## 3 Results
### Main Topics in Laboratory-Scale Experimentation of UHS
#### 3.2.2 Main Fluid Types  
The characteristics of the injected H\({}_{2}\) and the presence of other fluids such as brine, gas, or hydrocarbons also influence H\({}_{2}\) storage in geological formations. The transport/ mobilization of the gas(es) and liquids inside the rock, through the porous and/or fractured media, are important for understanding the distribution and mobility (high in the reservoir and very low in the seal) of hydrogen. This analysis is essential for assessing the capacity of the reservoir and the safety of retaining H\({}_{2}\) in a geological formation [66]. Hydrogen and Brine  
Due to the singular characteristics of hydrogen gas, its underground storage involves issues that are different from the storage of other gases like CO\({}_{2}\) or CH\({}_{4}\). Hydrogen's molecular weight is less than 5% of that of CO\({}_{2}\), 7% of N\({}_{2}\), and 12.5% of methane, and its kinetic diameter is smaller than either of those gases. As the diffusivity coefficient is inversely proportional both to the molecular weight and the kinetic diameter, hydrogen is far more diffusive than other gases, which means it is more likely to pass through certain caprocks by diffusion [67]. This raises one of the concerns regarding UHS integrity and hydrogen loss. In particular, diffusion of hydrogen at the molecular level through the rock matrix is a phenomenon of interest in formations with low permeability, where hydrogen can disperse slowly without migrating rapidly over long distances. This process may be more relevant in environments where rocks act as capillary barriers, slowing the escape of hydrogen but allowing its gradual diffusion [66]. In depleted gas fields, the high diffusivity of hydrogen could cause up to 2% hydrogen loss [68], as the hydrogen is dissolved in the formation water of the cap rock and diffuses through the cap rock. Besides temperature, pressure, and saturation, diffusion also depends on the presence of hydrocarbon phases, so it can vary from reservoir to reservoir [69], but it is only expected to be significant on longer time scales [70]. The dissolution of H\({}_{2}\) into the brine left behind from the leaching process in a salt cavern or the brine that occupies the pore space of a porous reservoir leads to H\({}_{2}\) loss and, therefore, is an unfavorable factor, unlike in CO\({}_{2}\) storage, where solubility trapping is one of the desired trapping mechanisms. Hydrogen solubility in brine is relatively low, and it is a function of pressure, temperature, and brine salinity. Increasing pressure and temperature generally results in an increase in H\({}_{2}\) dissolution, while higher brine salinity leads to lower H\({}_{2}\) solubility [71]. [PERSON] et al. [71], in their recent review paper, concluded that \"studies on fluid-fluid interaction between H\({}_{2}\) and reservoir fluids are very few in the literature. More studies of the interaction between hydrogen and formation brine are required to ascertain their interaction at the pore scale\". They also emphasize the need to study the effect of different brine compositions and the effect of organic acid concentration on H\({}_{2}\)-brine IFT behavior. The low density of H\({}_{2}\) when compared to other gases is another significant factor. On the one hand, it does not allow for underground storage in dense phases, as reaching those states would require extreme pressure and temperature. On the other hand, the substantial brine-H\({}_{2}\) density difference and high buoyancy of H\({}_{2}\) cause a strong gravity segregation effect in the reservoir, resulting in a greater upward movement of H\({}_{2}\) than other gases [30]. H\({}_{2}\) can also migrate within the rock mass by either single- or two-phase convective flow, especially in more porous formations, such as depleted hydrocarbon reservoirs. The permeability of rock salt regarding one-phase flow needs to be calculated experimentally, which is extremely challenging due to the very low values [72].