###### Abstract  
Tabe Oilfield, located in northwest China, is an unconventional fracture-vuggy carbonate reservoir. The foam-assisted nitrogen gas flooding technology has been proven to be a potential EOR technology. However, the flow behaviors of foam-assisted nitrogen gas in fracture-vuggy structures are not clear due to the complex fracture-vuggy structures and their strong heterogeneity. In this work, a three-dimensional visualized fracture-vuggy model is designed and fabricated to investigate the fluids behaviors of foam-assisted N\({}_{2}\) flooding and classify the residual oil types after foam-assisted N\({}_{2}\) flooding. Experimental results reveal that foam slug can enlarge the sweep efficiency, suppress the formation of nitrogen gas channeling, and detach the oil film. Additionally, the evolution processes of the gas-oil and oil-water interfaces are investigated and analyzed. Moreover, the residual oil types after foam-assisted N\({}_{2}\) flooding and nitrogen gas flooding, respectively, are classified and summarized. Compared to nitrogen gas flooding after water flooding, 12.36% more oil can be recovered through foam-assisted N\({}_{2}\) flooding. This work further studies the fluid flow behaviors of foam-assisted N\({}_{2}\) in the three-dimensional visualized fracture-vuggy carbonate model and also confirms the previous achievements. fluid flow behaviors; foam-assisted N\({}_{2}\) flooding; three-dimensional visualized model; residual oil types; unconventional fracture-vuggy reservoir +
Footnote †: journal: Applied Sciences  
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At the beginning, natural energy is utilized to recover crude oil from the subsurface to the surface. However, crude oil production declines very fast due to the rapid depletion of natural energy. The water cut for production wells increases quickly due to the occurrence of bottom water cone [13; 14]. Afterwards, the water-flood production method is applied to replenish reservoir pressure. This technology can recover a certain amount of crude oil, but some drawbacks such as early water breakthrough and a short stable crude oil production period still exist [7; 14; 15; 16]. Recently, to overcome these shortcomings deriving from water flooding, nitrogen gas (N\({}_{2}\)) has been injected to enhance crude oil recovery. N\({}_{2}\) can recover the attic oil by entering the top of fracture-vuggy unit due to its low density [17; 18; 19; 20]. A total of 20% of oil recovery can be achieved in two-dimensional visualized models during N\({}_{2}\) flooding [21]. Additionally, the phase flow behaviors of water and N\({}_{2}\) in a three-dimensional visualized fracture-vuggy model were studied and investigated [22]. Nr was proposed to describe the synergistic effects between the gas cap and bottom water. The field application in Tahe Oilfield has proven the potential of N\({}_{2}\) flooding for enhancing oil recovery [23]. However, gas channeling occurs easily during the N\({}_{2}\) flooding because of the low viscosity and density of N\({}_{2}\). Foam can control N\({}_{2}\) mobility and inhibit gas channeling during foam-assisted nitrogen flooding [24; 25]. Additionally, foam can improve oil displacement efficiency and sweep efficiency, contributing to oil recovery [26; 27]. The foam-polymer flooding method was also proposed to significantly enhance oil recovery in reservoirs [28]. Recent years, the foam-assisted N\({}_{2}\) flooding method, a combination of foam and nitrogen gas flooding, has attracted considerable attention to increase crude oil production [29; 30]. Foam with low mobility can virtually plug the high permeable channel [31; 32]. [PERSON] [33] systematically studied the plugging behaviors of foam at various conditions. A two-dimensional visualized physical model was used to study the migration behaviors of foam [34].