## 4 Results and Discussion  
### Displacement Characteristics Analysis of Low-Permeability Cores with Different Permeabilities  
(1) Pressure balance characteristics  
In the process of CO\({}_{2}\) injection into the cores before displacement, whether it is miscible or immiscible, for K > 1 mD cores, the outlet pressure will rise rapidly after gas injection, the inlet and outlet pressure will be balanced after 90 s, and the CO\({}_{2}\) flow is unimpeded. For 0.1 < K < 1 mD cores, after opening the inlet, it takes about 30-45 min to reach pressure balance; the pressure rise is slow first and then fast, and there is a specific resistance of CO\({}_{2}\) seepage. The pressure equilibrium time of K < 0.1 mD cores is more than 6 h, the resistance of CO\({}_{2}\) seepage is large, and the pressure rises slowly in the process. The lower the permeability and the lower the inlet pressure of the cores, the longer the pressure equilibrium time of the cores and the greater the resistance to gas seepage. (2) Characteristics of the degree of oil recovery  
Table 2 shows the final degree of oil recovery after 5 PV displacement of seven cores under miscible and immiscible conditions. As can be seen from the table, for the oil and gas two-phase flooding process, the final recovery degree of K > 1 mD cores is between 80% and 89%, and the miscible flooding is 18% to 23% higher than the immiscible flooding. The final recovery degree of ultra-low permeability cores is 74-77%, and miscible flooding is 26-29% higher than immiscible flooding. The last recovery degree of super-low permeability cores is only 36-47%, and D7 miscible flooding is only 8-12% higher. In oil and gas two-phase flooding, the higher the permeability is, the higher the final recovery degree is. In K > 0.1 mD cores, miscible flooding can improve recovery, but the improvement effect of ultra-low permeability cores (0.1 < K < 1 mD) is better than that of ordinary-low permeability cores (K > 1 mD). The pore-throat size is too tiny for super-low permeability cores (K < 0.1 mD), the CO\({}_{2}\) seepage resistance is large, and the recovery efficiency is lower. Even when the miscible phase is realized, and the interfacial tension of oil and gas is 0, it still has higher seepage resistance and lower recovery degree. Characteristics of cumulative output gas-oil ratio  
Figure 10 shows the cumulative injected PV number and cumulative gas-oil ratio under miscible and immiscible conditions. Whether it is miscible flooding or immiscible flooding, after 5 PV displacement, the cumulative gas-oil ratio increases rapidly with the decrease in permeability. The reduced core permeability impedes the flow of oil and gas, but the liquid (oil) is much more obstructed than the gas (CO\({}_{2}\)). Therefore, the lower the permeability, the higher the cumulative gas-oil ratio. It can also be seen from the cumulative oil recovery degree and cumulative gas-oil ratio diagram (Figure 11) that under the same cumulative production degree increment, the lower the permeability of the core, the higher the cumulative gas-oil ratio, indicating that the lower the permeability of the core, the lower the gas-carrying capacity. This phenomenon also reflects that the reduction of permeability weakens oil's flow capacity more than gas's. \begin{table}
\begin{tabular}{c c c c} \hline
**Core Number** & **Miscible** & **Immiscible** & **Difference** \\ \hline D1 & 89.86\% & 66.93\% & 22.93\% \\ \hline D2 & 84.97\% & 64.39\% & 20.58\% \\ \hline D3 & 80.77\% & 62.40\% & 18.37\% \\ \hline D4 & 77.31\% & 51.45\% & 25.86\% \\ \hline D5 & 74.47\% & 45.20\% & 29.27\% \\ \hline D6 & 47.44\% & 35.35\% & 12.09\% \\ \hline D7 & 36.40\% & 28.35\% & 8.05\% \\ \hline \end{tabular}
\end{table} 
Table 2: The final degree of oil recovery of 7 cores’ miscible and immiscible displacement after 5 PV. Figure 10: Cumulative injected PV numbers and cumulative gas–oil ratio of 7 cores under miscible and immiscible conditions. Figure 12 shows the cumulative gas-oil ratio comparison of seven cores under miscible and immiscible conditions.