# Experimental Study on the Impact of CO2 Treatment on Different Lithofacies in Shale Oil Reservoirs
## 3 Results and Discussion
### Porosity and Permeability Analysis of Different Lithofacial Types of Shale during CO2 Treatment  
In order to explore the effects of CO2 treatment on the porosity and permeability of different lithofacial types of shale, core soaking experiments are carried out. The physical properties of the shale core samples are listed in Table 3. Sample Q1 and Q2 are obtained from foliaceous shale formation. Sample Q3 and Q4 are obtained from laminated felsic shale formation. Sample Q5 and Q6 are obtained from laminated diamictic shale formation. As shown in Table 3, the porosities of core samples are extremely low and only 4.56-8.11% under confining pressure conditions. The porosity of foliaceous shale is the highest and the laminated felsic shale ranks the second, which is consistent with the SEM results above. Furthermore, the connectivity of shale is poor, and the permeability of core samples is extremely low and only 0.0417-0.0767 mD, making it difficult for shale oil to flow. The results of porosity and permeability after CO2 treatment are shown in Figures 8 and 9, and there are significant differences in the change rules for porosity and permeability among the three main types of shale. For foliaceous shale, CO2 treatment has both improving and damaging effects on porosity and permeability. With the increasing treatment time, the porosity and permeability of Q1 sample show a fluctuating, upward trend. After 7 days' treatment, the porosity and permeability of the Q1 sample increase by 8.14% and 62.93%, respectively. Meanwhile, with the increasing treatment time, the porosity and permeability of the Q2 sample decline noticeably. After 7 days' treatment, the porosity and permeability of the Q2 sample decrease by 13.17% and 63.43%, respectively. In combination with SEM analysis, it is revealed that there are many new, small dissolution pores, and precipitation and migration of minerals occur after CO2 treatment. When the dissolution effect dominates, CO2 treatment can improve porosity and permeability significantly (Q1). When the precipitation and migration effects dominate, CO2 treatment shows a damaging effect on porosity and permeability (Q2). Therefore, serious consideration should be given to potential reservoir damage when CO2 is injected into foliaceous shale formations. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Lithofacies**} & \multirow{2}{*}{**Treatment**} & \multicolumn{4}{c}{**Mineral Composition (\%)**} \\ \cline{3-6}  & & **Quartz** & **Felspar** & **Clay** & **Carbonate** & **Other** \\ \hline \multirow{2}{*}{foliaceous shale} & untreated & 26.6 & 11.2 & 52.9 & 6.1 & 3.2 \\  & CO2 treated & 34.3 & 6.8 & 50.2 & 3.3 & 5.4 \\ \multirow{2}{*}{ laminated felsic shale} & untreated & 31.1 & 19.3 & 40.6 & 4.1 & 4.9 \\  & CO2 treated & 39.3 & 12.1 & 38.9 & 2.1 & 7.6 \\ \multirow{2}{*}{ laminated diamictic shale} & untreated & 28.1 & 10.8 & 41.2 & 16.3 & 4.6 \\  & CO2 treated & 37.1 & 5.6 & 43.2 & 7.2 & 6.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mineral composition analysis of shale samples. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Diameter (cm)** & **Length (cm)** & **Porosity (\%)** & **Permeability (mD)** \\ \hline Q1 & 2.50 & 5.02 & 8.11 & 0.0572 \\ Q2 & 2.51 & 5.01 & 7.29 & 0.0443 \\ Q3 & 2.51 & 4.99 & 6.17 & 0.0628 \\ Q4 & 2.50 & 5.02 & 5.42 & 0.0767 \\ Q5 & 2.52 & 5.01 & 5.77 & 0.0417 \\ Q6 & 2.51 & 5.00 & 4.56 & 0.0421 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Physical properties of shale cores. For laminated felsic shale, both porosity and permeability improve significantly after CO\({}_{2}\) treatment. After 7 days' treatment, the porosities of the Q3 and Q4 samples increase by 12.48% and 18.27%, and their permeability increases by 93.31% and 174.97%, both respectively. In combination with SEM analysis, there are many large-sized mineral particles in felsic laminas, such as albite, potash feldspar, calcite and so forth.