# NMR Evaluation of Shale Oil Mobility: Combined Pyrolysis and CO\({}_{2}\) [PERSON]
## 3 Results and Discussion
### CO\({}_{2}\) Huff-N-Puff-NMR Experimental Results  
Shale plug samples were subjected to four cycles of CO\({}_{2}\) huff-n-puff experiments according to the procedure in Figure 2, and the NMR \(T_{1}\)-\(T_{2}\) spectrum was measured after each cycle. Figure 4 displays the \(T_{1}\)-\(T_{2}\) spectra of sample S42 in its original state, oil-saturated state, and at the end of each huff-n-puff cycle, normalized for easy comparison. The \(T_{1}\)-\(T_{2}\) spectrum signal of the sample was not significant in the original state, but it increased significantly after oil saturation, and subsequently decreased as the number of huff-n-puff cycles increased. Upon observing each huff-n-puff cycle individually, the most pronounced signal change occurred during the first cycle, with diminishing changes in subsequent cycles. This indicates that in the first huff-n-puff cycle, the oil in the large pores was initially produced, and as the number of cycles increased, the production gradually decreased. At this point, the injected CO\({}_{2}\) overcame the capillary pressure and entered the smaller pores, and the oil in these regions began to be produced. From the location of the fluid signal changes, the movable oil is mainly in the marked positions in Figure 4, that is, the large pore region where the change was most significant in the first huff-n-puff cycle. Figures 5 and 6 present the \(T_{1}\)-\(T_{2}\) spectra for samples S28 and S85 post oil saturation and after four CO\({}_{2}\) huff-n-puff cycles, with the most significant changes occurring in the movable oil regions. From the mechanism of CO\({}_{2}\) huff-n-puff experiments, CO\({}_{2}\) is soluble in oil. During the injection process, CO\({}_{2}\) comes into contact with and dissolves in the oil in the samples, causing the oil volume to expand and become movable oil. During the production process, this movable oil is produced due to fluid drive, increasing the recovery rate. Additionally, the dissolved CO\({}_{2}\) also reduces oil viscosity and acts as an extractant, further enhancing oil fluidity. As the amount of dissolved CO\({}_{2}\) in the oil increases, so does the system's energy. During the pressure reduction process, the dissolved gas rapidly expands and escapes, driving the oil to flow out and increasing production capacity. Figure 4: NMR \(T_{1}\)-\(T_{2}\) spectra of the plunger sample S42 in its original state, after saturation with oil, and four cycles of CO\({}_{2}\) huff-n-puff. Monitoring the mass after each CO\({}_{2}\) huff-n-puff cycle allowed for the calculation of oil production, as shown in Figure 7. After the first cycle, the oil content of the sample decreased significantly, while the changes in oil content during the subsequent three cycles were relatively small. This indicates that the movable oil was largely expelled after the first cycle, with the remaining oil primarily residing in small pores and adsorbed on the pore surfaces. Conventional CO\({}_{2}\) huff-n-puff experiments typically calculate recovery rates based on mass changes across different huff and puff cycles. Building on this, we employ a CO\({}_{2}\) huff-n-puff-NMR method to calculate recovery rates and compare them with conventional calculation methods. The gradual decrease in oil mobility over several cycles is primarily due to the near-complete production of strongly movable fluid components in the first two cycles under the experimentally set critical pressure. The remaining components are less movable and bound, making it difficult to produce oil through the huff-n-puff method. Table 1 compares the recovery rates of three samples, derived from both mass change and 2D NMR signal change methodologies [38]. The average recovery rate determined by mass change is 17.9%, while the average recovery rate derived from NMR signal change is 26.9%. This difference is attributed to the fact that the porosity employed in the recovery rate calculations reflects the total porosity of the samples.