## 5 Knowledge Gaps  
Underground hydrogen storage comprises a wide range of physicochemical processes driven by dynamic feedback mechanisms, which make comprehensive and accurate modeling very difficult. Long-term sustainability can only be accurately predicted if numerical models use reliable rates of changes of the physiochemical properties of subsurface strata at various conditions [145]. Laboratory experiments are among the main sources of this kind of data. Despite the rapidly growing number of studies in the field, there remain several knowledge gaps to be addressed. * Parameters determining fluid dynamics and, therefore, the movement of hydrogen in the reservoir are numerous and often difficult to determine. Although more and more data on the interfacial properties of \(\mathrm{brine/H_{2}/cushion}\) gas systems are available in the literature, the underlying interactions remain poorly explored, and a robust database is called for real-life implementations [146]. Researchers often use relative permeability and capillary pressure values from available models in their simulations because experimental measurements are very limited in number [32; 58; 73; 147] and reliable diffusivity and solubility values in different pore size ranges are also lacking [30; 32; 148]. The interfacial properties of \(\mathrm{brine/H_{2}/cushion}\) gas systems are difficult to generalize due to limited molecular-level insights and idealized experimental conditions. Variations in reservoir factors like temperature, pressure, and pore size further complicate their application and reliance on other models that may not fully represent the particular conditions. * Studies on fluid-fluid interaction between \(\mathrm{H_{2}}\) and reservoir fluids (e.g., dissolution, diffusion, chemical reactions, relative permeability) are very few in the literature, especially regarding the effect of different brine compositions and of organic acid concentration on \(\mathrm{H_{2}}\)-brine IFT behavior [71]. Hydrogen solubility in non-aqueous solutions (e.g., hydrocarbons and organic acids) is another critical and complex issue that needs further experimental results [76]. * The integrity of UHS projects is largely dependent on the performance of the caprock--may it be rock salt or a clay-rich detritic rock type. Gas migration by diffusion through both salt and clay-rich formations still remains underexplored [67; 69]. Rock salt is generally assumed to be impermeable; however, natural inhomogeneity and the processes induced by cavern construction and cyclic \(\mathrm{H_{2}}\) injection are factors that must be taken into account. [PERSON] and [PERSON] [72] published two-phase percolation-diffusion model parameters for intact and damaged rock salt. [PERSON] et al. [149] emphasize the potential problems in bedded salt beds due to the inhomogeneous physical properties and potential geochemical reactions with hydrogen and also point out the need to study experimentally the geomechanical properties of salt caverns under the conditions of cyclic operation. [PERSON] et al. [60] maintain that advective gas leakage through cavern walls should be negligible but warn about the possible contribution of diffusion of dissolved hydrogen into pore brine to hydrogen loss. * The cyclicity of gas injection and withdrawal also generates an additional stress to clay-rich caprock. Slow-fatigue stress due to adsorption-induced swelling and desorption-induced shrinkage is a significant research gap in the literature. The identification of sub-critical crack propagation in hydrogen-saturated caprock under reservoir conditions requires novel experimental methods [150]. Future research should focus on the long-term mechanical integrity of storage sites under injection cycles, especially creep in salt caverns and microfractures in porous rocks. Field-scale validation and advanced monitoring tools are essential to apply laboratory findings in real conditions.