# Investigation of Properties Alternation during Super-Critical CO\({}_{2}\) Injection in Shale
## 3 Results and Discussion
### Mineralogical Alterations  
The whole-rock XRD analysis was applied to detect the composition of the initial and Sc-CO\({}_{2}\) treated samples individually. We assume that the samples hold the similar mineral components since the chips were sliced from the same core plug and then powdered. Table 3 presents the constituent alteration with the varied Sc-CO\({}_{2}\) exposure duration. From the list, the major mineral components are quartz, with around 50%. Dolomite and K-Feldspar also account for a large proportion for the samples from the depth at 9848 ft, weighing 21% and 13.6 respectively. While for the samples from the depth at 9850 ft, the following minerals, analbite and calcite, weigh heavily on the mineral constituent. Minor amounts of other clays were observed in the samples. Comparing two different depth samples, we note that the sample at the depth of 9848 ft has the extra muscovite component with a proportion of 6.5% initially.  
As shown in Figure 9, the mineral compositions tend to change due to the varied Sc-CO\({}_{2}\) adsorption duration treatment. the concentrations of some certain type of minerals, such as K-Feldspar, calcite, dolomite and clay, start to decrease with the Sc-CO\({}_{2}\) flooding time increasing. While the quartz content gets increased with the duration extension, mainly due to the weigh portion of other minerals decrease and some amount of transformation from other components. Another characteristic phenomenon is that the percentage of muscovite get increase dramatically with the Sc-CO\({}_{2}\) flooding time increasing. It was believed that the generation of muscovite occurred during the solid-liquid contact process. The mineral components alteration was supported by the chemical reaction between the contents and the carbonic acid. When the Sc-CO\({}_{2}\) is dissolved into the water in the pore, the weaken carbonic acid will generate and then start to react with some minerals, then alternate the weight proportion in  
Figure 8: The scanning electron microscope (SEM) images of the sample before and after the CO\({}_{2}\) injection (**a**), (**b**) show the images of Sample 1 while (**c**), (**d**) show the images for Sample 2.  
the sample. A series of chemical reaction between the minerals and carbonic acid were initiated and mainly included as follows:  
\[\mathrm{CO_{2\,(aq)}+H_{2}O\leftrightarrow H_{2}CO_{3}\leftrightarrow H^{+}+HCO_{3} ^{-}} \tag{9}\]  
\[2\mathrm{NaAlSi_{3}O_{8}+H_{2}CO_{3}\rightarrow Na_{2}CO_{3}+2 HAlSi_{3}O_{8}} \tag{10}\]  
\[\mathrm{CaCO_{3}+H_{2}CO_{3}\leftrightarrow Ca^{2+}+2 HCO_{3}^{-}} \tag{11}\]  
\[\mathrm{Ca_{x}Mg_{1-x}CO_{3}+H_{2}CO_{3}\leftrightarrow xCa^{2+}+(1-x)Mg^{2+}+2 HCO _{3}^{-}} \tag{12}\]  
\[3\mathrm{K[AlSi_{3}O_{8}]_{(s)}+2H_{2}CO_{3}+12H_{2}O\leftrightarrow K^{+}+2 HCO _{3}^{-}+6H_{4}[SiO_{4}]+KAl_{2}[OH]_{2}/AlSi_{3}O_{10}]_{(s)}} \tag{13}\]  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample ID**} & \multirow{2}{*}{**Treatment Time**} & \multicolumn{6}{c}{**Mineral Content (\%)**} \\ \cline{3-8}  & & **Quartz** & **Analbite** & **Calcite** & **Dolomite** & **Muscovite** & **Clay** \\ \hline \multirow{4}{*}{9850} & 0 & 29.21 & 40.4 & 24.2 & 4.9 & - & 1.29 \\  & 10 & 30.93 & 39.7 & 24 & 4.41 & - & 0.96 \\  & 20 & 32.11 & 39.5 & 23.8 & 3.96 & - & 0.63 \\  & 30 & 33.13 & 39.4 & 23.7 & 3.5 & - & 0.27 \\ \hline \multirow{4}{*}{9848} & 0 & 47.85 & 13.6 & 6.5 & 21 & 6.5 & 4.55 \\  & 10 & 49.96 & 12.4 & 6.1 & 20.3 & 7.2 & 4.04 \\ \cline{1-1}  & 20 & 52.03 & 11.3 & 5.6 & 19.6 & 7.7 & 3.77 \\ \cline{1-1}  & 30 & 54.51 & 9.9 & 5.1 & 18.8 & 8.3 & 3.39 \\ \hline \hline \end