# Experimental Analysis of Pore and Permeability Characteristics of Coal by Low-Field NMR
## 3 Discussion
### NMR Porosity
#### 3.2.2 Movable Fluid Porosity  
The movable fluid saturation is an indicator of the permeability of the conventional reservoir and the coal reservoir, which refers to the proportion of movable fluid in the reservoir. The \(T_{2}\) cut-off method was usually used to calculate the free fluid index (FFI). Their \(T_{2}\) cumulative curves of the coal samples under two conditions of saturated water and centrifugation were obtained by NMR (Figure 7). Along with the maximum value of the cumulative curve of the centrifugal \(T_{2}\) plot, the parallel line to the \(X\)-axis intersected with the \(T_{2}\) cumulative curve of the saturated sample at the \(O\) point. At the \(O\) point, we made a perpendicular line to the \(X\)-axis and intersected at point \(A\). The relaxation time of the point \(A\) is the \(T_{\rm 2C}\) value. The FFI can be calculated based on the proportion of the accumulated signal area larger than \(T_{\rm 2C}\). The \(\varphi_{\rm FFI}\) is the product of FFI and the total porosity. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample ID** & **Volume (cm3)** & \(\rho\) (g/cm3) & \(S_{\rm w}\) & \(\varphi_{\rm t}\) (\%) & \(\varphi_{\rm p}\) (\%) & \(\varphi_{\rm mf}\) (\%) & \(\varphi_{\rm f}\) (\%) \\ \hline CZ-1 & 25.36 & 1.47 & 1031.45 & 8.26 & 8.18 & 0 & 0.08 \\ CZ-2 & 25.44 & 1.47 & 1016.76 & 8.14 & 7.96 & 0 & 0.17 \\ CZ-3 & 25.16 & 1.45 & 977.27 & 7.83 & 7.65 & 0 & 0.17 \\ CZ-4 & 25.82 & 1.49 & 955.97 & 7.66 & 7.56 & 0 & 0.10 \\ WY-1 & 24.84 & 1.42 & 372.77 & 3.05 & 2.58 & 0.36 & 0.11 \\ WY-2 & 24.80 & 1.40 & 334.02 & 2.75 & 2.34 & 0.30 & 0.10 \\ YJL-1 & 25.05 & 1.32 & 1900.47 & 15.12 & 9.43 & 5.53 & 0.16 \\ YJL-2 & 25.19 & 1.33 & 1831.65 & 14.58 & 8.67 & 5.78 & 0.13 \\ \hline \hline \end{tabular} \(\rho\) is the density of the coal samples; \(\varphi_{\rm t}\), \(\varphi_{\rm p}\), \(\varphi_{\rm mf}\), \(\varphi_{\rm f}\) is the porosity of the total, nanopores, micro-fissures, and fissures, respectively.  
\end{table} 
Table 2: NMR porosity of coal samples. Figure 7: Pore classification by \(T_{2}\) accumulate curve. Due to the differences in the pore and fissure structures in the coal samples from different mining areas, the different centrifugal force was used to test the centrifugal effect of the saturated coal samples, and the proper centrifugal force of each sample was obtained. Then, the \(T_{\text{2C}}\) and \(\varphi_{\text{FFI}}\) of each coal samples were calculated, and the results are shown in Table 3.  
At present, the pore size corresponding to \(T_{\text{2C}}\) is commonly used as the boundary between movable and bound water. Because the structure, distribution, and connectivity of the pores and fissures in the coal are very complex, there is a \"partial movable water pore\" interval in the \(T_{2}\) cumulative curve, which is the micro-fissure mentioned earlier. As shown in Figure 7, the minimum pore size of the micro-fissures begins at point \(B\) at the fork of the cumulative \(T_{2}\) curves of the centrifugal and saturated coal samples, and the maximum pore size is terminated by the maximum signal starting point \(C\) of the \(T_{2}\) cumulative curve of the centrifugal coal samples. The two ends of the micro-fissures are the nanopore and fissure segments, respectively. From Table 3, the FFI and porosity of coal samples in the same mining area were very close, but the FFI between the coal samples from different mining areas varied greatly, and the movable fluid porosity was much different. The movable fluid porosity of YJL-1 as the highest, up to 4.35%, and the \(\varphi_{\text{FFI}}\) of WY-2 was the lowest, only 0.63%. The difference between the vertical layer and the bedding sample was not obvious, and the FFI of the bedding coal samples was slightly larger than that of the vertical coal samples. In the logging process, the \(T_{2}\) spectra of the water-bearing coal seam can be measured, but the \(T_{\text{2C}}\) and \(\varphi_{\text{FFI}}\) cannot be obtained directly.