## 3 Results and Discussion  
### Pore Size Distribution, Porosity, and Absolute Permeability  
Pore size distributions for sub-volumes from samples A and B before (blue columns) and after (orange columns) their dissolution are shown in Figures 7 and 8. The equivalent pore radius was used as a characteristic size. Figure 6: Pore space (gray) of the sub-volumes extracted from cylindrical samples A and B. As shown in Figure 7, the number of small pores (\(<\)50 um) in all sub-volumes extracted from sample A decreased after rock dissolution. This indicates an increase in their radii due to the interaction of HCl solutions with the carbonate rock. Additionally, there was a noticeable increase in the number of large pores (those with radii greater than the average pore radii) after the rock dissolution, especially for sub-volumes A1, A2, and A4. These larger pores may have formed due to the widening of individual large pores or the merging of several small pores as a result of rock dissolution. Despite the different conditions of acid solution injection (see Table 2), a similar situation was observed in the sub-volumes of sample B (Figure 8). However, it can be noted that there are relatively small decreases in the number of small pores (Figure 8) compared to the sub-volumes of sample A (Figure 7). Table 3 provides a brief summary of pore statistics for the sub-volumes of both samples before and after their dissolution. As seen in this table, the lower boundary of the pore size distribution remained nearly unchanged for all sub-volumes except for sub-volume A5. In the case of A5, all interconnected small pores with radii below 23 um disappeared as a result of rock dissolution. Furthermore, the shift of the lower boundary of the pore radius range to the right indicates that the rock dissolution did not lead to the formation of new interconnected pores with radii smaller than the original small pores. It is also worth noting that the upper boundary of the pore radius range increased for all considered sub-volumes, except A2, where it decreased by less than 5% from the initial value. Figure 7: Pore size distribution of sub-volumes from sample A.  
Figure 8: Pore size distribution of sub-volumes from sample B. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sub-Volume**} & \multicolumn{2}{c}{**Min, \(\mu\)m**} & \multicolumn{2}{c}{**Max, \(\mu\)m**} & \multicolumn{2}{c}{**Mean, \(\mu\)m**} \\ \cline{2-7}  & **Before** & **After** & **Before** & **After** & **Before** & **After** \\ \hline A1 & 17.13 & 17.17 & 173.7 & 236.3 & 61.6 & 71.9 \\ \hline A2 & 17.13 & 17.17 & 194.9 & 185.9 & 62.7 & 69.0 \\ \hline A3 & 17.12 & 17.18 & 238.2 & 439.1 & 68.0 & 73.5 \\ \hline A4 & 17.12 & 17.18 & 220.7 & 340.8 & 65.4 & 72.0 \\ \hline A5 & 17.13 & 23.23 & 279.6 & 480.9 & 67.8 & 72.1 \\ \hline B1 & 17.10 & 17.18 & 178.8 & 392.4 & 63.9 & 68.6 \\ \hline B2 & 17.10 & 17.18 & 167.2 & 190.2 & 62.3 & 66.4 \\ \hline B3 & 17.09 & 17.18 & 211.3 & 216.3 & 62.6 & 67.53 \\ \hline B4 & 17.09 & 17.17 & 211.3 & 219.8 & 63.6 & 69.45 \\ \hline B5 & 17.10 & 17.18 & 180.9 & 193.0 & 60.3 & 65.17 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Brief statistics of the pore radius of sub-volumes from samples A and B. Overall, there was a significant increase in the maximum pore radius, indicating the formation of wide channels connecting large pores as a result of rock dissolution. The average pore radius of the sub-volumes increased from 6.5 to 16.8%. The smallest and largest increases were obtained, respectively, for sub-volumes A5 and A1 (Table 3). The effect of rock dissolution on the porosity and absolute permeability of the examined sub-volumes is demonstrated in Figure 9. The values of porosity and absolute permeability of the sub-volumes before and after their dissolution are given in Table 4. As Figure 9a shows, the porosity and absolute permeability of the sub-volumes changed differently as a result of rock dissolution.