## 7 Challenges, Risks, and Opportunities  
The challenge of H\({}_{2}\)-based energy is underground storage, particularly when using H\({}_{2}\) on a large scale is to be expected. To facilitate large-scale usage, it must be stored in geological formations, such as salt deposits, saline aquifers, and depleted hydrocarbon reservoirs. The UHS in these reservoirs offers the much-needed capacity to balance inter-seasonal discrepancies between demand and supply, decouple energy generation from demand, and decarbonize heating and transport, supporting decarbonization of the environment [109]. In porous reservoirs, H\({}_{2}\) displaces formation fluids, resulting in complex multiphase flows influenced by fluid and rock properties (e.g., viscosity, density, porosity, permeability). Accurate multi-phase fluid displacement predictions are crucial for H\({}_{2}\) storage operations. However, there are multiple challenges associated with UHS, such as storage capacity, availability, geological suitability, and uncertain behavior of H\({}_{2}\) in porous media from the perspective of Lithuanian geological formations. The properties of pure H\({}_{2}\) also poses challenges for UHS, including the following points. (a) H\({}_{2}\) boats high energy density per mass (\(\sim\)120 MJ kg\({}^{-1}\)) [110] and low density (0.084 kg m\({}^{-3}\) at 20 \({}^{\circ}\)C and 0.1 MPa), which necessitates more volumetric storage [111]. (b) Buoyancy, from its low density compared to formation brine, creates a H\({}_{2}\) plume beneath the caprock, which leads to leakage. (c) H\({}_{2}\)'s low-pressure behavior aligns with the ideal gas law, although high-pressure scenarios require a more complex equation of state [62]. (d) The low viscosity of H\({}_{2}\), which changes minimally with pressure and temperature, and high thermal conductivity (approximately three times that of CH\({}_{4}\) and CO\({}_{2}\) at 2 km depth, 65 \({}^{\circ}\)C and 20 MPa) also present challenges during subsurface storage [112]. (e) H\({}_{2}\) solubility in brine decreases with increasing temperature and increases with increasing pressure, though the type of salt present also influences it. (f) The interaction between H\({}_{2}\), brine, and sandstone can lead to mineral dissolution and precipitation, affecting minerals such as calcite, dolomite, illite, chlorite, pyrite, and K-feldspar [108]. (g) Salt precipitation (scale formation) can occur in the pore structure of sandstone over time, especially at higher salt concentrations, leading to loss of injectivity and operational challenges. Additionally, UHS in depleted hydrocarbon reservoirs presents risks such as biochemical reactions and microbial activity, which includes abiotic calcite dissolution, acetogenesis, methanogenesis, and biological souring. Moreover, methanogenesis and acetogenesis can cause hydrogen loss, but the extent depends on the growth kinetics of methanogens and acetogens. In contrast, sandstone reservoirs are a suitable candidate for H\({}_{2}\) storage due to their low reaction rate, good porosity, and permeability, but the potential interactions with sandstone minerals are not well understood. Therefore, advancements at lab- and field-scale levels are needed, and these challenges can be a further point of research to revolutionize energy storage from theory to practice. On the other hand, retrieval after UHS is also highly challenging, as retrieval efficiency is uncertain due to factors like leakage, reactivity with reservoir minerals, and diffusion losses that can reduce the usable hydrogen. UHS presents vast opportunities for supporting a sustainable hydrogen economy and renewable energy integration by enabling large-scale storage of surplus energy from wind and solar sources. Research opportunities lie in reservoir modeling, AI/ML applications, material science for rock interactions, and innovative monitoring technologies. UHS also supports CO2 sequestration, contributing to decarbonization, and aligns with policy and market development for the green hydrogen economy.