**4. Conclusions**

(1) Steam flooding assisted by three nonhydrocarbon gases, N2, CO2 and flue gas, could significantly improve the oil displacement efficiency, which was manifested as accelerated oil recovery and a prolonged high production period. Compared with that of pure steam flooding, the recovery factor was increased by 12.13%, 16.71% and 13.01%, respectively.

(2) The effect of enhancing the heat transfer of N2-assisted steam flooding was the best among the three nonhydrocarbon gases, and the CO2 effect was the worst. Compared with steam flooding, N2-, flue gas- and CO2-assisted steam flooding increased the temperature at the end of the sandpack by 14.3 ◦C, 13.1 ◦C and 8.8 ◦C, respectively. That is, N2 could bring more heat into the deep formation under the same conditions.

(3) N2 showed more evident assets in supplementing the formation energy. The maximum displacement pressure differences during steam flooding assisted by N2, CO2 and flue gas were 3.16 MPa, 2.86 MPa and 3.11 MPa, respectively.

(4) The oil content of the sands after undergoing CO2-assisted steam flooding was lower than that of N2-assisted steam flooding for the inlet and middle points of the sandpack, not the outlet, which indicated that CO2 had a more remarkable effect on oil displacement.

**Author Contributions:** Y.H. and W.X. mainly participated in the investigation of the research background and the integration of experimental data. S.C. and B.L. (Boliang Li) participated in the entire experimental process, including designing the experimental plan, recording the experimental detection data, etc. L.D. and B.L. (Binfei Li) mainly participated in the writing of the article. All authors have read and agreed to the published version of the manuscript.

**Funding:** The project was financially supported by National Natural Science Foundation of China (No. ZX20210203) and project: Research on the Mechanism and Parameter Optimization Design of Preliminary Non-hydrocarbon Gas Injection in Heavy Oil Old Areas.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We sincerely appreciate the researchers from the Shandong Engineering Research Center for Foam Application in Oil and Gas Field Development and Xinjiang Oilfield Engineering Technology Research Institute, Xinjiang Oilfield Heavy Oil Development Company for their assistance in this study. The valuable comments made by the anonymous reviewers are also sincerely appreciated. The authors would like to thank the reviewers for their helpful suggestions.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
