# The Influencing Factors of CO2 Utilization and Storage Efficiency in Gas Reservoir
## 6 Conclusions  
In this research, the influence of different parameters on CO\({}_{2}\) displacement and storage efficiency during the process of CO\({}_{2}\) injection in depleted gas reservoir was characterized, as well as the storage proportion of different storage mechanisms, through experiments and numerical simulations. 1. In the same injection pore volume, the lower the injection rate, the longer the injection and diffusion duration, the earlier the CO\({}_{2}\) breakthrough, and the lower the ultimate CH\({}_{4}\) recovery efficiency;
2. The lower the core permeability, the earlier the CO\({}_{2}\) breakthrough, and the lower the ultimate CH\({}_{4}\) recovery efficiency, due to the stronger fingering between CO\({}_{2}\) and CH\({}_{4}\);
3. The higher the injection spot (according to the dip angle), the earlier the CO\({}_{2}\) breakthrough, and the lower the ultimate CH\({}_{4}\) recovery efficiency, due to the stronger gravity segregation;
4. As the existence of irreducible water, the ultimate recovery and storage efficiency become higher due to the delay of CO\({}_{2}\) breakthrough caused by dissolution. 5. The density and viscosity ratio of CO\({}_{2}\)/CH\({}_{4}\) reach the maximum when the temperature is close to the critical CO\({}_{2}\) temperature (31 \({}^{\circ}\)C) and when the pressure is about 11 MPa. Therefore, the closer the reservoir condition to this temperature and pressure, the higher the CO\({}_{2}\) displacement and storage efficiency;
6. The higher purity CO\({}_{2}\) is injected, the more CH\({}_{4}\) is displaced and produced, so there is more space for CO\({}_{2}\) in the reservoir, and higher CO\({}_{2}\) storage efficiency will be achieved. When lower purity CO\({}_{2}\) is injected, the gas diffusion and fingering become stronger due to the lower compressibility, viscosity and dissolution of the impurities;
7. In comparison to the positive and compound rhythm reservoirs, the reverse rhythm reservoir has the greatest storage potential since it has the most recent CO\({}_{2}\) breakthrough, the highest CH\({}_{4}\) recovery efficiency, and the lowest storage stage pressure increase;
8. The CO\({}_{2}\) injection enhances the CH\({}_{4}\) recovery efficiency by 5.8%. Among different storage mechanisms, the CO\({}_{2}\) supercritical storage accounts for 83.78%; the CO\({}_{2}\) ionic storage accounts for 12.67%; and the CO\({}_{2}\) mineral storage accounts for 3.85%. Furthermore, part of supercritical storage will transform to mineral storage over time. 9. The urgent need for CO\({}_{2}\) emission reduction was revealed after COP27. With the continuous advancement of subsidy policies and CO\({}_{2}\) commercialization, CCUS should not be limited to oil reservoirs. Through the above studies, CCUS can also achieve fine recovery and storage effects in tilted gas reservoirs with good geological properties. In addition to the above conclusions, controversial factors such as residual water and injection rate need to be further investigated, and more influencing factors should be considered at the same time, which could provide more accurate references for implementing CCUS measures in actual gas reservoirs. Meanwhile, in the context of the further improving CCUS subsidy policies and accelerating the commercialization of CO\({}_{2}\) after COP27, further research is needed on adaptable and economic evaluation of CCUS for gas reservoirs. Conceptualization, Y.T. and Y.L. ; methodology, Y.L. and J.Q. ; software, J.Q. ; validation, formal analysis, investigation, writing--original draft preparation, Y.L. ; resources, data curation, J.C.; writing--review and editing, visualization, J.Q. and J.C.; supervision, project administration, funding acquisition, Y.T. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by National Natural Science Foundation of China (51974268), and the Sichuan Province Science and Technology Program (2019 YJ0423). **Institutional Review Board Statement:** Not applicable.