# Analysis and Characterization of Micro-Nano Pores in Coal Reservoirs of Different Coal Ranks
## 3 Results and Discussion
### Porosity  
In the coal pore space structure, there were connected and isolated pores [31]. To study the connectivity characteristics of the pore space structure of the coal samples in the Avizo software, the axis connectivity function was employed to extract the connected pores from the 2D slices scanned through micro-CT scanning, and the REV model was calculated through a nano-CT scan. The spatial structure is shown in Figure 6.  
The matrix and pores were separated through threshold segmentation. The total porosity of the sample was the volume of the pores divided by the total volume. After Avizo calculation, under the micro-CT scan, the total porosity of LL was 9.94%, the total porosity of XJ was 5.01%, and the total porosity of ZZ was 7.93%. Under nano-CT scanning, the total porosity of LL was 8.87%, the total porosity of XJ was 6.13%, and the total porosity of ZZ was 7.89%. The total porosity, connected porosity, isolated porosity, and the error with the mercury intrusion data of each sample are presented in Table 2. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**CT Type**} & **Sample** & **Total** & **Connected** & **Pressurized** & \multirow{2}{*}{**Error**} \\  & **Name** & **Porosity** & **Porosity** & **Mercury** & \\ \hline \multirow{3}{*}{Micro-CT} & LL & 9.94\% & 8.59\% & 9.31\% & 0.63\% \\  & XJ & 5.01\% & 4.64\% & 5.18\% & 0.17\% \\  & ZZ & 7.93\% & 6.10\% & 7.87\% & 0.06\% \\ \hline \multirow{3}{*}{Nano-CT} & LL & 8.87\% & 6.65\% & 9.31\% & 0.44\% \\  & XJ & 6.13\% & 1.71\% & 5.18\% & 0.95\% \\ \cline{1-1}  & ZZ & 7.89\% & 1.70\% & 7.87\% & 0.02\% \\ \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 2: Porosity distribution of each sample. Figure 6: Connected pore space morphology of coal samples. (**a**) Connected pore space morphology of micro-CT samples; (**b**) connected pore space morphology of nano-CT samples. Table 2 indicates that, under micro-CT scanning, the total porosity of the medium-rank coal LL was the highest, and the total porosity of the low-rank coal XJ was the lowest. Under nano-CT scanning, the total porosity of LL was the highest, and the total porosity of XJ was the lowest. The 2D sections obtained through scanning the two CT types were calculated using the Avizo software, and the error was smaller compared with the total porosity of the mercury intrusion experimental data, indicating that the REV model obtained under nano-CT conditions was relatively accurate. Upon comparing the connected porosity with the total porosity, it was observed that the connected porosity was lower than the total porosity under the micro-CT scan. Under nano-CT scanning, the connected porosity was lower than the total porosity because the total porosity contained isolated pores, and the connected porosity under nano-CT scanning was relatively lower compared with micro-CT scanning. The distribution of the surface porosity reflected the macro situation of the pores to a certain extent. Through the Avizo software, the 1000-layer 2D slice diagram of each micro-nano-CT was statistically analyzed by the porosity of each layer, and the surface porosity trend diagram of the xy direction slice of each sample was calculated. The results are shown in Figure 7.  
In Figure 6, it was observed that, under the micro-CT scan, the layer-by-layer pore rate of each sample exhibited a 'U' trend, and the porosity before and after 1000 2D slices was quite different; under nano-CT scanning, the layer-by-layer face rate of each sample exhibited a downward trend, and the layer-by-layer face rate of the high-order coal ZZ was relatively stable. Furthermore, compared with the layer-by-layer porosity under micro-CT  
Figure 7: Trend diagram of surface porosity of each sample layer by layer. and the layer-by-layer porosity under nano-CT, the former fluctuated greatly, indicating that the micron-level pores of each sample were more uneven than the nano-level pores.