# Experimental Study on the Impact of CO2 Treatment on Different Lithofacies in Shale Oil Reservoirs
## 2 Materials and Methods  
### Sample Preparation  
Our shale samples are from the Qingshankou Formation in the Songliao Basin (Figure 1), which is one of China's most important continental shale oil reservoirs. The Songliao Basin is located in northeast China, and the Qingshankou Formation in this basin develops high quality shales in which extensive hydrocarbon has been shown. The geological resources in this formation are about 54.6 \(\times\) 10\({}^{8}\) t and have huge exploration and development potential. The reservoir of the Qingshankou Formation is mainly shale, with a few sandstone and limestone interbeds, and the formation depth is about 1500-2500 m. The content of brittle minerals is high, and the proportion of clay minerals is 30-60%. The ionic composition of the brine used in the following experiments is shown in Table 1, measured from the produced water.  
The shale samples are processed into small pieces and huge bulk samples by mechanical cutting. Then, the cut surfaces of these bulk samples are polished by an automatic polish-grinding machine. After polishing, the bulk samples are put into an ultrasonic cleaner to remove contaminants and keep the cut surface of these samples flat and clean. These processed small pieces and huge bulk samples are shown in Figure 2a,b.  
The SEM samples are cut from target areas of huge bulk samples and the size of these samples is 0.5 cm \(\times\) 1 cm \(\times\) 1 cm, processed by mechanical cutting. Then, the SEM samples are fixed to sample holders and their top surfaces are polished with sandpaper. After mechanical polishing, the samples are put into an argon ion polisher for 4 hours' processing. Finally, the polished samples are plated with a thin layer of carbon on their surfaces. SEM samples are shown in Figure 2c.  
The shale samples are processed into core samples by numerically controlled wire cutting, and the size of these core samples is 25 mm in diameter and 50 mm in length. After processing, those core samples that have retained their integrity, have no damage and are fracture-free are chosen and polished with sandpaper to keep their surfaces smooth and their end surfaces flat. Then these samples are put into an oil-cleaning instrument for one week to clean off the crude oil and ensure the samples are clean and damage-free. Processed core samples are shown in Figure 2d and the direction of these samples is parallel to the bedding.  
Figure 1: Geographical location of shale samples collection sites.  
Figure 2: Photos of the processed samples: (**a**) crushed samples; (**b**) bulk samples; (**c**) SEM samples; and (**d**) core samples.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Ion** & \begin{tabular}{c} **Na\({}^{+}\) and** \\ **K\({}^{+}\)** \\ \end{tabular} & \begin{tabular}{c} **Ca\({}^{2+}\)** \\ \end{tabular} & \begin{tabular}{c} **Mg\({}^{2+}\)** \\ \end{tabular} & \begin{tabular}{c} **Cl\({}^{-}\)** \\ \end{tabular} & \
\begin{tabular}{c} **SO\({}_{4}\)\({}^{2-}\)** \\ \end{tabular} &
\begin{tabular}{c} **HCO\({}_{3}\)\({}^{-}\)** \\ \end{tabular} \\ \hline concentration (mg/L) & 2320 & 39 & 5 & 1450 & 307 & 3420 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Ionic composition of the brine.