# Analysis and Characterization of Micro-Nano Pores in Coal Reservoirs of Different Coal Ranks
## 3 Results and Discussion
### Pore Size Distribution  
In Avizo, the label analysis command was employed for the unit body after threshold segmentation, and then the data were counted, such as volume, area, equivalent diameter, etc., and checked in the command. The Avizo software had a built-in equivalent diameter formula, which directly calculated the required data, as shown in the micro-nano pore equivalent diameter parameter table of each sample in Table 3 and the pore equivalent diameter distribution histogram of each sample in Figure 8.  
Figure 8: Histogram of pore equivalent diameter distribution of each sample. (**a**) Histogram of pore equivalent diameter distribution of micro-CT samples; (**b**) histogram of pore equivalent diameter distribution of nano-CT samples.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**CT Type**} & \multirow{2}{*}{**Sample Name**} & \multicolumn{2}{c}{**Equivalent Pore Diameter (\(\mu\)m/nm)**} \\  & & **Max** & **Avg** & **Min** \\ \hline \multirow{3}{*}{Micro-CT} & LL & 22,444 & 117 & 51 \\  & XJ & 18,278 & 106 & 51 \\  & ZZ & 20,028 & 116 & 51 \\ \hline \multirow{3}{*}{Nano-CT} & LL & 10,0741 & 761 & 248 \\  & XJ & 63,889 & 1656 & 248 \\ \cline{1-1}  & ZZ & 63,861 & 454 & 248 \\ \hline \hline \end{tabular} Note: “LL” means medium-rank coal sample name; “XJ” means low-rank coal sample name; “ZZ” means high-rank coal sample name.  
\end{table}
Table 3: Pore equivalent diameter parameter table.  
From the statistics of the pore equivalent diameter in Table 3, it was observed that the pore equivalent diameter of each sample under the micro-CT scan was larger than the pore equivalent diameter under the nano-CT scan, whether it was the average value or the maximum and minimum values. Under micro-CT scanning, the average equivalent diameter of LL in the medium-rank coal was 117 \(\upmu\)m, which was the largest average equivalent diameter of the pores in the three samples. Under nano-CT scanning, the maximum equivalent diameter of the medium-rank coal LL was as high as 100,741 nm, which was larger than the maximum equivalent diameter of the low-rank coal XJ and high-rank coal ZZ. However, the average equivalent diameter was the largest for the low-rank coal XJ, 1656 nm. The minimum equivalent diameter of the micro-nano pores in each sample was the same, which was limited by the resolution of the CT scanning.  
The pore equivalent pore size distribution in Table 3 and the equivalent pore distribution histogram of each sample in Figure 7 were analyzed. Thus, under the micro-CT scan, the pores with an equivalent diameter greater than 300 \(\upmu\)m accounted for a small proportion in each coal sample, and the pores with an equivalent diameter of 50-200 \(\upmu\)m accounted for a large proportion in each coal sample. However, the average equivalent diameter of each sample was around 110 \(\upmu\)m, and the maximum and minimum equivalent diameters were quite different, indicating that the homogeneity of the pores was general. Under nano-CT scanning, each sample was distributed within 200-2000 nm, and the equivalent diameter of the low-rank coal XJ was evenly distributed in each size, attributable to the nano-equivalent diameter of the sample XJ, which was the largest among the three samples.