas reinjection rate, the carbon dioxide option in all cases results in a higher flowrate of tail gas reinjection to the reservoir. On the other hand, the assessed alternative cushion gas injection strategy had a relatively minor impact on results. Even though reservoir performance results are impacted when reservoir dip angles change, reservoir dip did not have an effect on the relative performance of the different cushion gases. Overall analysis suggests that a partial replacement of the hydrogen cushion gas with methane is the best option. Integrated project economics should be assessed to provide a final recommendation on which cushion gas alternative is best from an economic point of view.