# Experimental Analysis of Pore and Permeability Characteristics of Coal by Low-Field NMR
## 3 Discussion  
### Pore Type of NMR  
#### 3.1.1 Morphology and Variation of \(T_{2}\) Spectra  
The \(T_{2}\) spectra of the saturated coal were closely related to PSD, so their morphological characteristics reflected pore structure, PSD, and connectivity. The \(T_{2}\) of the saturated coal sample was positively correlated with the pore size. The \(T_{2}\) spectra peaks indicated that the corresponding pores were more developed. The peak width reflected the PSD range, and it can reflect the flow characteristics of the fluid in the pores in comparison with the results of the centrifugal coal samples [20].  
Figure 2 shows the \(T_{2}\) spectra of the 8 dried, saturated, and centrifugal samples, respectively. There were noticeable differences in the patterns of the \(T_{2}\) spectra of the coal samples from different mining areas, and their shapes differed obviously before and after the centrifugation.  
The \(T_{2}\) spectra of the dried coal samples were unimodal, and their peak intensity and width were different. The \(T_{2}\) spectra of samples from the same mining area were coincident or similar, and the intensity of CZ, YJL, and WY decreased greatly. The above characteristics indicated that there were some unbound hydrogen nuclei in the dried coal. The hydrogen atoms maybe belonged to the methane or gaseous water molecules adsorbed in the micropores. There was a slight difference between the amount of the hydrogen nuclei and the size of the pores absorbing methane or water vapor in the coal samples from different mining areas.  
There were noticeable differences in the number, position, and intensity of the \(T_{2}\) spectra peak of saturated coal samples from different mining areas. The \(T_{2}\) spectra of the CZ coal sample was bimodal, and the two peak positions were corresponding to the \(T_{2}\) of 1 ms and 100 ms, respectively. The left peak was an asymmetrical left monoclinic distribution, and its intensity was far higher than the right peak. The two peaks were completely separated, indicating that many small pores were developed in the coal samples (Figure 2a,b). The \(T_{2}\) spectra of the YJL coal sample had three peaks, they were located at 0.8 ms, 10 ms, and 300 ms, and their intensity decreased significantly with the increase of \(T_{2}\) (Figure 2d). The \(T_{2}\) spectra of the WY coal sample also had three peaks, and the peak positions were distributed in 0.4 ms, 15 ms, and 200 ms, respectively. They were of a continuous distribution and with no obvious crossover. The intensity of the peaks was significantly decreased with the increase of \(T_{2}\), and the amplitude was obviously higher than that of YJL coal sample (Figure 2c).  
After centrifugation, the variation of the \(T_{2}\) spectra of the coal samples from different mining areas was different. The first peak intensity of the CZ coal sample decreased slightly, and the secondary peak was completely disappeared. The first peak intensity of the YJL coal sample was invariable, and the secondary peak and the minimum peak disappeared. The first and secondary peak of the WY coal sample decreased to a certain extent, and the reduction of the minimum peak was the most obvious.  
The morphology and peak position of the \(T_{2}\) spectra of the bedding and vertical coal samples was basically the same. Only the first peak intensity of the WY vertical coal sample was obviously lower than that of bedding coal sample under the condition of saturated water and centrifugation.