## 4 CO2 and Crude Oil Imbibition Displacement Experiment
### Results and Analysis  
The experimental replacement conditions are presented in Table 7. Figure 12 shows the core after the replacement experiment. Before the immersion experiment, seven cores were tested using NMR, and the initial oil content of the cores was measured while obtaining the above calibration results of nuclear magnetic semaphore, as shown in Table 8. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**No.** & **Length/cm** & **Diameter/cm** & **Mass/g** & **Mode** & **Pressure/MPa** & **Temperature/\({}^{\circ}\)C** & **Time/h** \\ \hline
1 & 7.48 & 2.5 & 82.439 & Replacement & 40 & 90 & 24 \\
2 & 7.60 & 2.5 & 84.926 & Replacement & 30 & 90 & 24 \\
3 & 7.48 & 2.5 & 83.576 & Replacement & 20 & 90 & 24 \\
4 & 7.67 & 2.5 & 87.158 & Displacement & 20 & 90 & / \\
5 & 7.38 & 2.5 & 86.104 & Replacement & 20 & 90 & 6 \\
6 & 7.56 & 2.5 & 82.887 & Replacement & 20 & 90 & 12 \\
7 & 7.40 & 2.5 & 82.209 & Replacement & 10 & 90 & 24 \\ \hline \end{tabular}
\end{table} 
Table 7: The experimental conditions. Figure 12: The core after the replacement experiment. Figure 13 shows the T2 spectra of the original state of seven cores. The NMR test results of the original state of the seven cores are the basis of the experimental design. The table and figure show that the oil content and porosity of cores 1, 2, and 7 were close to each other; hence, they were used to determine the influence of pressure on the recovery rate. Furthermore, cores 3, 5, and 6 were also close and were used to study the change in recovery rate relative to time. The oil content of core 4 was between that of both groups, and the occurrence of crude oil was relatively close to that of core 3. Therefore, it was designated as a displacement under the same experimental conditions (20 MPa, 90 \({}^{\circ}\)C) as those of core 3, as a comparison of different experimental methods. Figure 14 shows the T2 spectra before and after displacement of seven cores. The imbibition displacement experimental results were obtained after the displacement experiment are presented in Table 9. Recovery curves based on the experimental results were drawn for groups 1, 2, 3, 7, and 3, 5, 6, respectively, as shown in Figures 15 and 16. It can be found that the increase in pressure led to an increase in the displacement capacity of supercritical CO2, and the amount of displaced crude oil was higher at the same time. However, the rate of increase gradually declined. The displacement persisted with time, although with a gradually declining displacement efficiency. Overall, the displacement occurred at a fast rate. Figure 14: T2 spectra before and after displacement of 7 cores. A comparison of the T2 spectra of core NMR before and after groups 1, 2, 3, 5, 6, 7, and 6 of the replacement experiments yielded the following observations:  
* Under the initial conditions, two wave peaks occurred in each of the six cores: 0.01 \(<\) relaxation time \(<\) the left wave peak and relaxation time of 100 \(>\) the right wave peak of 100. * After the replacement experiment, the left wave peak decreased while the right wave peak shifted to the left. The observation on the surface that the left shift of the right wave peak included two processes: first, the original right wave peak disappeared, which corresponds to the displacement of crude oil from the core in the large pore; second, the oil in the small pore represented by the left wave peak gradually migrated into the large pore, and in this continuous process, the oil in the middle pore was monitored. * Comparing cores 1, 2, 3, and 7, in the T2 spectrum after the replacement experiment of cores 1 and 2, the left wave peak not only decreased in its peak value, but also shifted to the left, a phenomenon that is not observed for cores 3, 7, 4, 5, and 6. The intuitive interpretation is that, under high-pressure conditions, some amount of oil is squeezed into smaller pores.