## 3 Results
### Effects of CH\({}_{4}\) Injection on EOR Efficiency
When 20% mole fraction of CH\({}_{4}\) is added into the CO\({}_{2}\) injection stream (Case 3), the deep blue region of Figure 4a changes to the light blue area of Figure 4b, indicating a 23.9% increase in the oil viscosity from 0.46 mPa-s to 0.57 mPa-s. The injected gas displaces the light and intermediate oil components from the initial oil, thereby increasing the oil viscosity after the multiple contact. For Case 1, the oil viscosity near the gas injector is zero because the oil is displaced by the high injection pressure and is miscible with the CO\({}_{2}\). In addition, Figure 4 shows that the injection gas front in Case 3 is more convex than that in Case 1. This is because less gas is acting as a solvent to reduce the oil viscosity, while the fraction of injected gas that does not come into contact with the oil creates a gas channel that leads to early breakthrough of produced gas, as indicated in Figure 5.  
Figure 3: Interfacial tension (IFT) with CH\({}_{4}\) addition into the CO\({}_{2}\) stream during the WAG process under reservoir conditions (T = 63 \({}^{\circ}\)C) for (**a**) 2007–2020, (**b**) 2010–2011. The cumulative gas injection under the reservoir condition obtained from the four CO\({}_{2}\)-CH\({}_{4}\) WAG cases is indicated in Figure 6. Note that although the gas injection rate at the surface is constant, the volume occupied by gas differs in each case because CH\({}_{4}\) is less compressible than CO\({}_{2}\); hence, the more CH\({}_{4}\) is added, the higher the gas injection rate at the reservoir ([PERSON] et al., 2017). In addition, the average gas saturation in each of the four cases for the ten-year WAG period is indicated in Figure 7. In the first WAG cycle (2010-2012), Case 1 displays the lowest average gas saturation, while Cases 2 to 4 indicate that the average gas saturation increases with increasing addition of CH\({}_{4}\) to the gas stream as CH\({}_{4}\) occupies more reservoir pore volume than does CO\({}_{2}\). Interestingly, this trend is reversed in the subsequent WAG cycles due to the poor sweep efficiency resulting from the CH\({}_{4}\) addition, so that the average gas saturation eventually becomes the lowest for Case 4 by the end of the final WAG cycle (2019). Meanwhile, the average reservoir pressure obtained from the four CO\({}_{2}\)-CH\({}_{4}\) WAG cases is indicated in Figure 8, where the greater pore occupation by CH\({}_{4}\) is seen to increase the average reservoir pressure due to the lower compressibility of CH\({}_{4}\) relative to CO\({}_{2}\). As a result, the co-injection of CO\({}_{2}\)-CH\({}_{4}\) generates higher oil recovery than the pure CO\({}_{2}\) WAG during the early stages of WAG  
Figure 4: WAG models of oil viscosity at 0.15 PV injection for (**a**) Case 1, (**b**) Case 3. Figure 5: Cumulative gas production obtained from the four CO\({}_{2}\)-CH\({}_{4}\) WAG cases under standard conditions. (2010-2012), but reduces the overall oil recovery at the end of the WAG process, as indicated in Figure 2:0 This is because the effects of displacement and reduction in sweep efficiency become more dominant than that of compressibility as the WAG proceeds. Figure 2:1 Cumulative gas injection obtained from the four CO\({}_{2}\)-CH\({}_{4}\) WAG cases at the reservoir condition (T = 63 \({}^{\circ}\)C). Figure 2:2 Average gas saturation obtained from the four CO\({}_{2}\)-CH\({}_{4}\) WAG cases.