## 1 Introduction  
Hydrogen is considered a highly convenient carrier to store energy from renewable sources and is foreseen to play a key role in a clean and secure energy future. In the so-called hydrogen economy, this gas would be used as the major carrier of energy, which would require its storage in large quantities and its withdrawal based on demand (e.g., storage cycle or seasonal requirements). Currently, this large-scale storage is one of the main obstacles in the implementation of the technology, together with the costs and inefficiencies associated with the production of green and blue hydrogen. Underground hydrogen storage (UHS), or H\({}_{2}\) geo-storage, has been suggested as a possible solution to overcome the first difficulty [1; 2; 3; 4]. Underground gas storage in adequate sedimentary geological formations has been used and thoroughly studied for several industrial applications. These formations are geographically widespread, potentially provide large storage space [5; 6; 7; 8], and, if properly sealed with a very low permeability caprock, constitute ideal reservoirs for natural gas, CO\({}_{2}\), CH\({}_{4}\) or H\({}_{2}\) (e.g., [5; 9; 10; 11]). Furthermore, large-scale underground hydrogen storage in suitable geological formations (e.g., caverns, abandoned hydrocarbon fields, aquifers, or salt domes) has been estimated to have much lower costs than other industrial solutions, such as liquefied H\({}_{2}\) storage or storage in high-pressure tanks [12]. Depending on the components of the energy cycle in question, underground hydrogen storage could happen in three different configurations: (i) underground storage of pure hydrogen [13]; (ii) underground storage of a mixture with natural gas lean in hydrogen [14]; and (iii) underground storage of a hydrogen-rich mixture with CO, CH\({}_{4}\), and CO\({}_{2}\) (syngas or town gas) [15]. Pure H\({}_{2}\) has been stored in the subsurface, for use in the chemical and aerospace industry, for about 80 years [16]. This was done in mined salt caverns (e.g., Praxair in Texas, Teeside in Great Britain, and Yakshunovskoe field in Russia) where, due to the chemical inertness of the salt, no relevant geochemical reactions were observed [14; 17]. The experience in storing hydrogen-carbonic mixtures is slightly more extensive due to the use of the so-called town gas (produced by coal gasification and containing about 50-60% of H\({}_{2}\), 15-20% of CO\({}_{2}\) + CO, 10-20% of CH\({}_{4}\), and traces of N\({}_{2}\)). Underground storage facilities of hydrogen-carbonic mixtures can be found, for example, in Kiel, Germany, with 62% of H\({}_{2}\) in a salt cavern; in Lobodice, Czech Republic, with 50% of H\({}_{2}\) in an aquifer; and in Beynes, France, with 50% H\({}_{2}\) in an aquifer [18]. Studies of similar cases have revealed that hydrogen-methane mixtures might provoke increases in reservoir pH and subsequent changes in the mineral composition of the reservoir rock, e.g., by dissolving dolomite and precipitating calcite and talc [19]. The so-called underground methanation reactor (UMR) is a rather unique phenomenon discovered in the storage of hydrogen-CO\({}_{2}\) mixtures. Certain methanogenic bacteria that consume hydrogen and carbon dioxide can generate methane in situ [20; 21; 22; 23], providing thus an economically favorable methane production method [24]. The possible interactions between the gas and the host rock are dissolution-precipitation of minerals and/or changes in permeability/porosity [25]. Studies on the storage of pure H\({}_{2}\)[26; 27] tend to conclude that hydrogen storage does not introduce major new features in comparison with natural gas storage [28], although lateral gas spreading outside the storage limits can be more pronounced because hydrogen is very light and diffusive [26] and H\({}_{2}\) is more reactive and consumed by bacteria than natural gas.