## 5 Conclusions  
A compositional numerical simulation was conducted to investigate the effects of CH\({}_{4}\) additions upon the coupled CO\({}_{2}\)-enhanced oil recovery (EOR) and carbon storage process. The oil properties were matched to the reference data from Weyburn W3 via the equation of state (EOS) parameter tuning. The minimum miscibility pressure (MMP) between the CO\({}_{2}\)/CH\({}_{4}\) mixture and the reservoir oil was estimated using the multiple-mixing-cell method. Based on the fluid model, the three-phase hysteresis model and Henry's law were applied for accurate simulation of WAG injection under dynamic conditions. The following conclusions were drawn from this study. Because adding CH\({}_{4}\) to the CO\({}_{2}\) stream increased the MMP, the interfacial tension (IFT) between the co-injected gases and the reservoir oil had a higher value than that of CO\({}_{2}\) alone. Further, the addition of CH\({}_{4}\) resulted in a less pronounced reduction in oil viscosity after multiple contacts with the gas stream, thus indicating the lower displacement efficiency. Since CH\({}_{4}\) is less compressible than CO\({}_{2}\), the CO\({}_{2}\)/CH\({}_{4}\) mixture occupied more pore volume than CO\({}_{2}\) in the first WAG cycle, but the reduction in sweep efficiency due to CH\({}_{4}\) addition reversed this tendency in subsequent cycles. Hence, although the CH\({}_{4}\) addition resulted in a higher EOR performance in the early stages due to the compressibility effect, the EOR performance was subsequently reduced due to the lower displacement and sweep efficiencies. The diminished sweep efficiency due to CH\({}_{4}\) addition led, in turn, to a reduction in the residual trapping performance of CO\({}_{2}\). By contrast, the increased reservoir pressure acted to offset the lower sweep efficiency for CO\({}_{2}\) solubility trapping, thus maintaining the performance even with increasing amounts of CH\({}_{4}\) addition. Taking the global warming potentials (GWPs) of the respective gases into consideration, the overall carbon capture and storage (CCS) effects were improved by 123%, 230%, and 313% by the use of CH\({}_{4}\) to CO\({}_{2}\) ratios of 0.1, 0.2, and 0.3 respectively, compared to the use of CO\({}_{2}\) alone. In conclusion, the developed model demonstrates that CH\({}_{4}\) has the combined effect of reducing EOR performance but increasing CCS performance. The above results indicate the significance of the integrated analysis for accurate CO\({}_{2}\) sequestration in depleted or depleting hydrocarbon reservoirs under EOR. **Author Contributions:** Conceptualization, J.C. and K.S.L. ; methodology, software, and validation, J.C., K.S.L., and B.M. ; formal analysis and investigation, J.C.; writing--original draft preparation, J.C.; writing--review and editing, B.M., G.P., S.K., and H.S.L. ; supervision, project administration, and funding acquisition, B.M. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was supported by the National Research Foundation of Korea (NRF) grants (No. 2018R1A6A1A08025520 and No. 2019R1C1C1002574). [PERSON] was partially supported by the NRF grant (No. 2020R1I1A1A01067015). **Acknowledgments:** The authors are grateful to the Computer Modelling Group Ltd. (CMG) for technical support. **Conflicts of Interest:** The authors declare no conflict of interest.