## 3 Results and Discussion
### Co-Solvent Performance Analysis on Assisting CO\({}_{2}\) Extraction  
Extraction solvent selection is based on the \"like dissolves like\" principle, which means that the solvent has good dissolving efficiency when its polarity is similar to that of crude oil. Thus, the polarity of a solvent mixture is significant to the extraction efficiency in crude oil extraction [21]. CO\({}_{2}\) is a non-polar solvent providing fast and efficient extraction of non-polar compounds such as alkanes. However, its large quadruple moment leads to an affinity for polar solutes such as polyaromatic hydrocarbons, aldehydes, esters, and alcohols [5]. Therefore, CO\({}_{2}\) helps to remove polar intermediate components. These reasons explain the phenomena in Figures 6 and 8, in which aromatic-intermediate components are extracted well by pure CO\({}_{2}\) injection. Crude oil is composed of not only non-polar component but also polar component. Co-solvent is selected and expected to assist CO\({}_{2}\) to extract polar component in crude oil that is unable to be extracted by CO\({}_{2}\), especially for heavy polar component. Co-solvent is responsible for increasing the CO\({}_{2}\) solubility in oil by enhancing the physical interactions such as dipole-dipole, dipole-induced dipole, and induced dipole-induced dipole. The effect of co-solvents on crude oil extraction is due to their participation in interactions with both solutes and solvents. Mechanisms to reach that interaction are by their capability to form hydrogen bonding [19] and their ability to interact with polar components. Table 4 displays the parameters of hydrogen bonding and polarity for ethanol and acetone. The notations of \(\beta\), \(\alpha\), and \(\pi^{\bullet}\) represent the tendency of solvent to be hydrogen acceptor, hydrogen donor, and polarizability of solvent, respectively [19]. According to the parameters, ethanol is a better hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) as compared with acetone. Thus, ethanol has a tendency to extract heavy components [18] through HBA, also higher-boiling fractions [5], and associated n-mers [11] influenced by ethanol polarity. Ethanol also slightly extracts aromatic components because they can form a fairly weak hydrogen bond. The strength is reduced as temperature increases due to breakage of the hydrogen bond [22]. For these reasons, ethanol is capable of assisting CO\({}_{2}\) to extract intermediate and heavy components, as shown in Figure 5. Acetone has less capability to form hydrogen bond as the value of \(\beta\) and \(\alpha\) are lower than those of ethanol. Meanwhile, acetone has high polarizability by which it is able to interact with polar and higher molecular weight hydrocarbons [19]. Thus, acetone is also effective in assisting CO\({}_{2}\) to extract polar-aromatic, aromatic [21], and asphaltene [5]. These reasons explain the phenomena in Figures 4b and 8, which show the capability of acetone to extract polar and aromatic-heavy components. tetrahedron-bonding angles of 109.5\({}^{\circ}\), which indicates that they are less stable and more reactive [20]. However, both solvents just slightly extract alkene component. According to the results of the previous analyses, it can be stated that acetone assists CO2 to extract heavy components. It is identified initially from the swelling factor and the IFT experimental results. The CO2-acetone mixture injection shows higher swelling factor and results in lower MMP than those by other cases. The analysis of the GC results supports this phenomenon through previous explanation. Ethanol is classified as a self-associated polar compound and acetone as a non-associated polar compound, because acetone is less likely to form hydrogen bonds than ethanol. Acetone has low cohesion forces because of the difficulty forming hydrogen bonds. Therefore, on the one hand, acetone and CO2 have high affinity molecular structures that can mix with each other. This may be another reason why acetone works well assisting CO2 to extract hydrocarbons.