Review on the loss during hydrogen storage in depleted gas reservoirs

Underground hydrogen storage (UHS) is essential for large-scale energy storage as it can address the intermittency of renewable energy sources and support the transition to a low-carbon energy structure. Compared with other suitable sites for UHS, depleted gas reservoirs may be the best due to their large storage capacity, numerous potential options and complete surface facilities. However, the lack of practice and theory about the loss during hydrogen storage in depleted gas reservoirs (HSDGs) hinders its development and large-scale utilization. This review aims to minimize the loss and improve withdrawal efficiency via analyzing and discussing previous works on hydrogen loss during HSDGs. To fill this gap, according to the mechanisms of hydrogen loss during HSDGs, the loss is classified into two categories in this work, real loss and fake loss. Real loss is the complete reduction in hydrogen caused by chemical reactions, gas diffusion, and geological leakage. Fake loss means that hydrogen stored in gas reservoirs can't be withdrawn under the specific conditions due to gas dissolution in formation water, residual trapping, adsorption in rock minerals, and other engineering factors, such as cushion gas, well perforation and cyclic operation. Given two categories of hydrogen loss, the pathways, mechanisms, quantity and influencing factors were evaluated and discussed critically, and specific strategies were also proposed to minimize the loss. Finally, the limitations of current valuable works were highlighted, and future works were developed. This review can help scientists and engineers maximize the withdrawal efficiency of HSDGs.