## 3 Results
### Main Topics in Laboratory-Scale Experimentation of UHS
#### 3.2.1 Main Rock Types
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**UHS Projects** & **Operator** & **Status** & **Country** & **Year** & \
\begin{tabular}{c} **Energy** \\ **Storage** \\ **Capacity** \\ \end{tabular} &
\begin{tabular}{c} **Type of** \\ **Reservoir Rock** \\ \end{tabular} \\ \hline HyStock-Phase I & Gasunie & Construction & Netherlands & 2028 & 216 GWh \\ \hline H\({}_{2}\) Gronau-Epe & RWE Gas Storage West & Feed & Germany & 2027 & 28 MM Nm\({}^{3}\) \\ \hline Aldbrough Hydrogen & Equinor, SSE & Feed & United Kingdom & 2028 & 320 GWh & Salt (e.g., halite) \\ \hline Spindletop & Air Liquide & Operating & USA & 2014 & 906,000 (m\({}^{3}\)) \\ \hline Moss Bluff & Praxair & Operating & USA & 2007 & 566,000 (m\({}^{3}\)) \\ \hline Teside & Subic & Operating & UK & 1972 & 210,000 (m\({}^{3}\)) \\ \hline  & RAG & & & & \\  & Austria AG, & & & & \\  & Axiom Angewandte & & & & \\  & Prozessetchnik GmbH, & & & & \\  & Energie AG Oberosterreich, & & & & \\  & Energieint at der & & & & \\ Underground & Johannes-Kepler-Universitat Linz, & & & & \\   Sun Storage 2030 & EVN AG, HyCentA Research & Feasibility & Austria & 2030 & 150 GWh \\  & GmbH, KI-MET GmbH, & & & & \\  & Vienna University of Technology, & & & & \\  & University of Natural Resources & & & & Porous rock formation (e.g., sandstone) \\  & and Life Sciences, VERBUND, & & & & \\  & Verein WIVA P\&G and voesalpine & & & (e.g., sandstone) \\ \hline Project Kestrel—Green & & & & & \\ Hydrogen @ Kinsale & ESB, dCarbonX & Feasibility & Ireland & 2021 & 3 TWh \\ \hline Hychico Underground & & & & & \\ Hydrogen Storage & Hychico & Operational & Argentina & 2016 & 120 Nm\({}^{3}\)/h \\ \hline Yela & Enagás & Concept & Spain & 2040 & 1700 GWh \\ \hline Hydrogen seasonal & & & & & \\ storage in Latvia & & & & \\ \hline Lacq Hydrogen & Enagás, Téégga, DH2, & & & & \\  & GazelEnergie & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main reservoir rock types in different UHS Projects based on International Energy Agency [47] and [PERSON] et al. [57]. Together with the petrophysical characteristics of the reservoir and caprock formations, their mineralogical and chemical composition is a key factor in reservoir integrity. Pure salt rock has been found to be inert when in contact with the stored hydrogen gas, but even low levels of impurities (i.e., 1%) might trigger geochemical reactions and alter the geomechanical properties within the reservoir complex (e.g., [61]). Similarly, siliciclastic porous formations are excellent reservoir rocks, not only because of their high porosity and permeability but also because sandstones of quartz, feldspar, hematite, and muscovite composition were found to be non-reactive under UHS conditions [58; 62]. However, if they contain pore-filling anhydrite or carbonate cement, they become chemically reactive when in contact with fluids [63]. The presence of pyrite and goethite can also have a negative geochemical impact on the reservoir (e.g., [64]), while clay minerals can possibly trap H\({}_{2}\)[65]. Possible reactions between rock-forming components and (injected and naturally present) fluids are discussed in detail in Section 3.2.3.