Gas cap oil reservoirs: A viable option for hydrogen storage

Gas cap oil reservoirs offer an economical and practical solution for underground hydrogen storage (UHS) due to their established porous media properties, sealing integrity, and existing infrastructure. Additionally, the natural gas within the cap can serve as cushion gas during storage. Storing hydrogen (H₂) in these reservoirs can help maintain reservoir pressure and potentially enhance oil recovery. However, the interactions between injected hydrogen, cushion gas, and crude oil remain underexplored. This study investigates the dissolution of H₂, methane (CH₄), and a natural gas mixture (representative of a gas cap) into light crude oil (38° API) under reservoir-representative conditions. Experiments were conducted to examine changes in oil properties due to gas dissolution across wide ranges of pressure (500–5000 psi) and temperature (30–90 °C). Additionally, oil–gas interfacial tension (IFT) measurements were performed at 90 °C and various pressures to assess miscibility. CH₄– and gas mixture–saturated oils showed average viscosity reductions of 3.6% and 6.3%, respectively, whereas H₂ saturation produced a modest average increase of 2.7%, attributed to molecular reorganization within the crude oil. Density changes further indicate limited H₂ dissolution (average reduction = 0.7%). In comparison, density reductions for CH₄ and the gas mixture were ∼2 × and ∼5 × larger, consistent with their stronger impacts on oil properties. IFT data further revealed that extremely high pressure is required for hydrogen to become miscible with light crude oil. Therefore, these findings suggest that gas-cap light-oil reservoirs can provide favorable conditions for UHS, with limited hydrogen loss to the oil phase.