## 3 Overview of UHS Globally and in the Baltics
### UHS Aspects in Lithuania
#### 3.1.1 Simulation Model Outcome  
A preliminary investigation was carried out, with a focus on a deep saline reservoir near Syderai. This part of the basin consists of the Cambrian sandstone reservoir, sealed by carbonaceous-shaly caproks and characterized by the sandstone lithology of the Deimena formation of the Middle Cambrian age [86,93]. The depth of the saline reservoir ranges from 0.3 km to over 2 km. It is one of the largest deep saline aquifer structures in Lithuania, covering an area of 26 km\({}^{2}\) with a thickness of 57 m and a top depth of 1458 m. The aquifer has a porosity of 16%, a permeability of 400 mD, and a net-to-gross value (NTG) of 0.75. In this study, we built a 3D mechanistic model using OGS stratus version 1.8 for the Syderai deep saline aquifer to infer the feasibility of UHS. The reservoir has a temperature of 50 \({}^{\circ}\)C and a pressure of 80 bar. The model's permeability varied in the x, y, and z directions to demonstrate heterogeneity. The cyclic injection and production are carried out using a single well located at the model's center, with six months of injection followed by six months of production over ten years beginning from January 2030 to January 2040. This cyclical system allows H\({}_{2}\) to be extracted during periods of high demand and injected during periods of excess production. The data mentioned in Table 3 have been used as input for the present simulation. \begin{table}
\begin{tabular}{c c c c} \hline
**Properties** & **Syderai** & **Vaskai** & **Gargzdai Hydrocarbon Zone** \\ \hline Stratigraphy & Middle Cambrian & Lowe-Middle Cambrian & Cambrian \\ Porosity (\%) & 16 & 15 & 10 \\ Permeability (mD) & 400 & 300 & 400 \\ Area (km\({}^{2}\)) & 26 & 20 & 200 \\ Top depth (m) & 1458 & 1465 & 1973 \\ Thickness (m) & 57 & 50 & 50 \\ Net to Gross (NTG) & 0.75 & 0.8 & 0.3 \\ \hline \end{tabular}
\end{table} 
Table 1: Properties of the explored saline aquifers and depleted hydrocarbon zone in Lithuania [85,86,88]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Reservoir Type** & **Reservoir Name** & **Sample Name** & **Porosity (\%)** & **Permeability (mD)** \\ \hline Deep saline aquifer & Syderai & S3 & 16 & 246 \\ \cline{2-5}  & Vaskai & V4 & 22 & 1309 \\ \cline{2-5} Depleted Hydrocarbon Reservoir & Pietu Siupariai & PS & 6.64 & 8 \\ \hline \end{tabular}
\end{table} 
Table 2: Petrophysical properties of the core samples extracted from the core samples of Lithuanian reservoirs [90, 91, 92, 93]. In Table 3, Dx, Dy, and Dz are the dimensions of the model (in meter) in the x, y, and z directions, respectively. Likewise, Nx, Ny, and Nz are the number of grids in the x, y, and z directions, respectively. BHPInj and BHPpro are the borehole injection and production pressure in bar, respectively. Methane (CH4) is used as a cushion gas in the present simulation. The cyclic H2 storage simulation is performed using two separate 3D models representing homogeneous and heterogeneous geological settings of the Syderai deep saline aquifer, as shown in Figure 7. Figure 8 highlights the changes observed in water saturation (S\({}_{w}\)) over 10 years for homogenous and heterogenous models and reflects significant reservoir dynamics. From Figure 9, the step pattern indicates the cyclical process of 6 months of injection followed by 6 months of production. In the homogenous model, the total injection (red line) has a consistently steeper slope compared to the total production (green line). In the heterogenous distribution, the steps are less uniform compared to the homogeneous model, suggesting variability in the injection and production rates. It is observed that at the end of the 10 years, the total amount of hydrogen injected is much higher than the total amount produced. Figure 8: Water saturation (S\({}_{w}\)) distribution (**a**) Initial at day 0, (**b**) at day 3652 for homogeneous, and (**c**) at day 3652 for heterogeneous. Figure 7: 3D model, homogeneous (**a**) and heterogeneous (**b**) used for cyclic H2 storage simulation showing permeability distribution.