# Experimental Study on the Impact of CO2 Treatment on Different Lithofacies in Shale Oil Reservoirs
## 3 Results and Discussion
### Micomorphology Analysis on Different Lithofacial Types of Shale during CO\({}_{2}\) Treatment  
In order to explore the effect of CO\({}_{2}\) treatment on the micromorphologies of different lithofacial types of shale, SEM analysis is carried out and the results are shown in Figure 7. The original pore structures and micromorphologies of foliaceous shale are shown in Figure 7a. After mechanical and argon ion polishing, the surfaces of the shale samples become smooth and tight, without mineral migration. There are a few small primary pores, and the pore connectivity is poor. After CO\({}_{2}\) treatment, the CO\({}_{2}\)-water-rock reaction significantly affects the surface structures of the shale samples, and many new dissolution pores are generated, as shown in Figure 7d. According to the XRD analysis, the main minerals of the shale samples are albite, potash feldspar, calcite, dolomite and chlorite, and the probable interaction mechanism during CO\({}_{2}\) treatment is shown as follows:  
\[2\text{NaAlSi}_{3}\text{O}_{8}+2\text{CO}_{2}+3\text{H}_{2}\text{O}\to 2 \text{Na}^{+}+2\text{HCO}_{3}{}^{-}+4\text{SiO}_{2}+\text{Al}_{2}\text{Si}_{2} \text{O}_{5}(\text{OH})_{4} \tag{1}\]  
\[2\text{KAlSi}_{3}\text{O}_{8}+2\text{H}^{+}+9\text{H}_{2}\text{O}\to 2 \text{K}^{+}+4\text{H}_{4}\text{SiO}_{4}+\text{Al}_{2}\text{Si}_{2}\text{O}_{5 }(\text{OH})_{4} \tag{2}\]  
\[\text{CaCO}_{3}+\text{H}^{+}\text{$\rightarrow$}\text{Ca}^{2+}+\text{HCO}_{3}{ }^{-} \tag{3}\]  
\[\text{CaMg}(\text{CO}_{3})_{2}+2\text{H}^{+}\rightarrow\text{Ca}^{2+}+\text{ Mg}^{2+}+2\text{HCO}_{3}{}^{-} \tag{4}\]  
\[[\text{Fe}/\text{Mg}]_{5}\text{Al}_{2}\text{Si}_{3}\text{O}_{10}(\text{OH})_{8 }+5\text{CaCO}_{3}+5\text{CO}_{2}\rightarrow\text{5 Ca}[\text{Fe}/\text{Mg}] (\text{CO}_{3})_{2}+\]  
\[\text{Al}_{2}\text{Si}_{2}\text{O}_{5}(\text{OH})_{4}+\text{SiO}_{2}+2\text{H} _{2}\text{O} \tag{5}\]  
Due to the small size and scattered distribution of mineral particles, the sizes of new dissolution pores are small and the improvement in pore connectivity is poor. Meanwhile, CO\({}_{2}\) treatment results in some new mineral precipitation and migration by geochemical reactions. Both the precipitation and migration of minerals block pores and throats and reduce the porosity and pore connectivity of shale samples, leading to a decrease in permeability. The original pore structures and micromorphologies of laminated feisic shale are shown in Figure 7b. The surfaces of the shale samples are smooth and tight, without mineral migration. There are a few small primary pores, and the pore connectivity is poor. After CO\({}_{2}\) treatment, the CO\({}_{2}\)-water-rock reaction significantly leads to many new dissolution pores, as shown in Figure 7e. In certain areas, rich in albite, potash feldspar, calcite and so forth, large-scale dissolution of these minerals occurs and results in many  
Figure 6: SEM analysis of shale samples: (**a**,**b**) foliaceous shale; (**c**,**d**) laminated feisic shale and (**e**,**f**) laminated diamatic shale. large-sized pores. Due to the large sizes of mineral particles in laminas, the sizes of new dissolution pores can reach 10-30 \(\upmu\)m in diameter. Although the precipitation and migration of minerals caused by CO2 treatment partially block pores and throats, the porosity and pore connectivity of the shale samples is still improved significantly under the existence of large-sized dissolution pores, leading to an increase in permeability. The original pore structures and micromorphologies of laminated diamictic shale are shown in Figure 7c. The surface of the shale samples becomes smooth and tight, without mineral migration. Due to the fine diagenetic cementation, there are hardly any micropores, and the pore connectivity is poor. After CO2 treatment, the CO2-water-rock reaction significantly leads to many new dissolution pores, as shown in Figure 7f. There is a large-scale dissolution of some minerals, such as albite, potash feldspar, calcite and so forth.