# Investigation of Properties Alternation during Super-Critical CO\({}_{2}\) Injection in Shale
## 3 Results and Discussion
### Pore Structure Alteration
Furthermore, the surface of the sample after the CO2 treatment tends to decrease with the flooding time increasing. Table 2 lists the detailed surface area alteration corresponding to the varied Sc-CO2 injection time. For the test sample from the depth of 9848 ft, the surface area was 5.595 m\({}^{2}\)/g initially, while decreases to 5.238 m\({}^{2}\)/g for the 10 days' treatment, then to 4.055 m\({}^{2}\)/g as the Sc-CO2 flooding time up to 30 days. For the test sample from the depth of 9850 ft, the surface area value deducted 27.04% to 4.998 m\({}^{2}\)/g for 10 days' treatment, and 43.58% deduction to 3.865 m\({}^{2}\)/g with the 30 days' Sc-CO2 injection. The main reason for the surface area deduction gradually with the Sc-CO2 flooding time increment may due to the corrosive property of dissolved CO2. The acidizing behavior of some minerals will connect some micro pore to the meso-pore type. The pore size distribution (PSD) was analyzed for the samples with different Sc-CO2 flooding times. In this paper, based on density functional theory (DFT), the adsorption branch of the isotherm curve was chosen for the PSD analysis. Figure 7 represent the PSD vibration corresponding to the Sc-CO2 flooding time variation. It was observed that the PSD curves of all samples present the multimodal characteristic. Several observations were obtained through the comparison of the PSD curve via time duration. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Sample ID**} \\ \cline{2-5}  & \multicolumn{2}{c}{**9848 ft**} & \multicolumn{2}{c}{**9850 ft**} \\ \hline
**Time, Day** & **Surface Area, m\({}^{2}\)/g** & **Deduction Per., \%** & **Surface Area, m\({}^{2}\)/g** & **Deduction Per., \%** \\ \hline
0 & 5.595 & - & 6.85 & - \\
10 & 5.238 & 6.38\% & 4.998 & 27.04\% \\
30 & 4.055 & 27.52\% & 3.865 & 43.58\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Surface area vs. various Sc-CO2 injection time. Figure 6: BET plot of 1/[W((P\({}_{0}\) / P) \(-\) 1)] versus P\({}_{0}\)/P. (**a**) sample at depth of 9848 ft; (**b**) sample at depth of 9850 ft. The Sc-CO\({}_{2}\) injection mainly affect the meso-pore volume rather than the micro and macro pore. The PSD vibration mostly located at the meso-pore zone, where the pore diameter is from 10 nm to 50 nm roughly. The Sc-CO\({}_{2}\) flooding time variation has a significant effect on the meso-pore size distribution. Figure 7a illustrates that with the Sc-CO\({}_{2}\) injection time increases, the percentage of the meso-pore accrescent, and the porosity was increased from 6.3% to 8.5% roughly. While for the samples from the depth of 9850 ft (Figure 7b), the increment of the injection time demonstrated a negative effect on the meso-pore size distribution, and the porosity dropped slightly from 5.1% to 4%. In order to prove that the pore structures of the shale samples changed after the injection. Some SEM images were taken before and after the injection. Figure 8 shows some images. We can clear see some pores appeared after the CO\({}_{2}\) injection (Figure 8b,d) compared with the images before the injection (Figure 8a,b)  
Figure 7: Pore size distribution (PSD) with Sc-CO\({}_{2}\) injection time variation. (**a**) sample at depth of 9848 ft; (**b**) sample at depth of 9850 ft.