## 3 Results
### Geochemical Reactions  
Studies on depleted hydrocarbon reservoirs have shown that abiotic interactions (e.g., dissolutions as carbonate, sulphate, feldspars, clay, or precipitation as illite, sulphide, pyrrhotite), except for pyrite dissolution, generally occur under extreme reservoir conditions or over extended periods, conditions typically not found in depleted sandstone hydrocarbon reservoirs [69]. In depleted sandstone gas reservoirs, H\({}_{2}\) is reported to behave as an inert gas [58]. This author [58] conducted an experimental and numerical study of abiotic reactions in sandstone exposed to H\({}_{2}\) (10 MPa, 100-200 \({}^{\circ}\)C, 6-24 weeks, dry and water saturated); only a limited number of reactions between the sandstone and hydrogen were observed. However, some authors reported the possibility of abiotic reactions occurring in sandstone reservoirs due to changes in pore fluid chemistry caused by H\({}_{2}\) injection (e.g., [69]). This can lead to the dissolution of anhydrite and carbonate cement, which are common in sandstone and shale reservoirs and in caprocks. On the other hand, significant alterations in pore structure have been observed due to geochemical reactions with H\({}_{2}\) in reservoirs with a high percentage of carbonate and anhydrite [63]. These researchers [63] exposed rock samples (sandstones -reservoir- and siltstone-caprock-) to H\({}_{2}\) (10-20 MPa, 100-130 \({}^{\circ}\)C and 6-24 weeks, pure H\({}_{2}\)) and their experimental results revealed that abiotic reactions \"might occur in reservoirs due to the alteration of the pore fluid's chemical composition upon hydrogen injection\". Probably, dissolution of pore-filling anhydrite and carbonate cement (abundant in most sandstone and siltstone). According to [PERSON] et al. [91], in their studies on abiotic reactions, sandstones only exhibited mineralogical changes after interacting with H\({}_{2}\) under the following conditions: \(>\)15 MPa, \(>\)100 \({}^{\circ}\)C, and \(>\)200 g/L. Accordingly, [PERSON] et al. [86], in their experimentation in sandstones (4-20 MPa, 40-120 \({}^{\circ}\)C, and 6-24 weeks, batch experiments), found that no significant mineralogical changes occur (partial dissolution of sulphate and carbonate cement) due to geochemical interactions in sandstone reservoirs, regardless of their mineralogy, at low pressures, temperatures, and salinities. This study [PERSON] et al. [86] provided additional evidence that in conditions of low pressure, temperature, and salinity, there are no observable geochemical interactions or alterations in the mineral composition of sandstone reservoir rocks, regardless of the mineral types present. Geochemical reactions and subsequent mineralogical changes in shales and mudstones are more significant than in sandstone,highlighting the risk associated with caprock integrity [64]. It is important to highlight that geochemical reactions can occur when injecting hydrogen into gas reservoirs in the reactive zone [84; 86; 87; 88]. This reactive zone is characterized by highly saline brine, carbonate rock, and anhydrite. Notably, according to [PERSON] et al. [63], this zone can change with reservoir operations, such as injection and production. Salt caverns mainly consist of halite (NaCl) and anhydrite (CaSO4) salt layers, with these two types of salt being the primary source of brine composition in salt caverns [86]. Under normal conditions, halite is chemically inert, and no reaction with hydrogen is expected during UHS in salt caverns. However, the chemical inertness of halite (99%) during UHS can be influenced by other factors like impurities (anhydrite, <1%) in the salt cavern or the storage conditions [89]. Among the other impurities (<1%) such as quartz (SiO2), dolomite (CaMg(CO3)2), pyrite (FeS2), gypsum (CaSO4-2H2O), and calcite (CaCO3), each could potentially react with hydrogen. Moreover, some common ions of these impurities (e.g., Fe3+, Mg2+, CO2-, Ca+2, SO4-2-, Fe2+, K+, Cl-, and Ba2+) could enhance microbial activity [64; 90; 92].