## 5 Mechanism Analysis of Heavy-Oil Recovery by Injecting Non-Condensate Gas
### Mechanism Analysis of Heavy-Oil Recovery by Non-Condensate Gas Injection  
The main stimulation mechanisms of conventional steam flooding are viscosity reduction, thermal expansion, steam distillation, and oil-phase permeability improvement. Non-condensate-gas-assisted steam injection to develop heavy-oil reservoirs not only has a steam flooding mechanism, but also has the following flooding mechanisms:  
1. **Insulate and reduce heat loss**. The density of CO\({}_{2}\) is less than the density of steam [92]. N\({}_{2}\) has a low thermal conductivity. Hence, both gases provide excellent heat insulation. Therefore, injecting a certain amount of non-condensate gas during steam injection can act as a heat insulation in the oil casing annulus and reduce heat loss in the wellbore. Non-condensate gas will preferentially occupy the top space of the reservoir to form the overlying phenomenon, so that the steam heats the cold oil area of the stratum, slows down the steam overlying effect, expands the steam sweep range, and improves the heat utilization [93]. Figures 4 and 5 show the temperature field distribution after N\({}_{2}\) injection into formation [94]. Figure 4A shows that the steam chamber extends from the injection well to the production well, Figure 4B shows that the steam chamber mainly expands in the horizontal direction, and Figure 4C shows that the thermal front reaches the production well. Figure 5A shows the expansion of the thermal front, Figure 5B shows the promotion of fluid override, and Figure 5C shows the occurrence of fluid channeling. 2. **Demulsification and interfacial tension reduction**. It is easy to cause high-temperature emulsification in the process of steam injection for heavy-oil and ultra-heavy oil, forming water-in-oil emulsion, which greatly increases the viscosity of crude oil and reduces the fluidity of crude oil. However, the non-condensate gas has a high diffusion coefficient in oil and water. Its diffusion effect can redistribute the gas itself and destroy the water-in-oil emulsion formed by steam injection, stabilize the phase equilibrium of the system, and further reduce the tension at the oil-water interface. In addition, due to the high solubility of CO\({}_{2}\) in crude oil, adding CO\({}_{2}\) to steam can increase the affinity of oil and water, and improve the interfacial activity. It can also break the oil film adsorbed on the rock surface into small movable oil droplets, and greatly reduce the interfacial tension of oil and water [95]. 3. **Increase formation pressure and replenish formation energy**. A certain amount of nitrogen can effectively replenish the formation energy and maintain the pressure during steam injection, thus prolonging the huff-and-puff period and increasing the average pressure drop [92]. The mixed injection of hot steam and CO\({}_{2}\) makes full use of the characteristics of large elastic energy in gas flooding, which can make up for the pressure reduced by steam condensation, so as to maintain the formation pressure. At the same time, CO\({}_{2}\), as a non-condensate gas, can continuously replenish the formation energy, improve the formation pressure, and accelerate the flowback rate of fluid in the process of the end of the soaking well [96]. Figure 6 shows the SAGP technology of a heavy-oil reservoir [97]. 4. **Help to remove blockage and increase seepage capacity. ** N\({}_{2}\) has good displacement property, gas lift, and drainage functions due to its good expansibility. CO\({}_{2}\) is weakly acidic when dissolved in water, which becomes more acidic at high temperature. It reacts with the formation matrix, and acid decomposes some impurities, and removes the pollution near the well zone. In addition, it can also dissolve the mineral components such as calcite and dolomite in the rock, thus enlarging the pores between mineral particles and enhancing the seepage ability of the rock [98].