## 3 Results
### Main Topics in Laboratory-Scale Experimentation of UHS
#### 3.2.1 Main Rock Types  
Detrital rocks (i.e., sandstone) and salts (i.e., halite) are the most common rock types used for temporary hydrogen storage, with distinct concepts employed: in rock salt, cavities are engineered for storing the gas, while in detrital rocks, the hydrogen is stored in the porous space. Rock salt is generally considered safe due to its impermeability and its ability to form stable underground caverns (natural or artificial) and for its chemically inert nature, while pure sandstone (i.e., of dominantly quartz composition) is preferred for its high porosity and permeability, low reactivity and expected lower costs of storage. The existence of an adequate rock type together with an appropriate geological structure is fundamental to secure the necessary conditions to retain gas. According to Mi  
Figure 5: Evolution of keywords related to laboratory studies over time. Figure 6: (**A**) Typical underground hydrogen storage reservoir rocks and structures. Porous rock formations (e.g., depleted hydrocarbon reservoirs and saline aquifers), salt (e.g., salt dissolution caverns) and other rock types (e.g., mined rock caverns). (**B**) Schematic of laboratory-scale experimentation in batch and flow setups. Own work. geological structures for underground gas storage are: (i) saline domes (salt caverns), (ii) porous media (depleted hydrocarbon reservoirs and saline aquifers), and (iii) mined rock caverns (Figure 6A). Salt formations within saline domes are a safe option for geological hydrogen storage. Salt has virtually zero permeability, which makes it an ideal geological seal. It is also plastic under pressure, which means that it can adapt and close small fractures, reducing the risk of leakage. In salt domes, it is possible to develop artificial caverns by leaching (dissolving salt with water), creating large underground spaces to safely store hydrogen (e.g., [2]). The \"Clemens-Conoco Philips-, Moss Bluff-Praxair-, and Spindletop-Air Liquid-\" projects in the United States stand out as a notable endeavor investigating the viability of storing hydrogen in former salt deposits [48; 49]. Hydrogen storage in porous media requires a reservoir rock and a seal within an appropriate geological structure that favors structural trapping. These structures are usually saline aquifers or hydrocarbon reservoirs. The most common porous reservoir rock types are sandstones and limestones. Certain sandstones are characterized by the presence of interconnected pores and high permeability that facilitates the movement of fluids within the reservoir and a predominantly siliciclastic composition that can be considered chemically inert under hydrogen storage conditions [49; 50; 51]. As such, these sandstones are generally considered adequate porous reservoir rocks for UHS. Limestone, on the other hand, might pose a direct risk to the success of the storage operation due to the presence of organic matter and its potential interaction with hydrogen [52] and therefore is generally rejected as reservoir rock for UHS. H\({}_{2}\) storage in porous media requires, for trapping, the presence of a seal that prevents gas leakage, acting as a low-permeability diffusion barrier. The cap rock is usually a fine-grained, low-permeability detrital rock, such as mudstone or shale. According to [PERSON] et al. [48], while hydrogen storage within porous media has recently seen growing attention, the only two pilot studies that have injected and recovered hydrogen to date are green methane projects in Austria and Argentina. Argentina's \"Hychico\" project concentrates on the utilization of porous rock formations located in subsurface environments for hydrogen storage [53]. In Austria, the \"Underground Sun Storage\" project represents a pivotal effort focused on the underground storage of hydrogen in a depleted reservoir [54]. These porous geological formations have garnered attention as potential reservoirs for hydrogen due to their widespread distribution and large storage capacity.