## 4 Results and Discussion
### Displacement Characteristics Analysis of Low-Permeability Cores with Different Permeabilities
The gas flow characteristics of cores with different permeability levels are different. The ordinary-low permeability core first goes through the low-gas oil recovery stage (gas-oil ratio less than 50). It enters the rapid rise stage of the gas-oil ratio after 1-2 PV displacement, and finally, gas breakthrough is formed. However, the ultra-low and super-low permeability cores directly enter the stage of the rapid rise of the gas-oil ratio in the initial stage of displacement, and there is no stage of low-gas oil recovery. Due to the presence of the back pressure valve, gas/liquid cannot be produced when the core outlet pressure does not reach the specified pressure (25 MPa/18 MPa). The pressure balance process is when oil and gas migrate within the cores and accumulate to a specific pressure at the cores' outlets. It takes more than 30 min for ultra-low and super-low permeability cores to achieve pressure balance, during which displacement has begun; the absence of gas/liquid production makes it impossible to record the initial displacement's no-gas and low-gas oil recovery stages. In contrast, the ordinary-low permeability cores reach pressure equilibrium in 90 s. They can start displacement immediately, so a relatively complete displacement process can be recorded, and there are no-gas and low-gas oil recovery stages. It can be seen from Figure 12 that the gas-oil ratio at the late stage of miscible displacement is always higher than that of immiscible displacement. There is almost pure CO\({}_{2}\) seepage at the later displacement stage, and the relative permeability of oil is extremely low. Under the same temperature (70 \({}^{\circ}\)C), the injection pressure of miscible flooding is high, the density of injected gas is high (the density of 25 MPa is 11.50 mol/L, the density of 18 MPa is 16.744 mol/L), and the amount of substance under the same volume is large. Since the injection rate is the same (0.2 mL/min), compared with immiscible flooding, the gas produced at the late stage of miscible flooding has a higher gas volume at 23 \({}^{\circ}\)C/0.1 MPa in the same amount of time. In addition, in both ultra-low and super-low permeability cores, the cumulative gas-oil ratio at the initial stage of immiscible displacement is higher than that of miscible displacement. For the same core, under both miscible and immiscible displacement states, the gas injection flow rate is the same, so the same volume of gas has a larger amount of substance under miscible pressure. On the contrary, the cumulative gas-oil ratio of immiscible flooding in the initial displacement is higher than that of miscible flooding. This indicates that more gas is injected in the immiscible displacement pressure equilibrium stage of ultra-low and super-low permeability cores. The interfacial tension of oil and gas  
Figure 11: The cumulative oil recovery degree and the cumulative gas–oil ratio of 7 cores under miscible and immiscible conditions. in miscible flooding is 0, while there is a two-phase interface of oil and gas in immiscible flooding. In the immiscible displacement pressure equilibrium process, the interfacial tension must be overcome, so the energy loss is more significant, and more gas needs to be injected to maintain the stability of the inlet pressure (constant pressure injection is held at the core inlet during the pressure equilibrium stage). With the formation of gas seepage channels in the late displacement period, the accumulated gas in the pressure equilibrium stage is slowly discharged, resulting in the cumulative gas-oil ratio of the miscible phase being gradually higher than that of the immiscible phase. An ordinary-low permeability core only needs 90 s to achieve pressure balance; there is almost no pressure loss, and no additional CO\({}_{2}\) injection is required in the pressure equilibrium stage, so the cumulative gas-oil ratio of miscible flooding is always higher than that of immiscible flooding. Figure 12: Comparison diagram of the cumulative gas-oil ratio of the miscible and immiscible output of 7 cores.