## 3 Results
### Effects of CH\({}_{4}\) Injection on Carbon Storage Efficiency  
As indicated in Figure 7, the addition of CH\({}_{4}\) to the CO\({}_{2}\) stream reduces the WAG sweep efficiency and gas saturation, thus decreasing the \(S_{gm}\) (Equation (4)) to lower the trapped gas saturation, \(S_{gr}\). As a result, the residual trapped CO\({}_{2}\) by hysteresis (Figure 10) is seen to decrease as the ratio of CH\({}_{4}\) to CO\({}_{2}\) in the gas stream increases. In addition, the amount of solubility-trapped CO\({}_{2}\) decreases as the concentration of CH\({}_{4}\) increases, as indicated in Figure 11. However, this decrease is primarily due to the reduced volume of injected CO\({}_{2}\) and does not mean that the addition of CH\({}_{4}\) lowers the performance of solubility trapping. As mentioned in Section 3.2, the injected total gas rate is set to a constant of 2,265 m\({}^{3}\)/day, so that the amount of injected CO\({}_{2}\) decreases with increasing CH\({}_{4}\) addition. Figure 8: Average reservoir pressure obtained from the four CO\({}_{2}\)-CH\({}_{4}\) WAG cases. Figure 9: Oil recovery factor obtained from the four CO\({}_{2}\)-CH\({}_{4}\) WAG cases. The contributions of the residual trapping and solubility trapping mechanisms to geological carbon sequestration is examined in Figure 12 by comparing the mole proportions of injected and remaining CO\({}_{2}\). Note that movable CO\({}_{2}\) also has the potential to be trapped over time. Thus, compared to Case 1, the residual trapping performances of Cases 2, 3, and 4 are seen to decrease by 13.2%, 20.5%, and 27.4% respectively, due to the lower hysteresis effect and gas saturation. By contrast, the solubility trapping efficiencies remain relatively constant as the amount of injected CO\({}_{2}\) is reduced with CH\({}_{4}\) addition. This is because the CO\({}_{2}\) solubility trapping is affected not only by gas saturation but also by reservoir pressure. Since the added CH\({}_{4}\) increases the reservoir pressure, as indicated in Figure 8, this balances out the negative effect of the reduced CO\({}_{2}\) injection, thereby resulting in the similar CO\({}_{2}\) solubility trapping performance. Meanwhile, the amount of movable CO\({}_{2}\) remaining in the reservoir decreases with increasing concentration of CH\({}_{4}\) due to the gas breakthrough effect. Figure 11: Solubility-trapped CO\({}_{2}\) obtained from the four CO\({}_{2}\)-CH\({}_{4}\) WAG cases. Figure 10: Residual CO\({}_{2}\) trapped by hysteresis obtained from the four CO\({}_{2}\)-CH\({}_{4}\) WAG cases. Although the above results elucidate the aspect of CO\({}_{2}\) sequestration, it is also necessary to consider the contribution of CH\({}_{4}\) storage because the GWP of CH\({}_{4}\) is about 28 times that of CO\({}_{2}\). The GWP is an index of the relative amount of heat trapped by a unit mass of other GHGs compared to CO\({}_{2}\). In the present study, the GWP is transformed into the index per \(10^{7}\) moles of GHG. GWP of CH\({}_{4}\) per \(10^{7}\) moles is ten times higher than that of CO\({}_{2}\) with consideration for the molecular weight of each gas. In other words, the effect of 1 mole of CH\({}_{4}\) sequestration upon the prevention of global warming is the same as that of 10 moles of CO\({}_{2}\) sequestration. Thus, the relative amounts of trapped GHGs are presented in Figure 13, where \(10^{7}\) moles of trapped CO\({}_{2}\) and CH\({}_{4}\) are quantified as 1 and 10, respectively. For Case 1, the net GWP of trapped gases is \(3.94-2.13=1.81\), under the assumption that CH\({}_{4}\) is only produced without re-injection into the reservoir. As the CH\({}_{4}\) concentration increases, the trapped GWPs for the co-injection cases are increased by 123%, 230%, and 313% respectively, compared with Case 1 due to the CH\({}_{4}\) sequestration effect in spite of the reduction in CH\({}_{4}\) storage.