## 3 Results  
### NMR Spectrum  
#### 3.1.1 Sealed Mudstone  
The internal water of mudstone includes interlayer water inside clay mineral particles and pore water between rock skeleton particles and clay particles. According to the principle of the NMR test, the \(T_{2}\) pattern is determined according to the content of H protons in the sample, so the pore water referred to in the NMR test includes interlayer water between the mudstone skeleton particles, between the clay particles, between the skeleton particles and clay particles, and between the internal layers of the clay mineral particles.  
As seen from Figure 4, the \(T_{2}\) pattern of mudstone in the initial state is a single-peak type. The maximum value of the NMR signal corresponds to a chirp time of 0.34 ms; the chirp time is mainly distributed between 0.05 and 1.29 ms, and its area accounts for 99.49% of the spectral region. The other peak total area accounts for 0.51%, which can be ignored, and the characteristics of the \(T_{2}\) pattern indicate that microporosity is dominant within the mudstone sample.  
The size of the \(T_{2}\) spectral area is positively correlated with the internal pore water content of mudstone sealed samples, and the change of mudstone pore volume can be reflected by the change of \(T_{2}\) spectral area under different freeze-thaw cycles. The distribution of the \(T_{2}\) spectral area can calculate the rock porosity and pore distribution, while the evolution of porosity can reflect the change of freeze-thaw damage of rocks. As can be seen from Figure 5, with the increase in the number of freeze-thaw cycles, the \(T_{2}\) spectral area of mudstone samples increased, indicating that the original cracks within the mudstone under freeze-thaw conditions produce expansion or new cracks, and the cracks gradually develop under the repeated cyclic action and ultimately lead to damage.  
Figure 4: \(T_{2}\) spectral of mudstone in the initial state.  
The peaks of the \(T_{2}\) spectra were counted for different number of cycles of the test to obtain Figure 6. From Figures 5 and 6, it can be seen that for the confined mudstone, with the increase in freeze-thaw cycles, the distribution of \(T_{2}\) spectral curves is the same shape; all of them are single-peak type. The NMR peak signal corresponds to a significant increase in the chilling time, from 0.280 ms (initial state) to 0.300 ms (1 freeze-thaw cycle), 0.321 ms (5 freeze-thaw cycles), 0.601 ms (10 freeze-thaw cycles). Meanwhile, the NMR \(T_{2}\) spectrum area increased significantly from 30,077 (initial state) to 30,394 (1 freeze-thaw cycle), 35,946 (5 freeze-thaw cycles), 52,117 (10 freeze-thaw cycles), and 53,548 (15 freeze-thaw cycles), with an increase of 1.05%, 19.51%, 73.27%, and 78.03%, respectively. The growth shows the law of rising first and then decreasing with the increase in freeze-thaw cycles when the number of freeze-thaw cycles is 6-10 times, and the average increase in pore volume of the confined mudstone samples is the largest with 7.327% per freeze-thaw cycle, which indicates that the pore volume expansion caused by water-ice phase change in the freezing process of the confined mudstone under the action of freeze-thaw cycles is different. With the increase in freeze-thaw cycles, the pore volume increases. The freeze-thaw damage accumulates, the pore volume grows with the increase in freeze-thaw cycles, and the freeze-thaw damage accumulates. With the rise in freeze-thaw cycles, the pore volume increases, and the freeze-thaw damage increases. Still, the pore volume increase rate shows the law of increasing first and then decreasing, and there is a maximum interval, which is between 6 and 10 times the freeze-thaw cycles in this test. With the continuous accumulation of freeze-thaw damage, the damaged pores gradually decrease under the condition that the total number of pores is certain.  
Figure 5: \(T_{2}\) spectra of sealed mudstone under different freeze–thaw cycles.