## 4 Results and Discussion
### Analysis of Pore-Throat Utilization Characteristics of Cores with Different Permeabilities  
Figure 13 shows the T\({}_{2}\) spectra of the seven cores before displacement, after immiscible displacement, and after miscible displacement. From immiscible displacement to miscible displacement, except for D6 and D7 (K \(<\) 0.1 mD), the larger pore-throat (T\({}_{2}\) \(>\) 10 ms) in other cores (K \(>\) 0.1 mD) is more thoroughly utilized, and the utilization degree is greatly improved compared to that of the finer pore-throat (0.1\(<\) T\({}_{2}\) \(<\) 10 ms). In contrast, in D6 and D7 cores (K \(<\) 0.1 mD), miscible flooding poorly improves the overall pore-throat utilization degree. Even in the large pore-throat of D7, only the size of the oil droplets decreased (the peak T\({}_{2}\) on the right side decreased), and the number of oil droplets increased (the peak signal value on the right side increased). This indicates that large oil droplets are divided into small oil droplets by CO\({}_{2}\) after displacement and remain in the pore-throat. Based on the converted pore-throat radius, seven pore-throat radius intervals of \(<\)3.7 nm, 3.7-10 nm, 10-100 nm, 0.1-0.5 um, 0.5-1 um, 1-2.5 um, and \(>\)2.5 um are set. As can be seen from Figure 14, the radius ranges of D1, D2, and D3 are concentrated in the  
Figure 13: T\({}_{2}\) spectra of 7 cores before displacement, after miscible displacement, and after immiscible displacement. range of 1-2.5 \(\upmu\)m, D4 is 0.5-1 \(\upmu\)m, and the pore-throats of D5, D6, and D7 are 10-100 nm and <3.7 nm. From the point of view of the utilization degree of the pore-throat, the utilization degree of miscible flooding in each pore-throat region is higher than that of immiscible flooding. Except for D7, the maximum pore-throat radius of other cores can be used by more than 80%, and the degree of use decreases with the decrease of pore-throat size. The oil signal in the 0.1-0.5 \(\upmu\)m pore-throat of D7 is unchanged, and the utilization degree is only 2.74-5.96%. The 0.1-0.5 \(\upmu\)m interval was divided into two intervals of 0.2-0.5 \(\upmu\)m and 0.1-0.2 \(\upmu\)m. After D7 displacement, the oil signal of 0.2-0.5 \(\upmu\)m decreased, and the oil signal of 0.1-0.2 \(\upmu\)m increased, showing that large oil droplets dispersed into small oil droplets after displacement, corresponding to the T\({}_{2}\) spectrum after displacement. According to the pore-throat distribution results before displacement, the pore-throat <3.7 nm accounted for 43% in D7, which was dominant, while the highest pore-throat accounted for only 26% in the other six cores. The large pore-throat is scattered in the small pore-throat,  
Figure 14: Pore-throat distribution column diagram and pore-throat utilization degree line diagram of 7 cores. and the difficulty of oil droplets being carried out by the core increases sharply under the obstruction of the small pore-throat. At this time, the gas passing through the large pore-throat mainly separates oil droplets, so the utilization degree of the pore-throat is low. Figure 15 shows the line diagram of the pore-throat utilization degree of seven cores under miscible and immiscible flooding conditions. Under immiscible flooding, the utilization degree of <3.7 nm is 8.65-21.43%, and the utilization degree of 3.7-10 nm is 21.75-35.28%. The utilization degree of the pore-throat of <10 nm is low, and the range is relatively concentrated. In contrast, the utilization degree of miscible flooding <3.7 nm is 20-60%, 3.7-10 nm is 50-73%, and the utilization degree of the <10 nm pore-throat is significantly increased. This phenomenon indicates that immiscible flooding has a poor utilization degree for <10 nm pore-throats, while miscible flooding has a better utilization degree for <3.7 nm. In immiscible flooding, the gas seepage resistance is large, and it is challenging to use <10 nm pore-throats in the cores, so the utilization of the <10 nm pore-throat is low and concentrated.