## 4 Experimental Studies at Laboratory Scale
### Case Studies of Experimental Research  
Laboratory investigations are crucial for gaining insights into the behavior and properties of different geological materials under energy storage conditions [135; 136]. These analyses are vital for assessing the integrity of storage structures and identifying potential geomechanical and geochemical consequences [137]. In a controlled laboratory setting, various tests and experiments can be conducted using representative rock (see some examples in Figures 8-10). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Experiment** & **Pressure (MPa)** & **Temperature (\({}^{\circ}\)C)** & **Time (Days)** & **Cycles** & **Rock/Gas/Fluid** & **References (Laboratory)** \\ \hline  & 5 to 10 & 92 & 120 & 1 & \
\begin{tabular}{c} Sandstone \\ H\({}_{2}\), CH\({}_{4}\) \\ \end{tabular} &
\begin{tabular}{c} University of \\ Edinburgh, \\ Scotland, UK \\ \end{tabular} \\ \hline
20 & 10 to 20 & 40 to 130 & 42 & 1 & \
\begin{tabular}{c} Silestone, Sandstone \\ 100\% H\({}_{2}\) \\ \end{tabular} &
\begin{tabular}{c} [PERSON] \\ University Jena. \\ Germany \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._Figure 8: Exposition of a calcarenite (reservoir) to H\({}_{2}\)-rich brine. UNDERGY Project (Misiones, Spain). (**A**) Before H\({}_{2}\). (**B**) After H\({}_{2}\) (conditions, 40 \({}^{\circ}\)C, and 50 MPa, for 30 days). Source: Authors’ unpublished experimental work. Figure 10: Exposition of salt H\({}_{2}\)-rich brine. H\({}_{2}\)Salt Project. (**A**) Before H\({}_{2}\). (**B**) After H\({}_{2}\) (conditions, 35 \({}^{\circ}\)C, and 40 MPa, for 15 days). Source: Authors’ unpublished experimental work. Figure 9: Exposition of a marl (caprock) to H\({}_{2}\)-rich brine. UNDERGY Project (Misiones, Spain). (**A**) Before H\({}_{2}\). (**B**) After H\({}_{2}\) (conditions, 40 \({}^{\circ}\)C, and 50 MPa, for 30 days). Source: Authors’ unpublished experimental work. In research on hydrogen interactions with detrital rocks, varying degrees of reactivity to injected H\({}_{2}\) have been observed under specific reservoir conditions. Petrographic and petrophysical analyses have shed light on these interactions, especially emphasizing the influence of cementing materials within the rock matrix. According to [PERSON] et al. [63], experiments on certain sandstone samples indicated a minimal reactivity between hydrogen and the rock material. This lack of significant reaction was largely due to the scant presence of pore-filling carbonate and anhydrite cements in their samples. These observations suggest that under moderate reservoir conditions, characterized by relatively low temperatures and pressures, along with moderate salinity levels in the formation fluids, the potential for reactivity is limited. Petrophysical measurements taken before and after hydrogen exposure experiments support this finding, with variations remaining within the bounds of methodological error. In contrast, sandstones rich in carbonate and anhydrite cements showed considerable changes (dissolution) [63] and clay mineral transport [138] after exposure to hydrogen. Changes in petrographic characteristics and petrophysical properties indicate a heightened reactivity in these rocks towards H\({}_{2}\), driven by their cement content. Factors such as high reservoir temperatures (\(\geq\)100 \({}^{\circ}\)C), working pressures (\(\geq\)15 MPa), and elevated salinity levels of the formation fluids (TDS \(\geq\) 288 g/L) contribute to this increased reactivity [139]. In the study of [PERSON] and [PERSON] [140], where they exposed sandstone and limestone samples to an H\({}_{2}\)-CH\({}_{4}\) mixture, at 75 \({}^{\circ}\)C, and 10 MPa, for 90 days, insignificant changes were detected at the petrophysical level (by nuclear magnetic resonance and gas porosity analysis), concluding that the potential geochemical reactions (dissolution/precipitation) between the H\({}_{2}\)-CH\({}_{4}\) mixture and rock minerals are very weak. Similarly, the laboratory-scale study of [PERSON] et al.