Modelling underground hydrogen storage in depleted hydrocarbon reservoirs: Exploring early adoption and large-scale seasonal storage possibilities

In the face of intensifying climate change, the global energy industry must transition from its dependence on fossil fuels to renewable sources. Underground hydrogen storage (UHS) in depleted hydrocarbon reservoirs offers a practical way to use existing energy infrastructure to address renewable energy intermittency. This thesis advances the feasibility of UHS by developing and evaluating innovative strategies to improve operational performance and enable both early-stage deployment and large-scale application. High upfront capital expenditures associated with cushion gas and uncertainties in H2 supply and demand present major barriers to early deployment. To address these, the first study develops a scalable approach—beginning with small pilot projects that can be progressively expanded—providing a pathway to prove technical feasibility and de-risk investment. This approach is tested on a depleted oil field to assess its feasibility. Results show that standard cushion gas to working gas ratios are insufficient; connectivity between depleted layers caused by faulting necessitates progressively higher cushion gas requirements. To mitigate this, a CO2 cushion gas strategy was developed to reduce pressure dissipation, lower expensive H2 requirements, and avoid mixing issues while retaining flexibility for upscaling. Subsequent studies investigated large-scale, seasonal UHS in laterally extensive reservoirs analogous to the Southern North Sea gas fields. Using numerical modelling, a lateral separation strategy was developed in which cushion and working gases were separated laterally to limit contact surface area and, consequently, mixing. This work was then compared to a conventional vertical separation strategy. The analysis provides valuable insights into (i) capacity estimations of CO2 storage and H2 withdrawal, (ii) macro-scale fluid dynamics, and (iii) the effects of gas mixing trends on H2 purity. The results underscore key trade-offs between cushion gas separation strategies, CO2 storage volumes, and H2 withdrawal and purity.