Evaluating the Potential for Underground Hydrogen Storage (UHS) in Lithuania: A Review of Geological Viability and Storage Integrity  
[PERSON]  
Department of Mathematical Modeling, Kaunas University of Technology, 51368 Kaunas, Lithuania; apoorv:[EMAIL_ADDRESS] (A.V. ); [EMAIL_ADDRESS] (S.M.) [PERSON]  
Department of Mathematical Modeling, Kaunas University of Technology, 51368 Kaunas, Lithuania; apoorv:[EMAIL_ADDRESS] (A.V. ); [EMAIL_ADDRESS] (S.M.) May 19, 2022 May 19, 2022 May 19, 2022 May 19, 2022 May  
the yearly global average concentrations of CO\({}_{2}\), nitrous oxide (N\({}_{2}\)O), and methane (CH\({}_{4}\)) reached unprecedented levels of 414.7 ppm, 0.3343 ppm, and 1.8957 ppm, respectively [6; 8]. The most recent data indicate that CO\({}_{2}\) emissions lead the world's greenhouse gas (GHG) emissions, contributing 74.4% of total GHG emissions. Next in line are CH\({}_{4}\) and N\({}_{2}\)O, with 17.3% and 6.2% of total GHG emissions, respectively. Therefore, to reduce CO\({}_{2}\) emissions globally, we need to shift towards sustainable and renewable energy sources to achieve net zero emissions by 2050 [9; 10]. However, some renewable and sustainable energy sources like solar, wind, and hydroelectric power are currently available options but are subject to fluctuations in supply due to diurnal cycles, and seasonal and climate changes [11; 12; 13; 14]. The solar intermittency has been amplified over nearly 85% of the global land due to climate change [15]. Also, H\({}_{2}\), which is a non-carbon-based renewable energy, can secure up to 70% of stored energy. Green H\({}_{2}\) is produced from renewable sources, providing a clean solution to these issues, and transforming renewable energy sources into secondary energy carrier [16; 17]. Globally, storage of H\({}_{2}\) energy is still under development or somewhat limited specifically underground hydrogen storage (UHS). Development in H\({}_{2}\) storage provides a solution by allowing excess or curtailed renewable energy to be stored and utilized when needed, as recently highlighted in multiple studies [18; 19; 20; 21; 22; 23; 24; 25]. Additionally, H\({}_{2}\) storage offers the potential for large-scale energy storage, addressing the issue of seasonal energy storage and ensuring a continuous energy supply. Various underground geological settings can be utilized for UHS, such as salt deposits (salt domes, salt beds, and salt caverns) and porous media (depleted oil and gas reservoirs and saline aquifers) [16; 26; 27]. UHS in depleted gas reservoirs has the potential to play a significant role in the transition to a low-carbon energy system by providing a flexible and scalable solution for energy storage [28]. The utilization of depleted gas reservoirs for H\({}_{2}\) storage can leverage existing infrastructure and reduce the need for new storage facilities, thereby minimizing costs and environmental impacts. H\({}_{2}\) can be stored in depleted gas reservoirs through various mechanisms, including physical adsorption, dissolution, and chemical reactions [29]. The storage capacity of depleted gas reservoirs for hydrogen can be significant, depending on factors such as reservoir size, porosity, and permeability [30]. Advances in technologies for H\({}_{2}\) production from subsurface, purification, and utilization can further enhance the feasibility and efficiency of UHS in depleted gas reservoirs [31; 32; 33]. [PERSON] et al. [34] identified depleted gas reservoirs as potential sites for UHS due to their existing infrastructure and geological characteristics. The injection and withdrawal of H\({}_{2}\) in depleted gas reservoirs can be optimized to maximize storage efficiency and minimize operational costs [35; 36; 37]. However, H\({}_{2}\) loss in depleted gas reservoirs can be attributed to factors such as diffusivity, mineral dissolution, precipitation, gas mixing, gravity segregation, and microbial activities [38; 39; 40].