## 7 Conclusions  
This research reviewed the enhanced heavy-oil-recovery technologies using CO\({}_{2}\) and N\({}_{2}\) in China. Both the mechanisms and applications of these enhanced heavy-oil-recovery technologies were reviewed in detail.  
1. Heavy-oil reservoirs are generally developed by thermal recovery, which mainly involves steam injection development, hot water injection development, reservoir fire, and so on. Steam injection development accounts for 97% of the total heavy-oil production. However, in the process of steam injection development, there are many problems, such as the large heat loss of wellbore, serious cross-flow between wells, influence of steam overburden, and unequal production of profile, which result in the difficulty of producing the remaining oil of heavy-oil reservoirs and the increasingly poor production effect.
2. Heavy-oil production by injecting non-condensate gases such as CO\({}_{2}\) and N\({}_{2}\) into the formation and relying on the principles of gas expansion and viscosity reduction, evaporation extraction, and formation energy replenishment is a replacement development technology for the problems of insufficient formation energy and low steam sweep coefficient in the late stage of steam huff-and-puff development, which can effectively improve the steam huff-and-puff effect in high-wheel wells.
3. The condensate-gas-assisted steam injection has more obvious effect than a single gas injection, and the condensate-gas-assisted steam injection technology can greatly improve the physical characteristics of a single vapor phase, making the heat-carrying fluid compressibility stronger, and, with the expansion of the higher performance, can effectively reduce the viscosity of crude oil, improve the seepage flow characteristics of heavy oil, and improve the effect of reservoir development.
4. At present, the thermal composite development technologies, based on steam-non-condensate gas, steam-chemical agent, and steam-organic solvent, are the key technologies for the efficient development of heavy-oil reservoirs in the late stage of steam thermal recovery and difficult-to-produce heavy-oil reservoirs. The development of new technology will bring a qualitative leap to the petroleum industry.  
As the first author, [PERSON] wrote the main manuscript text. [PERSON] and [PERSON] are the main authors of the review, and completed the collection and analysis of the relevant literature. [PERSON], [PERSON] and [PERSON] participated in the analysis and sorting of the literature materials and contributed to the review and editing of this work. As the corresponding author, [PERSON] made substantial contributions to the conception/design of the work and approved the final version to be published. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Science Foundation of China (51774256) and Scientific Research and Technology Development Project of the China National Petroleum Corporation (2021 DQ0105-02) (Standardization research of Chinese, English, and Russian paraphrases or common terms in the oil and gas industry--Phase I).  
Not applicable.  
The authors would like to thank the research team members for their contributions to this work.