## 3 Results and Discussions
### Classification of Residual Oil after Foam-Assisted N2 Flooding  
Figure 11 shows the residual oil types in the three-dimensional visualized fracture-vuggy physical model after foam-assisted N2 and nitrogen flooding. Most residual oil can be observed in the three-dimensional visualized fracture-vuggy physical model after nitrogen gas flooding compared with foam-assisted nitrogen flooding. In other words,  
Figure 10: Mobilizing compound residual oil after water flooding in the fracture–vuggy unit controlled by well TK426: (**a**) Formation of various residual oil, (**b**) Mobilization of oil film and bypassed oil, (**c**) Formation of residual oil after foam-assisted nitrogen gas flooding.  
Figure 9: Graph depicting the position of the GOI around well T401 with time.  
foam-assisted nitrogen flooding technology has better oil displacement capacity than nitrogen gas flooding technology.  
Figure 11 shows the different residual oil types after foam-assisted N\({}_{2}\) flooding and nitrogen gas flooding, respectively. The detailed descriptions and formation progress for the residual oil are exhibited as follows:  
1. Residual oil types after nitrogen gas flooding;
2. Residual bypassed oil  
Most residual bypassed oil can be found in isolated or poorly connected fracture-vuggy units. The displacing phases (water or nitrogen gas) preferentially enter the fracture-vuggy units with excellent connectivity. Once the mainstream channel is formed, the oil in the isolated or poorly connected fracture-vuggy units is locked by displacing phases. As a result, the residual bypassed oil is formed, seen in Figure 11a,b.
3. Residual oil film  
The oil has strong adhesion to the oil-wet surface of model. The injected water and nitrogen gas have no ability to change the IFT between oil and the model's surfaces. Hence, many oil films are left after water and nitrogen flooding because of the high IFT, seen in Figure 11a,b. However, the injected foam can release surfactants to the water phase to reduce IFT. The reduction in IFT is conducive to detaching oil film, shown in Figure 11c,d.
4. Residual attic oil  
Residual attic oil represents the residual oil at the fracture-vuggy unit's top. As the water cut reaches 100%, or the dominant gas channeling is formed, the residual attic oil at the fracture-vuggy unit's top is left, especially in some isolated or poorly connected fractures and vuggs, as shown in Figure 11a,b.
5. Residual oil cluster  
The continuous oil phase can be broken up into large-scale size oil clusters by displacement phases. As the oil is recovered continuously, some of the oil clusters are left in the corner, seen in Figure 11a.  
1. Residual oil types after foam-assisted N\({}_{2}\) flooding;
2. Residual oil in the corner  
Figure 11: Residual oil types in three-dimensional visualized fracture–vuggy physical model: (**a,b**): after nitrogen gas flooding and (**c,d**): after foam-assisted N\({}_{2}\) flooding.  
During foam-assisted N2 flooding, there is almost no oil film left because of the IFT reduction of oil and solid surface. However, some oil film in the corner that is not in the main flow field cannot be detached and displaced, seen in Figure 11c,d.  
Residual oil drop  
Residual oil drops refer to the oil droplets with a smaller size compared to the residual oil cluster. The oil droplets are formed because the IFT of oil and water can be decreased as a result of the presence of surfactants, shown in Figure 11d.