Mixing dynamics and recovery factor during hydrogen storage in depleted gas reservoirs

Large-scale hydrogen storage is strategically important for society and depleted gas reservoirs have been proposed as a potential solution. However, the mixing with remaining hydrocarbon gas during injection/production cycles and the impact of different reservoir parameters on the hydrogen recovery factor (RFH2) have not been fully understood. We investigate mixing dynamics and analyze the effects of initial hydrocarbon recovery factor, reservoir permeability, well perforation length, intelligent completion, and fractures on RFH2. Fine-grid gas reservoir models with compositional simulation were used to simulate cyclic hydrogen storage. Mixing of injected hydrogen with remaining hydrocarbon gas occurred in three distinct phases in each cycle: 1) displacement of remaining hydrocarbon gas through pressure-driven flow during hydrogen injection, 2) density-driven flow of gases during the idle time after hydrogen injection, and 3) pressure-driven flow of gases towards the production intervals during the hydrogen production phase. Higher RFH2 s were obtained when the storage process was initiated at higher hydrocarbon gas recovery factors. Storing hydrogen in reservoirs with lower permeability also led to higher RFH2 s, provided the well pressure limits were not an issue. In conditions of a 5× to 15× increase in permeability, the injected hydrogen moved upward and spread laterally, positioning it farther from the wellbore. This made hydrogen production more difficult and placed the mixing zone and hydrocarbon gas closer to perforations. Shorter perforation lengths at the top of the formation resulted in the best RFH2 s. Additionally, intelligent completions (that closed perforations producing gas with low hydrogen content) improved RFH2 by providing the possibility of purer hydrogen production. The presence of natural fractures significantly reduced hydrogen recovery, especially in the first few cycles. However, the influence decreased over time as highly conductive flow paths provided by fractures can contribute to the production of the accumulated unrecovered hydrogen from previous cycles.