Underground hydrogen storage in a partially depleted gas condensate reservoir: Influence of cushion gas

Hydrogen gas as a clean and renewable kind of energy can be considered to supply electricity demand during peak usage times. Actually, excess electricity cannot be stored in great quantities, but it can be converted to hydrogen, which can be stored and converted to electricity as a peak shaving. However, due to the very low hydrogen energy density in terms of volume, a huge capacity is needed for its storage. Therefore, underground hydrogen storage (UHS) can be evaluated as a solution. This study investigated the effect of cushion gas on underground hydrogen storage in a partially depleted gas condensate reservoir in a real case which is located in the Middle east. To the best of our knowledge, it is the first time the feasibility of underground hydrogen storage in a gas condensate reservoir has been investigated. The effect of some parameters such as cushion gas type, namely; methane, nitrogen, carbon dioxide, condensate existence, the implementation time of storage, hydrogen injection initialization stage, and hydrogen injection/production rate was investigated on hydrogen heating value and recovery during the underground hydrogen storage operation. The results of replacing alternative gases as part of cushion gas showed that the highest amount of hydrogen recovery and purity could be obtained by injecting the nitrogen. While carbon dioxide was the most effective alternative gas to improve condensate production but in terms of hydrogen recovery and purity was not. Also, implementing hydrogen storage in the gas condensate reservoir leads to higher hydrogen recovery than dry gas reservoirs due to the trapping of alternative gases in the condensate phase. However, most of the injected carbon dioxide was produced during hydrogen production, which is unfavorable. This study shows that 60% depletion of the gas condensate reservoir is the best condition to start underground hydrogen storage. Further, applying a hydrogen injection initialization stage indicated that the hydrogen recovery and heating value could be increased. However, it requires a detailed economic evaluation to consider the cost/benefit of initialized hydrogen loss and recovery.