## 4 Experimental Studies at Laboratory Scale  
### Overview  
UHS laboratory tests can be carried out under either batch or flow conditions (Figure 7). Batch experiments focus on studying hydrogen adsorption, contact angle, and chemical reactions in different rock types. Flow experiments mainly inject pure hydrogen or hydrogen mixtures with other gases (e.g., CO\({}_{2}\), N\({}_{2}\), CH\({}_{4}\)) to determine the dynamic behavior of the gases and the rock, including properties such as permeability and diffusion. Table 3 shows some batch and flow experiments described in the literature, and a more detailed version can be found in Supplementary Materials, Table S1. Table 4 presents some examples of batch experiments with the principal parameters of experimentation. [PERSON] et al. [110] carried out more than 250 batch reaction experiments on different reservoir sandstones simulating subsurface conditions to investigate reactions associated with hydrogen in geological storage, comparing each with an identical, hydrogen-free control to directly link any observed reactions to the presence of hydrogen. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Storage Structures** & **Countries** & **E** &
\begin{tabular}{c} **Gas/Fluid** \\ **Mixtures** \\ \end{tabular} & **References** \\ \hline \multirow{4}{*}{Depleted oil and gas reservoir} & Austria, Australia, China, Germany, Iran, Qatar, Saudi Arabia, United States & B/F & H\({}_{2}\), CH\({}_{4}\), N\({}_{2}\) \\  & & & H\({}_{2}\), H\({}_{2}\)O \\  & & & H\({}_{2}\), CH\({}_{4}\), N\({}_{2}\), CO\({}_{2}\) \\  & & & H\({}_{2}\), natural gas \\ \hline \multirow{4}{*}{Salt cavern} & China, France, Germany & B/F & H\({}_{2}\), CH\({}_{4}\), N\({}_{2}\), CO\({}_{2}\), He & [72; 119; 120; 121] \\  & & & H\({}_{2}\), He, air \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Examples of UHS flow and batch laboratory experiments. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Experiment** & **Pressure (MPa)** & **Temperature (\({}^{\circ}\)C)** & **Time (Days)** & **Cycles** & **Rock/Gas/Fluid** & **References Laboratory** \\ \hline \multirow{4}{*}{250} & \multirow{4}{*}{11 to 20} & \multirow{4}{*}{59 to 80} & \multirow{4}{*}{14 to 56} & \multirow{4}{*}{1} & Five sandstone specimens & \multirow{4}{*}{[110]} \\  & & & & (diameter: 0.35 to 4 mm of & \\ \cline{4-5}  & & & & disaggregated sample) & \\ \cline{4-5}  & & & & N\({}_{2}\), H\({}_{2}\), (purity 99.99 vol \%). & \multirow{4}{*}{University of } \\  & & & & Water (salinities 0, 35, 100, & \\ \cline{4-5}  & & & & and 250 ppt of NaCl). & England, UK \\ \hline \multirow{4}{*}{1 to 30} & \multirow{4}{*}{6 to 30} & \multirow{4}{*}{2 to 10} & e.g., & \multirow{4}{*}{Twelve samples of salt} & \multirow{4}{*}{[60]} \\  & & & Sample & & cavern (diameter: 50.1 to & \\ \cline{4-5}  & & & & 100.3 mm) & \\ \cline{4-5}  & & & & (thermal cyclic & He & Lorraine, France \\ \cline{4-5}  & & & & & \\ \cline{4-5}  & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Examples of batch experiments and experimental parameters. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Storage Structures** & **Countries** & **E** &
\begin{tabular}{c} **Gas/Fluid** \\ **Mixtures** \\ \end{tabular} & **References** \\ \hline \multirow{3}{*}{Saline aquifer} & \multirow{3}{*}{Australia, France, Norway} & \multirow{3}{*}{B/F} & H\({}_{2}\) & \\  & & & H\({}_{2}\), N\({}_{2}\) & \\ \cline{4-5}  & & & H\({}_{2}\), H\({}_{2}\)O & \\ \hline \multirow{3}{*}{Other material} & \multirow{3}{*}{Australia, China, Germany, Iran, Iraq, Malaysia, Qatar, Saudi Arabia} & \multirow{3}{*}{B/F} & H\({}_{2}\) \\  & & & H\({}_{2}\), CH\({}_{4}\), CO\({}_{2}\) & [126; 127; 128; 129; 130; 131; 132; 133] \\ \cline{4-5}  & & & & H\({}_{2}\), CH\({}_{4}\), He \\ \hline \hline \end{tabular} Note: E (experiment type). F (flow). B (batch).  
\end{table} 
Table 3: _Cont._  
Figure 7: Multi-scale parameters and their interrelationship in underground hydrogen storage. Own work.