Managing reservoir dynamics when converting natural gas fields to underground hydrogen storage.

On the path to a net-zero-carbon economy, large-scale energy storage will be an essential part of a renewable energy system. Hydrogen generated from renewable or zero-carbon sources has significant potential as an energy storage medium, by allowing for long-duration and high-capacity energy storage. At the volumes anticipated, however, underground storage systems will likely be essential. Much like the current natural gas system, underground hydrogen storage would provide a safe and cost-effective complement to smaller surface storage infrastructure. Recently, there has been significant interest in the possibility of converting existing gas storage systems to hydrogen storage, though many uncertainties exist in such a process. This paper explores the reservoir dynamics of natural gas/hydrogen gas mixtures and the resulting impacts on storage conversion and gas deliverability. Two specific topics are addressed. First, a novel viscosity model for hydrogen-containing gas mixtures is developed, which shows better agreement with experimental measurements than the widely-used Lohrenz-Bray-Clark model. Second, detailed reservoir simulations are used to investigate cost-effective injection/withdrawal strategies for converting a natural gas storage system to one that stores hydrogen/natural-gas blends. The critical role of buoyancy and gravity override is described, and recommendations are made regarding key reservoir characteristics and injection strategies for managing this behavior. • This study focuses on the conversion of an operational natural gas storage field to hydrogen storage. • A novel viscosity model is developed for hydrogen-containing gas mixtures. • Reservoir simulations evaluate the effect of geologic and engineering parameters on composition and rate of recovered gas. • Gravity segregation significantly influences the severity of fluctuations in the produced gas composition. [ABSTRACT FROM AUTHOR]