## 3 Results and Discussions
### Foam-Assisted N\({}_{2}\) on Mobilizing Residual Oil Formed from Water Flooding  
(1) Residual attic oil mobilization  
The vug, located at the top of the fracture-vuggy unit controlled by T401, has poor connectivity. A lot of residual attic oil is left after being water-driven, as shown in Figure 8a. The attic oil represents the residual oil at the fracture-vuggy reservoir's top. The injected foam and N\({}_{2}\) gradually spread into the poor connectivity vug with time. N\({}_{2}\) migrates to the vug top and forms an N\({}_{2}\) cap because of the density difference of oil and N\({}_{2}\). With the N\({}_{2}\) and foam injection, the N\({}_{2}\) can accumulate at the vug top and decrease the GOI height. The foam also accumulates to suppress the N\({}_{2}\) channeling owing to the coating effects on N\({}_{2}\), displayed in Figure 8b. The GOI height decreases further with the N\({}_{2}\) cap expansion downward. However, the OWI position remains unchanged, although the GOI  
Figure 7: Synergistic effects of foam-assisted N\({}_{2}\) and bottom water on interface evolution: (**a**) Foam invasion, (**b**,**c**) Foam migration, (**d**) Formation of N\({}_{2}\) cap. height decreases downward (Figure 8c). The higher pressure existing between the GOI and OWI is released from the production well T401. As a result, most attic oil is recovered with the release of pressure. Additionally, compared to the FOI in Figure 7, the GOI always keeps horizontal, corresponding to the discoveries in two-dimensional fracture-vuggy models [29; 30; 37]. Figure 8d shows a stable OWI and a significant decrease in the GOI with the N\({}_{2}\) cap further expansion. Moreover, the position of GOI around well T401 with time is depicted in Figure 9. The slope of this graph represents the decline rate of GOI height. It shows that the GOI height in the vug top of the fracture-vuggy unit controlled by T401 decreases at a constant rate (0.018 mm/s). The result is different from the results obtained from the OWI height decline trends (Figure 6). The GOI height decline rate (0.018 mm/s) is lower than that of the OWI (steep stage, 0.028 mm/s) due to the different distances between their fracture-vuggy structures and the injection well TK467. The closer the distance, the greater the downward driven force, resulting in a fast decline rate [29; 30]. The OWI decline rate (gentle stage, 0.008 mm/s) is lower than that of the GOI (0.018 mm/s), which results from the different connectivity between their fracture-vuggy structures and around structures. The vug, which locates at the top of the fracture-vuggy unit, has poor connectivity. Therefore, the accumulated N\({}_{2}\) and foam are less likely to spread to other fractures and vugs. On the contrary, the accumulated N\({}_{2}\) and foam in an excellent connectivity vug can migrate into other fractures and vugs, causing a lower OWI height decline rate. Figure 8: Mobilization of residual attic oil in the fracture–vuggy unit controlled by T401: (**a**) residual attic oil after water-driven, (**b**) Foam accumulation, (**c**,**d**) Decrease of gas-oil interface. Residual oil film and bypassed oil mobilization  
The fracture-vuggy unit controlled by well TK426 has complex fracture and vug structures and is far away from the injection well TK467. Various residual oil types are formed in the fracture-vuggy unit controlled by TK426, such as oil film and bypassed oil (Figure 10a). The oil film is mainly left in fractures, while the bypassed oil is primarily left in blind vugs, which cannot be mobilized during water flooding. Foam from the adjacent fracture-vuggy unit gradually migrates to the top of the fracture-vuggy unit controlled by TK426. The upward migration behaviors of foam and N2 result from the density difference between oil and N2 foam. The foam slug moves downward with the action of the N2 cap and can detach oil film and mobilize bypassed oil, seen in Figure 10b. In addition, the foam between N2 and oil can suppress N2 channeling and increase the sweep efficiency of N2 due to the foam plugging effect.