## 4 Vettability Alteration Due to Rock Dissolution and Precipitation
### Wettability Alteration Due to Rock Dissolution  
A previous study [71] investigated how mineral dissolution affects multiphase flow properties in carbonate rocks, demonstrating changes in relative phase permeability. Two distinct carbonate rocks, Ketton and Estaillades, were used based on their different pore structures. High-resolution X-ray imaging and mercury porosimetry were employed to characterize their properties, which are shown in Table 4. In the study, nitrogen (N\({}_{2}\)) was used as the non-wetting phase, and deionized water was used as the wetting phase. Eight stages of dissolution were considered, during which 0.5% of the mineral volume was uniformly dissolved into the core sample using an acid solution at a controlled temperature. According to the relative phase permeability curves (Figure 19), the wettability index was calculated using the Lak method. The wettability index ranges from 0.177 to 0.401. Values close to 0.3 indicate a neutral-wet or weakly water-wet rock. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Rock** & **Diameter (cm)** & **Length (cm)** & **Porosity** & **Permeability (mD)** \\ \hline Ketton & 38 & 16.4 & 0.218 & 2840 \\ \hline Estaillades & 38 & 16.5 & 0.279 & 196 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Properties of the samples [71]. Figure 18: Effect of carbon dioxide on droplet shape and contact angle on limestone surface [65]. To study the impact of porosity on wettability, a graph was plotted showing the relationship between the Lak index and porosity (Figure 20). The highest Lak index value (0.40) is observed at stage 8 of dissolution, corresponding to the highest porosity (0.25). This suggests an increase in wettability at high dissolution stages (Figure 20). This is also evident from the relative phase permeability curves, as the residual oil saturation shifts to lower values (Figure 19). This behavior can be attributed to changes in pore structure in the sample. In the Ketton carbonate rock, a decrease in the proportion of pores with throat sizes r > 3.16 \(\upmu\)m was observed with increasing dissolution, while the proportion of pores with throat sizes r < 0.1 \(\upmu\)m increased (Figure 21). The mineral dissolution process led to an increase in the proportion of the smallest pores and a decrease in the proportion of the largest pores, which resulted in increased overall porosity. This change in rock structure created new pathways for the flow of the non-wetting phase in areas that were previously blocked, thereby enhancing relative permeability and bringing the rock's wettability closer to a more water-wet state. Figure 19: Relative permeabilities for Ketton (**top**) and Estaillades (**bottom**) carbonates at different dissolution stages [71]. Porosity gradually increased with advancing dissolution stages for Estaillades. This indicates that the dissolution process of Estaillades rock leads to an increase in porosity. The wettability index calculated from the relative permeability curves (Figure 19) ranges from \(-0.032\) to \(0.286\) (Figure 22). These values indicate varying degrees of wettability: a negative value at stage 0 suggests weak oil-wet conditions, while positive values at other stages indicate neutral wettability. Figure 21: Pore size distribution for Ketton (**left**) and Estaillades (**right**) carbonates at different dissolution stages. The top row shows the raw data, while the bottom row presents pore sizes divided into three categories to more clearly illustrate trends in pore structure evolution [71]. The mineral dissolution process led to a reduction in the proportion of both the smallest and largest pore size groups, while the proportion of medium-sized pores increased (Figure 21). Consequently, the separation of wetting and non-wetting phases became less effective in both fine and large pore networks, as evidenced by changes in the wettability index. Figure 23 shows changes in the Land trapping coefficient as a function of porosity for Ketton carbonates (top) and Estaillades carbonates (bottom).