Dynamic hydrogen working gas storage capacity in compartmentalised gas fields: The UK Rough storage site as a case study

The compartmentalised Permian sandstone reservoirs in the Southern North Sea are a prime candidate for potential future large-scale underground hydrogen storage (UHS) due to the oil and gas infrastructure in place, the advanced knowledge of the reservoirs and caprocks and their operational history as underground gas storage sites. Building on this potential, this study introduces a new method for estimating dynamic working gas storage capacity for depleted gas fields and existing natural gas storage sites, applied to the Rough field, the UK's largest gas storage site. This methodology, which produces standardised dynamic capacities and hence allows the comparison of different sites, is based on the establishment of a pressure equilibrium between hydrogen injected and withdrawn, which gives an improved representation of the hydrogen storage capacity of porous reservoirs. For this methodology, we use operational data, including injection and withdrawal pressure ranges, and propose the re-use of infrastructure to provide an improved first order dynamic capacity estimate for depleted gas fields or gas storage sites for hydrogen storage, but the user can change the configuration and undertake more site-specific investigations. The testing of the impact of technical changes on the estimated capacity, is illustrated by sensitivity analysis using different numbers of wells. The results show that while increasing the number of wells initially boosts working gas capacity and lower the cushion gas to working gas ratio efficiency, these benefits tail off beyond a certain number of wells added. Using this new methodology, we estimate for the Rough field a dynamic hydrogen storage capacity of 7.5 TWh (∼79 BCF) and a cushion to working gas ratio of 2.1, using all 29 existing vertical wells. Reducing the well number shows a rapid decline in working gas capacity and an increase of the cushion gas to working gas ratio (e.g. down to 6.1 TWh (64 BCF) and up to 2.8, respectively, for scenarios with 15 wells). Additional factors, such as the low permeability of the underlying formations, the unrecovered natural gas in the Rough field, as well as the dependence of the capacity on accurate multi-phase fluid flow data, are also discussed. The site-specific working gas capacity can be further optimised by utilising a specifically designed storage development plan for the Rough storage site. • A method to estimate dynamic underground hydrogen storage capacity is presented. • The UK Rough field gas storage site is investigated as a hydrogen storage site. • The methodology highlights the importance of operational parameters. • The cushion gas to working gas ratio depends highly on the number of wells.