## 3 Results and Discussion
### Changes of Physical Properties
#### 3.1.2 Changes in Volume, Porosity and Permeability  
Table 2 shows the final changes of the effective volume and porosity of the cylinder samples from the two groups after the reaction. The permeability of the samples from the two groups at different reaction times is summarized in Table 3. \(P_{c}\) is the confining pressure when the permeability was measured. It is clear that the volume, porosity and permeability of the specimens of Group control barely changed during the test, which is comparable to the results of mass. After 15 days, the volume of the specimens of Group CO\({}_{2}\) exhibited a minor reduction (about 0.14-0.79 cm\({}^{3}\)) in volume, which caused a 0.57-3.15% increase in the porosity. However, the permeability of the samples of Group CO\({}_{2}\) showed a significant increase regardless of the limited volume change of the samples. The permeability also increased exponentially in time. For example, the initial permeability for the 1-1 sample was 0.65 mD when the \(P_{c}\) was 1 MPa, but it increased to 11.40 mD at the end of the trial. Hence, although a tiny amount of rock dissolved during the reaction, it had a tremendous impact on the permeability. Figure 8: Change of Ca\({}^{2+}\) concentration of the solutions from the two groups with time. To obtain further insights into the process of the CO\({}_{2}\)-seawater-limestone reaction, we carried out a detailed analysis of the results in Group CO\({}_{2}\). Figure 9 shows the changes of the effective volume, porosity and permeability of the BUT samples with increasing reaction time. The changes of the volume and porosity were presented as percentages, while the changes of permeability were presented as differences. The error bars in the figure present the standard deviations. In Figure 9, the trend of the permeability with time was similar to the trends of the porosity with time under all confining pressures. The relationship may be explained by the fact that permeability is an intrinsic property of the rock that depends mainly on the effective porosity, and it may experience a bigger change corresponding to a slight alteration of the porosity [38; 39; 40; 41]. Further, the smaller the confining pressure, the easier the seepage, and the higher the permeability [42]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{**Group**} & \multirow{2}{*}{**Number of Sample**} & \multirow{2}{*}{**Size**} & \multicolumn{6}{c}{**Volume(cm\({}^{3}\))**} & \multicolumn{3}{c}{**Porosity (\%)**} \\ \cline{3-8}  & & & **Before** & **After** & \(\Delta\)**V(cm\({}^{3}\))** & **Before** & **After** & \(\Delta\)**P** \\ \hline \multirow{3}{*}{CO\({}_{2}\)} & 1-1 & & 24.45 & 23.66 & \(-\)0.79 & 3.05 & 6.20 & 3.15 \\  & 1-2 & BUC & 23.59 & 23.45 & \(-\)0.14 & 6.98 & 7.54 & 0.57 \\  & 1-3 & & 24.53 & 24.19 & \(-\)0.34 & 2.96 & 4.31 & 1.34 \\ \hline \multirow{3}{*}{Control} & 1-4 & & 22.82 & 22.76 & \(-\)0.02 & 5.32 & 5.58 & 0.26 \\  & 1-5 & buc & 22.56 & 22.52 & \(-\)0.06 & 5.63 & 5.82 & 0.19 \\ \cline{1-1}  & 1-6 & & 22.29 & 22.24 & \(-\)0.02 & 7.70 & 7.90 & 0.21 \\ \hline \end{tabular}
\end{table} 
Table 2: The final changes in the effective volume and porosity of the cylinder samples from the two groups after 15 days of reaction time.