## 3 Experimental Method and Procedure
### Experimental Procedure  
The cores used in displacement have low permeability, and the porosity and core volume are small, resulting in the pore volume also being small. Therefore, the volatilization of oil should be strictly controlled during the experiment. Due to the high discharge pressure, the fine-tuning valve must strictly control the gas velocity in the subsequent pressure relief process. If the pressure relief pressure difference is too large, the CO\({}_{2}\) flow rate in the core will be too high, and then the residual oil after displacement in the core will be carried out, resulting in a serious deviation of the NMR results from the displacement results.  
The displacement experiment flow is as follows:  
1. The cores are directly saturated with simulated oil after drying, weighing, and vacuuming for ten hours.
2. After the saturated oil is treated, the cores are taken out and weighed, and the pore volume is calculated according to the simulated oil density.
3. The cores are quickly loaded into the gripper after weighing, applying 2 MPa confining pressure, closing the inlet and outlet, and heating the cores to the specified temperature (70 \({}^{\circ}\)C) with the heating incubator (the core heating time is 4 h). No confining pressure will make the oil escape into the ring space of the gripper; not closing the inlet and outlet will cause the oil steam to run out of the gripper. In this way, the oil loss during heating can be minimized.
4. After the core is heated to the specified temperature, the gas in the piston must be pressurized to the specified pressure. The gas source pressure is fixed at 5 MPa, and the booster pump can pressurize the low-pressure gas to the specified pressure and inject it into the piston. When the gas in the piston reaches the specified pressure (25 MPa and 18 MPa), the pressurization can end.
5. Before gas injection into the cores, we use the hand pump to increase the confining pressure to 27 MPa/20 MPa, and at the same time, increase the pressure of the back pressure to 25 MPa/18 MPa, and then open the inlet and outlet of the gripper to prepare for gas injection. (6) Displacement can only begin when the cores' inlet and outlet pressures are balanced. Due to the energy loss at the inlet end during the pressure balance process, the pump is always in the constant pressure injection mode. (7) We record the oil output, gas output, and pressure at any time during the oil displacement process, stop the experiment after injecting 5 PV, and then close the inlet and outlet of the gripper, stop heating, and prepare for pressure relief. (8) The fine-tuning valve is placed at the inlet end of the core to strictly control the discharge speed of CO\({}_{2}\) (if the fine-tuning valve is placed at the outlet end, it is easy to carry the remaining oil out). After measurement, it is found that when the gas discharge speed is 150-200 mL/min, the emitted CO\({}_{2}\) does not contain oil, which can better retain the remaining oil in the core. (9) One hour after pressure relief, the gripper temperature drops, and the confining pressure will decrease rapidly. Once the confining pressure is lowered below the core pressure, the oil and gas will immediately escape the cores and be discharged through the trimmer valve. After the temperature drop causes the confining pressure to drop, the confining pressure needs to be increased again until it is higher than the core pressure to ensure that the oil and gas do not escape from the core during the subsequent pressure relief process until the outlet pressure drops to 0 MPa. The ordinary-low permeability cores pressure relief (K > 1 mD) needs about 2 h, the ultra-low permeability cores (0.1 < K < 1 mD) need about 12 h, and the super-low permeability cores (K < 0.1 mD) need about 24 h. During this period, it must be ensured that the confining pressure is always higher than the cores' pressure. After the pressure relief, the remaining NMR oil scan can be carried out. At this point, the experiment is over, and subsequent data processing can be carried out.