## 4 Potential Changes in the Caprock Integrity with a Vertical Injector  
Figure 4 brings up the motivation for this study. It compares the injection of 7000 tonnes of CO\({}_{2}\) (3.5% of the initial pore volume) over 30 years through a vertical well perforated at the bottom of the aquifer (Figure 4A) and two cases with horizontal wells (400 m and 800 m in length indicated by \(L_{w}\); Figure 4B,C), simulated in the homogeneous aquifer model. The horizontal wells were located at the same depth of the shallowest perforation of the vertical well at 1170 m. In this figure, the CO\({}_{2}\) in terms of global mole fraction, after 3000 years of redistribution, contacted and penetrated, mainly by diffusion, the caprock, when the injection well was vertical. On the other hand, the longer the horizontal well, the further away the CO\({}_{2}\) plume was from the caprock. \begin{table}
\begin{tabular}{c c c} \hline  & **Sandstone** & **Siliceous Shale** \\ \hline \multirow{2}{*}{**Mineral**} & **Normalized Volume** & **Normalized Volume** \\  & **Fraction** & **Fraction** \\ \hline quartz & 0.614 & 0.277 \\ kaolinite & 0.021 & 0.015 \\ calcite & 0.020 & 0.073 \\ illite & 0.011 & 0.374 \\ albite & 0.210 & 0.063 \\ anorthite & 0.076 & 0.115 \\ chlorite & 0.048 & 0.038 \\ dolomite & 0.000 & 0.021 \\ pyrite & 0.000 & 0.023 \\ \hline \end{tabular}
\end{table} 
Table 6: Mineral composition for the sandstone aquifer and the shale caprock. Figure 4: Cross-sectional view of CO\({}_{2}\) redistribution after 3000 years through a vertical well (**A**), a horizontal well with \(L_{w}\) = 400 m (**B**), and a horizontal well with \(L_{w}\) = 800 m (**C**). The dashed black line marks the top of the sandstone aquifer. From Figure 4, in the vertical case, the upward movement of CO2 was more prominent, leading to contact with the caprock, which had much lower permeability. This resulted in the CO2 plume spreading extensively at the interface between the caprock and the host sandstone formation. In contrast, in the horizontal well case, the CO2 movement exhibited two distinct peaks, representing the toe and heel of the horizontal well. These peaks indicated areas of higher injectivity compared to other sections of the well, primarily due to the larger open area available for fluid flow. The presence of these peaks in the horizontal well configuration suggests that the CO2 distribution and injectivity were not uniform along the entire length of the well. A more significant CO2 spreading with a horizontal well allowed more CO2 to be trapped due to hysteresis. Figure 5 in the study illustrates the CO2 inventory after a period of 3000 years of redistribution. The figure utilizes the following terminology to depict the different forms of the CO2 inventory:  
* % free Sc-CO2: free CO2 as a super-critical fluid;
* % CO2 adsorbed: CO2 adsorbed on the rock by chemical trapping;
* % CO2 in water: dissolved CO2 in aqueous phase, CO2 (aq);
* % residual CO2: CO2 trapped as a residual gas due to relative permeability and capillarity hysteresis. Figure 5 highlights the smaller percentage of free CO2 as a supercritical fluid (1.6% of free Sc-CO2 in purple) and the greater amount as a residual fluid (37% of residual CO2 in yellow) provided when a horizontal injector well was used (Figure 5B). The free Sc-CO2 increased to 13% with a vertical well, and the residual CO2 reduced to 24% (Figure 5A). The CO2 dissolution in the brine (% CO2 in water in blue) was roughly the same in both cases (vertical and horizontal wells), in agreement with the conclusions from [PERSON] et al. [17] for short-term storage evaluations. The higher amount of CO2 adsorbed (4.1% of CO2 adsorbed in green) in Figure 5A with a vertical injector is due to the CO2 penetration in the caprock, which contained 22% of the mass injected. As discussed before, adsorption was much more relevant in the shale rock compared to in the sandstone aquifer minerals, and this was confirmed by Figure 6, with the inventory for each region (caprock and aquifer).