## 2 Research Background  
The geological heterogeneity of naturally fractured-cavity reservoirs is extremely strong. It was proven that several types of reservoir spaces coexist, mainly including karst cavities, tectonic fractures, and dissolved pores. For typical fractured-cavity carbonate reservoirs that are widely distributed, there is a lack of efficient methods to obtain a high oil recovery during exploitation of these difficult-to-produce reserves due to great lateral and vertical heterogeneity. Figure 1a shows the lateral amplitude distribution retrieved from seismic data of a typical fractured-vuggy carbonate reservoir. The deeper the red color, the greater the amplitude, indicating a larger possibility to represent a cavity. Figure 1b displays the vertical amplitude distribution of a typical fractured-cavity body selected from Figure 1a. Figure 1c reflects the 3-D porosity field of the typical fractured-cavity body as shown in Figure 1b, where the deeper the red color, the higher value the porosity of cell grid in the 3-D geological model of the typical fractured-cavity body.  
The production performance in the early stage depends heavily on formation energy. To keep oil production stable, water injection is usually selected to supply energy into reservoir space. However, the decline rate of oil production remains stubbornly high due to a poor understanding of reservoir heterogeneity and configuration patterns of multiple fractured-cavity units, leading to a great amount of oil remaining unexploited in the fractured-cavity reservoir. N\({}_{2}\) huff-n-puff is proven to be an efficient technique to further tap the potential of remaining oil. Figure 2 displays the underlying mechanisms of N\({}_{2}\) huff-n-puff in a fractured-cavity carbonate reservoir. It is found that three steps are necessary to carry out one cycle of N\({}_{2}\) huff-n-puff: gas injection, soaking, and production. When N\({}_{2}\) gas is injected into a typical fractured-cavity carbonate reservoir, driven by gravity segregation, it will gradually move to an upper position of the targeted cavity. The secondary gas cap will gradually move down the oil-water interface, and a large amount of crude oil will flow into the bottom hole of producing wells. The supply of formation energy by N\({}_{2}\) huff-n-puff can effectively inhibit the decline of oil production and rise of water cut.  
Figure 3 displays the production performance of N\({}_{2}\) huff-n-puff for a producer in a typical fractured-cavity reservoir. It indicates that N\({}_{2}\) huff-n-puff achieves a better oil production rate even if the producer is shut down because of abrupt water-rising. However, the actual effect of increasing oil production by N\({}_{2}\) huff-n-puff differs greatly. It is difficult to achieve an adaptive prediction of EOR performance by N\({}_{2}\) huff-n-puff in a fractured-cavity carbonate reservoir. Many factors can affect the performance of enhanced oil recovery by N\({}_{2}\) huff-n-puff in karst reservoirs (e.g., remaining oil saturation, reservoir thickness, formation pressure, cumulative gas storage rate, daily oil production, natural fracture permeability, injection-production strategy, etc.). By taking all influential factors into account, the actual effect of N\({}_{2}\) huff-n-puff enhanced oil recovery in karst reservoirs can be accurately evaluated, but it is difficult to implement. Selecting certain controlling factors provides an alternative to solve this issue.  
Figure 1: Geological characteristics of typical fractured-cavity reservoir. (**a**) lateral distribution of amplitude; (**b**) vertical distribution of amplitude; (**c**) 3-D geological model.  
Figure 3: Actual production performance of N\({}_{2}\) huff-n-puff for a typical producer.  
Figure 2: Mechanisms of N\({}_{2}\) huff-n-puff in karst reservoir (red: oil; blue: water; yellow: gas).