## 3 Results and Discussion
### Extraction Phenomena Due to Co-Solvent  
According to the GC results in Figure 4a, it is obvious that the intermediate and heavy components of oil are removed effectively by the CO\({}_{2}\)-ethanol mixture injection at 131 \({}^{\circ}\)F. Meanwhile, based on Figure 4b, the CO\({}_{2}\)-acetone mixture focuses on extracting more heavy components of crude oil at 158 \({}^{\circ}\)F. From these phenomena, co-solvents obviously assist CO\({}_{2}\) to extract intermediate and heavy components during injection. However, if pure CO\({}_{2}\) injection at 131 \({}^{\circ}\)F and 158 \({}^{\circ}\)F are compared, heavy components are extracted more effectively at a low temperature than at a high temperature. This happens because CO\({}_{2}\) solubility decreases as temperature increases [13]. Moreover, a certain amount of acetone and ethanol are left within the remaining oil instead of being vaporized with CO\({}_{2}\) in all proportion during the experiment. \begin{table}
\begin{tabular}{c c} \hline \hline
**Case** & **Explanation** \\ \hline Case 1 & Original oil (without CO\({}_{2}\) injection) \\ Case 2 & Pure CO\({}_{2}\) injection at 131 \({}^{\circ}\)F \\ Case 3 & CO\({}_{2}\)-ethanol mixture injection at 131 \({}^{\circ}\)F \\ Case 4 & Pure CO\({}_{2}\) injection at 158 \({}^{\circ}\)F \\ Case 5 & CO\({}_{2}\)-acetone mixture injection at 158 \({}^{\circ}\)F \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Case studies and their explanations. As discussed earlier, the physical properties, such as molecular weight that is represented by carbon number and molecular structure of crude oil, affect oil extractability [5; 10]. In accordance with the available data, carbon number could be classified into low- (C\({}_{1}\)-C\({}_{2}\)), intermediate- (C\({}_{3}\)-C\({}_{6}\)), and high- (C\({}_{7+}\)) carbon number components. On the basis of molecular structure, hydrocarbons are split into two main classes, which are aliphatic and aromatic. Aliphatic hydrocarbons are further divided into alkane, alkene, alkyne, and their cyclic analogs (cyclo-aliphatic) families [20]. In this work, the structures that were analyzed are alkane, alkene, cyclo-aliphatic (especially cycloalkane), and aromatic. In addition, branched-chain alkanes do not exhibit the same physical properties as the straight-chain alkanes. The different physical properties are caused by the difference in intermolecular forces that is directly affected by hydrocarbon chain arrangement [20]. Therefore, alkanes are further classified, based on the arrangement of hydrocarbon chain, into straight-chain alkanes and branched-chain alkanes. In this case, C\({}_{7+}\) components were excluded from analysis based on chemical structures, due to less detailed information of C\({}_{7+}\) composition. As shown in Figure 5, Case 3 (CO\({}_{2}\)-ethanol mixture injection) removes high carbon numbers even slightly better than Case 2 (pure CO\({}_{2}\) injection at 131 \({}^{\circ}\)F). Meanwhile, Case 2 focuses on extracting intermediate-carbon number components. Low carbon number components such as methane and ethane do not appear on the remaining oil. The higher mole percent of high carbon number of Case 2 and Case 3 than that of Case 1 represents better extraction on intermediate component than heavy component by pure CO\({}_{2}\) and CO\({}_{2}\)-ethanol mixture injection. Because the data is provided in percent, deduction of intermediate composition will increase the mole percent of high carbon number components. According to Figure 6, ethanol assists to extract non-polar structures such as alkanes and cycloalkanes. In addition, ethanol slightly extracts molecules of alkene. Nevertheless, CO\({}_{2}\) is able to work on extraction of all these structures. Figure 4: Gas chromatography (GC) results at (**a**) 131 \({}^{\circ}\)F and (**b**) 158 \({}^{\circ}\)F.  
Figure 5: Remaining oil composition of CO\({}_{2}\)-ethanol mixture injection based on carbon number classification.