## 1 Introduction  
There are about 4 billion tons of onshore heavy-oil resources in China, which reserves more than 20% of the total oil resources in China. At present, more than 70% of heavy-oil fields have been discovered in 12 basins [1; 2], including the Liaohe, Shengli, Henan, and Xinjiang Oilfields. Heavy-oil production accounts for about 10% of total crude oil production each year in China. China is the world's fourth-largest producer of heavy oil after the United States, Canada, and Venezuela. Conventional mining methods are difficult to exploit for heavy-oil reservoirs, which generally adopt thermal production or cold production. Thermal production mainly includes steam injection, hot water injection, and the in situ combustion. Cold production mainly includes microbiological recovery, gas injection, oil displacement, etc. The steam injection method accounts for 97% of the total recovery of heavy oil [3]. As shown in Figure 1, it is the main steam injection thermal recovery method. However, the performance of the steam injection method is limited by the heat loss of wellbore, cross-flow between wells, steam overburden, and uneven production of profile. As a result, it is difficult to produce the remaining oil in heavy-oil reservoirs [4]. Therefore, in the face of the increasing water cut and increasing exploitation difficulty in the later stage of reservoir development, the heavy-oil reservoir changed its development mode, and experimental research and field practices of injecting non-condensate gas were carried out [5,6]. There are many kinds of non-condensate gases, including CO\({}_{2}\), N\({}_{2}\), CH\({}_{4}\), and flue gas. The main components of flue gas are N\({}_{2}\) and CO\({}_{2}\). CH\({}_{4}\) is not commonly used in petroleum exploitation. Therefore, common types of gases include N\({}_{2}\) and CO\({}_{2}\) due to abundant gas sources and lower prices [7]. Compared with water vapor, N\({}_{2}\) is more readily available due to its high concentration in air, which is conducive to low-cost and efficient development under the situation of low oil price. CO\({}_{2}\) emission in the atmosphere is the main cause of the greenhouse effect. In the application of the non-condensate gas injection technology, CO\({}_{2}\) is injected to the reservoir to enhanced oil recovery and reduce the carbon concentration in the atmosphere through CO\({}_{2}\) storage. The injection of non-condensate gas could reduce the viscosity of heavy oil, increase the volume coefficient of crude oil, and expand the sweep range of waterflooding and the degree of recovery. The injection of N\({}_{2}\) and CO\({}_{2}\) would change the PVT characteristics of the heavy-oil system, such as the viscosity, dissolved gas-oil ratio, bubble point pressure, density, and volume coefficient [8]. As one of the commonly used heavy-oil-recovery technologies, the non-condensate gas injection method has been successfully implemented in some heavy-oil reservoirs, such as the Shengli, Xinjiang, and Bohai Oilfields. It is especially suitable for reservoirs in the late stage of steam injection production, such as steam huff and puff, steam flooding, and SAGD (Steam-Assisted Gravity Drainage) production technology [9]. The non-condensable gas injected in the SAGD production process is SAGP (Steam and Gas Push). Compared with traditional SAGD, adding non-condensable gas can maintain the effective expansion of a mature SAGD steam chamber, which will have higher oil-displacement efficiency. Figure 2, shows the development of a steam cavity after injecting non-condensate gas during the SAGD development of heavy-oil reservoirs. Non-condensate gas injection stimulation technology would not only effectively improve oil recovery, but could also help to achieve carbon neutrality, which has a very broad application prospect in the future oil recovery, energy utilization, environmental improvement, and other aspects. Therefore, this research reviewed the enhanced heavy-oil-recovery technologies using CO\({}_{2}\) and N\({}_{2}\) in China.