# Experimental Analysis of Pore and Permeability Characteristics of Coal by Low-Field NMR
## 3 Discussion
### Pore Type of NMR
#### 3.1.3 Influence of Pore Type on Fluid  
To understand the influence of the size, shape, and connectivity of the pores on the water migration in the process of saturation and centrifugation, we selected CZ-4 and YJL-2 to saturate in different times (24 h, 48 h, 120 h) and centrifuge by different rotational speeds (4000 rpm, 6000 rpm, and 8000 rpm), and carry out the NMR scanning to obtain their \(T_{2}\) spectra (Figures 4 and 5).  
Figure 4 is the \(T_{2}\) spectra of the CZ-4 and YJL-2 coal samples under different saturated times. With the increase of saturated time, there was no obvious change in the shape and peak intensity of YJL-2, which showed that the mass and distribution of the water in the fissures and pores were not changed obviously after 24 h. In 24 h, the water had fully entered the fissures, micro-fissures, and partial nanopores. Because the micro-fissures developed very well in the YJL coal samples, water could enter larger nanopores through micro-fissures, while some gaseous water molecules diffused to small nanopores through the micro-fissures. With the increase of saturated time, the peak position in the \(T_{2}\) spectra of CZ-4 had no obvious change, but the peak intensity increased significantly. In 24 h, the water was also full of fissures. However, the micro-fissures in this coal were not developed, so it was lacking an effective migration channel for the liquid water molecule seepage and gaseous water molecule diffusion. The adsorption process of liquid water into the larger nanopores was slow, so the intensity of the pores peak increased continuously.  
Figure 5 is the \(T_{2}\) spectra of centrifugal coal samples at different rotational speeds of CZ-1 and YJL-2. Under the rotational speed of 4000 rpm, the nanopore peak intensity of CZ-1 decreased slightly, and the fissure peak intensity decreased significantly. When the rotational speed reached 6000 rpm, the nanopore peaks continued to decrease, and the fissure peaks disappeared completely. When the rotational speed increased to 8000 rpm, there was no obvious change in the spectra. After centrifugation, the nanopore peak of YJL-2 had not changed, and the variation of fissure peak was similar with CZ-1.  
Figure 4: \(T_{2}\) distribution under different full water times.  
Figure 5: \(T_{2}\) distribution of different centrifugal states.  
With the increases of centrifugal speed, the micro-fissure peak intensity decreased gradually and disappeared in 8000 rpm.  
The fissure width in the coal was large, and their connectivity was good, so most of the water in the fissures was discharged under the action of centrifugal force. Because coal belongs to a water wetting medium, a very small amount of membrane water still adsorbed on the fissure wall (Figure 6a). The process of water migration in the fissures was in agreement with the variation of the fissure peaks in centrifugation. The micro-fissures had small width and poor connectivity, and the water could migrate effectively only under greater centrifugal force (the YJL coal samples), so the micro-fissure walls would still absorb a small amount of membrane water (Figure 6b). In some coal, the structure of the micro-fissures was complex, so even if the centrifugal speed was up to 8000 rpm, only part of the water could discharge, which showed the decrease of the intensity of the micro-fissure peaks (the WY coal samples). The signal of the nanopore peaks of the \(T_{2}\) spectra was the superstition of liquid water and gaseous water molecules absorbed in small nanopores wall. In centrifugation, only the liquid water in the larger pores with good connectivity discharged effectively, which reduced the intensity of the nanopore peaks, such as in the WY samples. Most of the water in the nanopores in the centrifugal process did not move, such as the CZ and YJL coal samples, and their spectral peaks were basically unchanged (Figure 6c).