## 1 Introduction  
In response to the significant challenges posed by global climate change, China has taken a leading role in proposing the \"3060 dual carbon\" targets. To meet these objectives, a transition from a fossil fuel-based energy system to one primarily reliant on renewable energy sources is essential [1]. However, renewable energy sources such as wind, solar, and tidal power suffer from inherent intermittency, leading to periods of both surplus and shortfall in energy production. This cyclic variability poses challenges to the stable operation of the power grid. Energy storage, especially underground storage, can improve the grid management and guarantee energy security by balancing supply and demand [2]. A promising solution proposed by researchers involves using surplus renewable energy to electrolyze water, producing hydrogen, which can then be stored in depleted oil and gas reservoirs, aquifers, or other suitable underground geological formations, as illustrated in Figure 1. From a geological standpoint, underground storage is particularly suitable for hydrogen due to the capacity of such formations to securely contain gases under pressure and their natural sealing properties. Moreover, Underground hydrogen storage (UHS) is a technology that utilizes underground geological structures (such as salt caverns, depleted gas fields and gas reservoirs, and aquifers) for large-scale hydrogen storage, providing an effective solution for large-scale, long-term energy storage [3]. Currently, UHS facilities in China are not yet widely deployed, and most related research remains theoretical. There are several technical challenges that still need to be addressed, particularly concerning the unstable displacement and two-phase seepage processes during hydrogen injection and extraction. To enable efficient UHS, it is essential to ensure both adequate storage capacity and sealing integrity. This requires reservoirs to possess high porosity to accommodate the necessary gas volumes and sufficient permeability to facilitate effective gas injection and extraction at adequate rates [4]. It has been proven that geological formations suitable for UHS include salt caverns, aquifers, and depleted oil and gas reservoirs. Salt caverns are currently the most developed storage option due to their low cover permeability and stable mechanical properties, though they are prone to significant creep and volumetric contraction over long-term operation [5]. Aquifers, which are abundant in sedimentary basins, are porous and permeable layers of underground rock filled with either fresh or saline water. However, aquifers present challenges in terms of exploration and site selection [6]. In contrast, depleted oil and gas reservoirs offer large storage volumes, widespread distribution across China, and natural trap structures with well-developed reservoir-cap rock combinations [7]. Compared to other geological formations, depleted oil and gas reservoirs offer distinct advantages and considerable development potential. There has been extensive research and practical experience both domestically and internationally in the storage of these reservoirs, with several global CO\({}_{2}\) storage projects underway to reduce greenhouse gas emissions and  
Figure 1: A brief description of the operation mechanism of underground hydrogen storage. foster the development of related technologies. Australia's Gorgon project, one of the world's largest CO2 sequestration initiatives, mitigates emissions from liquefied natural gas (LNG) production by injecting CO2 into underground salt caverns [8], and similarly, in Ordos, where CO2 is injected into underground formations for long-term storage. This initiative is aimed at providing emission reduction solutions to support the clean utilization of coal in the region [9]. The key to successful UHS lies in effective \"injection, extraction, and sealing\" [10; 11].