## 3 Enhanced Heavy-Oil Recovery by Gas Injection
### Review of N\({}_{2}\) Injection Study in Heavy-Oil Reservoirs  
At present, most reservoirs are developed by waterflooding, and facing the problem of high water cut at the late stage of reservoir development [40]. These types of reservoirs are usually characterized by complex residual oil distribution and rapid increase in water cut, which will affect water injection development performance. It is difficult to maintain effective development through waterflooding. Hence, gas injection has been applied in low permeability reservoirs, fracture-vuggy carbonate reservoirs, tight reservoirs, heavy-oil reservoirs, and fault-block reservoirs instead of waterflooding [41; 42; 43; 44]. As early as the 1970s, numerous laboratory and field pilot tests of N\({}_{2}\) injection to enhanced oil recovery were conducted in the United States and Canada [45]. In cooperation with TOTAL of France, the Yanling Oilfield of China has been conducting feasibility technical demonstrations of enhanced oil recovery in the late stage of carbonate reservoir development since 1986. In 1994, the first field pilot test of N\({}_{2}\) injection flooding was conducted, and a certain stimulation effect was achieved. After N\({}_{2}\) injection in the north area of Yanling Oilfield, the oil-water interface decreased, the comprehensive water cut decreased, and the output increased. The annual oil production for nine consecutive years was higher than that before N\({}_{2}\) injection. In addition, the general trend of gas flooding development is to replace hydrocarbon gas with non-hydrocarbon gas. The application of N\({}_{2}\) in oil and gas development is mainly reflected in the following two aspects: (1) N\({}_{2}\) or mixture of N\({}_{2}\) and other gases injection to enhanced oil recovery mainly contributed to the mechanism of increase formation pressure. Since the thermodynamic characteristics of N\({}_{2}\) are not considered, it is considered as non-thermodynamic enhanced oil recovery [46]; (2) N\({}_{2}\) is injected with steam or viscosity reducer, or N\({}_{2}\) is injected in the annulus to reduce heat loss with the consideration of the low thermal conductivity of N\({}_{2}\), and it is considered as thermodynamic enhanced oil recovery [47]. [PERSON] et al. [48] proposed N\({}_{2}\) injection technology to be applied in the late stage of the steam huff-and-puff process in 2002 and conducted field pilot tests in the Karamay 96 th Area. The research shows that N\({}_{2}\) injection can effectively improve the development performance in the late steam huff-and-puff process, and can be applied to all stages of the heavy-oil steam huff-and-puff process. In 2002, [PERSON] [49] studied the technology of N\({}_{2}\)-controlled bottom water coning and hot N\({}_{2}\) mixed injection to improve the thermal recovery of a heavy-oil reservoir in Shanjiasi. The results show that N\({}_{2}\) injection can effectively control the bottom water coning after multiple cycles of the huff-and-puff process and improve the thermal recovery efficiency in the heavy-oil reservoir with high bottom water thickness. For karst cave carbonate reservoirs in Northwest China, [PERSON] et al. [50] demonstrated the feasibility of N\({}_{2}\) injection in 2008, and [PERSON] et al. [51] conducted a test by using N\({}_{2}\) injection to displace high-position oil in 2013. The test results suggest that N\({}_{2}\) will migrate to the high position under the gravity effect, and forms a secondary gas cap to displace crude oil downward. At the same time, the formation energy is restored and the attenuation of the formation energy is slowed, which would effectively control the bottom water coning, and enable the oil that cannot be driven by waterflooding. In addition, N\({}_{2}\) flooding pilot tests have been conducted in some heavy-oil reservoirs. In 1996, an injection rate of 600 m\({}^{3}\)/h and injection pressure of 10.0 MPa were applied to wells Shan 2-3 and Shan 8-16 in the Shengli Shan Jiasi heavy-oil reservoir.