## 4 Discussion  
### Mineral Dissolution and Precipitation  
Feldspars and carbonates are known to be easily corroded by acidic fluids during CO\({}_{2}\) injection [2; 30; 31], as confirmed by numerical simulations [32; 33; 34], in situ, real-time field monitoring [35; 36], and natural analogies [37; 38]. Changes in fluid ion contents and SEM core observations in the physical simulation confirm that feldspar and carbonate were altered by CO\({}_{2}\) injection. This is consistent with the numerical simulation, which also indicated dissolution of feldspars and carbonates (Figure 7). Both simulations indicate that Si and K, and Ca and Mg exhibit similar trends with ongoing reaction (Figure 4). Statistical analysis of Si, K, Ca, and Mg data using SPSS (Statistical Program for Social Sciences) software indicates correlation coefficients >0.5 (Table 4). Ion contents are thus likely controlled by a common reaction mechanism, as follows. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Correlation Matrix** & **K** & **Ca** & **Mg** & **Si** & **Fe** & **Al** \\ \hline K & 1.000 & & & & & \\ Ca & 0.943 & 1.000 & & & & \\ Mg & 0.989 & 0.979 & 1.000 & & & \\ Si & 0.921 & 0.932 & 0.955 & 1.000 & & \\ Fe & 0.877 & 0.978 & 0.938 & 0.931 & 1.000 & \\ Al & \(-\)0.035 & \(-\)0.143 & \(-\)0.126 & \(-\)0.301 & \(-\)0.330 & 1.000 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Correlation coefficient matrix of the outlet solution ions. Figure 6: Porosity changes over time in the numerical simulation. The main mechanism controlling these reactions involves the formation of H\({}_{2}\)CO\({}_{3}\) from dissolved CO\({}_{2}\), causing the formation water to become acidic (Equation (4)), with reducing pH. Reactions between the acidic fluid and core minerals, especially carbonates and feldspars (Equations (5)-(8), below), buffer formation-water pH, causing an increase in pH of fluid produced during the experiments [39]. This process is described by the following equations:  
\[\text{CO}_{2}+\text{H}_{2}\text{O}\rightarrow\text{H}^{+}+\text{HCO}_{3}{}^{-} \tag{4}\]  
\[\text{CaCO}_{3}\text{ (calcite)}+\text{H}^{+}\rightarrow\text{Ca}^{2+}+\text{HCO}_{3}{}^{-} \tag{5}\]  
\[\text{CaMg (CO}_{3})_{2}\text{ (dolomite)}+2\text{H}^{+}\rightarrow\text{Ca}^{2+}+ \text{Mg}^{2+}+2\text{HCO}_{3}{}^{-} \tag{6}\]  
\[2\text{KAlSi}_{3}\text{O}_{8}\text{ (K-feldspar)}+2\text{H}^{+}+9\text{H}_{2} \text{O}\rightarrow\text{Al}_{2}\text{Si}_{2}\text{O}_{5}\text{(OH)}_{4}\text{ (kaolinite)}+2\text{K}^{+}+4\text{H}_{4}\text{SiO}_{4}\text{(aq)} \tag{7}\]  
\[\Delta\text{G}^{0}=18\text{ KJ mol}^{-1},\Delta\text{S}^{0}=73\text{J mol}^{-1}\]  
\[\text{NaAlSi}_{3}\text{O}_{8}\text{ (albite)}+\text{CO}_{2}+\text{H}_{2} \text{O}\rightarrow\text{NaAlCO}_{3}\text{ (OH)}_{2}\text{ (dasownite)}+3\text{SiO}_{2}\text{ (chalcedony)} \tag{8}\]  
\[\Delta\text{G}^{0}=-132\text{ KJ mol}^{-1},\Delta\text{S}^{0}=-101\text{J mol}^{-1}\]  
where \(\Delta\text{G}^{0}\) is the Gibbs free-energy change and \(\Delta\text{S}^{0}\) is the entropy change. Figure 7: Mineral changes over time in the numerical simulation. The degree of dissolution of albite is significantly greater (by a factor of \(\sim\)2) than that of K-feldspar (Figure 7), possibly due to differences in their \(\Delta G^{0}\) and \(\Delta S^{0}\) values. Albite \(\Delta G^{0}\) and \(\Delta S^{0}\) values are both negative, with albite therefore needing little energy to dissolve, whereas K-feldspar values are positive, with more energy input needed for dissolution. The numerical simulation indicated that some kaolinite (up to 0.25 mol m\({}^{-3}\)) and quartz (up to \(19.8\times 10^{-6}\) mol m\({}^{-3}\)) precipitated after 4 d of reaction (Figure 7), although quartz precipitation can obviously be ignored. Equations (7) and (8) indicate that kaolinite precipitation restrains the reactions, leading to reduced K-feldspar dissolution. This is consistent with the results of other studies [15; 40].