## 3 Results
### Effects of Impurities in CO\({}_{2}\) Streams on EOR Efficiency
#### 3.2.2 Displacement Efficiency  
Above the MMP, the injected gas is completely mixed with the oil and the interfacial tension (IFT) between the injected gas and fluid would be negligible, affecting the relative permeability curves. The density and viscosity also decrease, leading to miscible displacement. The effect of adding impurities on the IFT between gas and oil in the grid block coordinates (12, 110) was investigated. The lowest IFT of Case 1 was 0.10 mN/m between the CO\({}_{2}\) and reservoir fluid, whereas for Case 2 it was 0.65 mN/m, a 547% higher value. The addition of impurities increased the IFT of Case 2 as shown in Figure 3, indicating that higher pressure would be required to achieve the miscible condition. In the same grid block of (12, 1, 10), the effect of reducing the density of oil was hindered as impurities were added to the CO\({}_{2}\) stream at the beginning of the injection period. After multiple contacts, Case 1 gave the lowest oil density value of 785 kg/m\({}^{3}\) (2.5% reduction in initial oil density), while Case 2 gave a value of 799 kg/m\({}^{3}\) (0.83% reduction in initial oil density). However, at some point, the oil density of Case 1 was greater than that of Case 2 since the intermediate and heavy components of the oil were displaced (Figure 4).  
The cumulative gas injection under reservoir conditions obtained from CO\({}_{2}\) and impure CO\({}_{2}\) WAG is indicated in Figure 5. Impurities in the CO\({}_{2}\) stream have low critical temperatures and have relatively lower compressibility under the reservoir conditions [13]. CO\({}_{2}\) containing impurities would cause an increase in molar volume, i.e., impure CO\({}_{2}\) would occupy more volume than pure CO\({}_{2}\) under the same conditions. Although the injection rate was the same at the surface condition, the total amount of injected gas of Case 2 was greater than Case 1 under reservoir conditions. The gas moves faster in the upper layer in Case 2 than in Case 1, which causes an early gas breakthrough, as shown in Figure 5. This phenomenon occurs because the gas does not lower the viscosity of the oil compared with pure CO\({}_{2}\), and some of the injected gas forms a gas channel, causing early gas breakthrough in the production well, as shown in Figure 6.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Model** & **Viscous Gravity Number, \(N_{gv}\)** \\ \hline Case 1 & 2.1 \\ Case 2 & 5.2 \\ \hline \hline \end{tabular}
\end{table}
Table 10: Viscous gravity numbers of Cases 1 and 2.  
Figure 3: Interfacial tension between CO\({}_{2}\)-impure gas and the oil during CO\({}_{2}\) and impure CO\({}_{2}\) WAG injection.  
The average reservoir pressure is presented in Figure 7. As the impurities are less compressible than CO\({}_{2}\), they occupy a larger volume in the pore, increasing the average reservoir pressure. However, due to significant reduction in the density of the injected stream caused by impurities, a higher flow pressure is required to achieve the same bottom hole pressure in the pure CO\({}_{2}\) case. Therefore, co-injection of impurities and CO\({}_{2}\) causes a relatively higher oil production compared with CO\({}_{2}\) injection at the early stage of WAG cycles. This trend is reversed due to the decrease in vertical sweep and displacing efficiency from the impurities, and the oil recovery is higher for Case 1 as shown in Figure 8. As a result, impurities in CO\({}_{2}\) reduce the oil recovery by 9.2%.  
Figure 4: Oil density during CO\({}_{2}\) and impure CO\({}_{2}\) WAG injection.  
Figure 5: Cumulative gas injection during the CO\({}_{2}\) and impure CO\({}_{2}\) WAG injection under reservoir conditions.