Role of Cushion Gas on Underground Hydrogen Storage in Depleted Oil Reservoirs

Hydrogen storage in underground structures is an appropriate way for keeping the balance between the energy production and consumption. Indeed, excessive electrical energy can be converted, through electrolysis, to chemical energy of hydrogen molecules, which can then be temporarily stored in underground structures. Later on, when the demand for energy rises, the process can be reversed to satisfy the demand. Hydrogen has distinctive characteristics compared to other gases. It possesses good potentials for energy generation and exhibits widely different behaviors in porous media. In the present study, underground hydrogen storage (UHS) in a depleted oil reservoir was numerically simulated and the results were investigated. The UHS requires base or cushion gas to retain the reservoir pressure high enough as the hydrogen is being retrieved out of the reservoir. We herein studied gaseous forms of carbon dioxide, nitrogen, and methane as candidate cushion gas. Various scenarios, in terms of the type and composition of the cushion gas flow, were simulated and the results were evaluated in terms of hydrogen recovery factor, unwanted fluid production, and hydrogen loss. Moreover, an attempt was made to present the mechanism of hydrogen behaviour based on different hydrodynamic reservoir phenomena, such as gravity segregation and overriding, under various scenarios. Our findings show that the maximum hydrogen recovery (89.7%) was anticipated when methane was injected as the cushion gas. In the absence of any cushion gas, the hydrogen loss was estimated at 15.5%. Prior to water breakthrough, the pressure buildup due to water injection was found to improve the hydrogen recovery in annual cycles, but the effect on the UHS performance was seen to be the opposite since after the water breakthrough.