## 3 Discussion of Experimental Results  
### Huff and Puff Experimental Process and Results  
#### 3.1.1 First Round of Huff and Puff Experimental Process and Results  
The throughput parameters were optimized by conducting four rounds of huff and puff experiments on the experimental cores, and the content of crude oil assigned to the cores was calculated using on-line NMR equipment throughout the throughput experiments.  
In the first huff and puff process, CO\({}_{2}\) was first injected to bring the system pressure up to 25 MPa (Figure 4a), and then the well was stewed for 65 h (Figure 4b), after which the outlet end was opened for production (Figure 4c-e). From the simmering process, it can be seen that the bimodal pattern of the NMR curve changed in signal volume, with the first peak displaying a decrease, while the second peak displaying an increase and a shift to the right. Through the conversion relationship between the relaxation time and pore size, the oil content of the different pore sizes is plotted as shown in Figure 5, from which it can be seen that the crude oil originally endowed in 0-0.02 \(\upmu\)m is displaced into the pore sizes of 0.02-20 nm with the end of the coking process. This means that by CO\({}_{2}\) stewing, wells can displace crude oil from small holes into larger holes, giving it better flow paths, easier production, and improved recovery.  
It can be seen from the first production process that as the system pressure decreases, the first peak of the NMR curve has a significant decrease, and the second peak has an insignificant trend of change. It can be seen from Figure 4f that the crude oil in the small hole is continuously extracted, while the oil content in the 0.02-20-micron-diameter hole displays an insignificant change and a different trend; for example, when the system pressure decreases from 23 MPa to 20 MPa, there is a tendency to increase the oil content in this hole diameter. At this stage, the system pressure decreases to 74% of the original pressure, and with the decrease in system pressure, the extraction degree of crude oil in the first round of production shale is 6.11%, as can be seen in Table 3, calculated by the signal amount of NMR.  
Figure 3: Schematic diagram of the flow of the experiments of segment plug injection of CO\({}_{2}\) huff-n-puff. 1. CO\({}_{2}\) gas cylinder; 2. air booster pump; 3. NMR experimental device; 4. confining pressure pump; 5. intermediate vessel (CO\({}_{2}\)); 6. constant pressure and speed pump; 7. pressure gauge; 8. back pressure valve; 9. graduated cylinder.  
Figure 4: NMR results of the first round of huff and puff experiment. **(a)** The process of injecting CO\({}_{2}\) into the well (the red line is the NMR curve after drying the rock sample, the green line is the NMR curve after saturated oil, and the blue line is the NMR curve after injection of 25 MPa CO\({}_{2}\)); **(b)** the process of stewing the well for 65 h (the green line is the NMR curve after 65 h in the well); **(c–e)** the changes in NMR curves under different production pressures; **(f)** the changes in NMR curves from the beginning of stewing the well to the end of the first cycle of huff and puff (23 MPa, 20 MPa, 18.5 MPa are the NMR curves corresponding to the system pressure in the production engineering).  
Figure 5: Variation in oil content in each pore size of the core during the first huff and puff round (the blue bar graph shows the pore size distribution after stewing the well for 65 h; 23 MPa, 20 MPa, 18.5 MPa are the pore distributions at different production pressures).