## 5 Mechanism Analysis of Heavy-Oil Recovery by Injecting Non-Condensate Gas  
### Mechanism Analysis of Heavy-Oil Recovery by Gas Injection  
Many scholars have studied the main mechanism of non-condensate gas enhanced oil recovery in heavy-oil reservoirs, including the foam oil formation, crude oil viscosity reduction, crude oil volume expansion, interfacial tension reduction, water phase wettability increase, and influence of three-phase relative permeability [85; 86; 87; 88]. In heavy-oil reservoirs, it is difficult for the injected CO\({}_{2}\) and N\({}_{2}\) to miscible with crude oil because of the very low minimum miscible pressure (MMP) of non-condensate gas and heavy oil. In addition, the interfacial tension between non-condensate gas and crude oil does not decrease significantly. Since this process is an immiscible process, the main mechanisms can be summarized as oil expansion, viscosity reduction, and formation energy replenishment [89]. The main mechanism of non-condensate gas to enhance oil recovery in heavy-oil reservoirs can be summarized as follows:  
1. **Crude oil volume expansion.** Non-condensate gases such as CO\({}_{2}\) and N\({}_{2}\) dissolve in crude oil and expand the volume of crude oil. Compared with other gases such as N\({}_{2}\), CO\({}_{2}\) has a larger expansion coefficient of crude oil. The volume expansion of crude oil can not only improve the formation elastic energy, but also make the expanded crude oil free from the constraints of rock and formation water, thus increasing the oil production [88]. At the same time, gas dissolution expands the volume of crude oil, and the expanded oil pushes water out of the pore space, so that the drainage oil-phase relative permeability is higher than the imbibition water-phase relative permeability, and the relative permeability transformation occurs, which is conducive to the flow environment of oil flow.
2. **Crude oil viscosity reduction**. There is significant crude oil viscosity reduction after the dissolution of CO\({}_{2}\) and N\({}_{2}\). Laboratory experiment results show that the crude oil viscosity decreases with the continuous dissolution of CO\({}_{2}\). There is greater crude oil viscosity reduction after CO\({}_{2}\) dissolution when the original crude oil viscosity is higher. The viscosity of crude oil can be reduced to about one tenth of the original value; that is, the viscosity-reduction range can reach 90%. Reducing the viscosity of the oil phase increases the oil mobility, making it easier to flow through the wellbore. At the same time, it also reduces the remaining oil saturation, thereby increasing production per well [7,8].
3. **Component extraction**. Non-condensate gas can evaporate and extract crude oil. When the injected gas contacts the oil in the reservoir, the hydrocarbons in the oil will evaporate due to the component extraction. The gas phase will be enriched continuously. The injected gas is more likely to miscible with crude oil and improve the oil displacement efficiency [90].
4. **Formation energy replenishment**. Gas injection can effectively replenish formation energy and maintain reservoir pressure, and N\({}_{2}\) has good displacement property, gas lift, and drainage functions due to its good expansibility. N\({}_{2}\) can enter the low-permeability formation and small pores where water finds it difficult to enter. The residual oil left in the reservoir after waterflooding can be displaced from the small pores. In addition, the purpose of maintaining formation pressure by gas injection is to keep the condensate reservoir pressure above the dew point pressure, so as to avoid the reverse condensate phenomenon below the dew-point pressure and reduce condensate and crude oil recovery [91].