# Analysis and Characterization of Micro-Nano Pores in Coal Reservoirs of Different Coal Ranks
## 4 Discussion  
The study employed micro-nano-CT scanning to analyze the pore space structures of low-, medium-, and high-rank coal samples. The scanning data volume was processed using Avizo 3D visualization software 2020.1, facilitating the establishment of the 3D reconstruction of the coal samples. Specifically, the micro-CT coal sample was determined at 1000 voxels, while the nano-CT coal sample was at 1000 voxels per volume unit (REV). The changes in porosity, fractal dimension, pore connectivity, and other parameters in the micro-nano scale of the coal samples were analyzed in detail. The specific conclusions are as follows:  
1. The comparison with the mercury injection experimental data indicated a small error in the 2D slices obtained by the CT scans, indicating the accuracy of the REV model obtained under the conditions of the nano-CT scanning. It can be seen from the pore space morphology that the middle- and high-rank coal samples are located in the central and southern part of Qinshui Basin and the eastern margin of Ordos Basin, which belong to the high yield and enrichment area of coalbed methane in China. Generally, they have favorable geological conditions, such as the enrichment of coalbed methane resources and high permeability, so the pores are more developed, while the low-rank coal samples are located in the southern margin of Junggar Basin. In some areas, the coal seams are rich in CO\({}_{2}\), and the geological conditions are complex, so the pores are not developed. The interval of the surface porosity distribution was large, indicating strong heterogeneity with each sample. 2. The micro-CT scanning revealed pores with an equivalent diameter of 50-200 \(\upmu\)m, accounting for a large proportion in each coal sample. However, the average equivalent diameter of each sample was ~110 \(\upmu\)m, and the maximum and minimum equivalent diameters were quite different, indicating general pore homogeneity. Whereas, under nano-CT scanning, each sample was distributed within the range of 200-2000 nm, and the equivalent diameter XJ was evenly distributed in each size. 3. The volume fractal dimension of each sample, observed under micro-nano-CT scanning, was ~2.5, with the surface fractal dimension exhibiting characteristics of fluctuation and general uniformity. The analysis of the fractal dimensions indicated that XJ exhibited the highest uniformity, while LL exhibited the roughest pore structure and lowest uniformity. 4. The pore equivalent radius and throat equivalent radius of each sample exhibited unimodal distributions. Despite the topological structure of each sample being relatively complex, the coordination number proportion of 0-20 was the largest, indicating excellent connectivity and suitability for seepage simulation. The large-medium pores of the medium-rank coal were more developed compared with the low- and high-rank coal. Thus, the low- and high-rank coal exhibited relatively more micro-pores. The analysis of the pore size distributions indicated a wide span and uneven distribution. Figure 13: Distribution histogram of equivalent throat radius of each sample. (**a**) Histogram of throat equivalent radius distribution of micro-CT samples; (**b**) histogram of throat equivalent radius distribution of nano-CT samples. 5. This study covers the coal samples of three low-medium-high coal ranks, but the different geographical locations will lead to some pore structure differences in coal samples of the same coal rank. Therefore, the pore structure characteristics of each coal rank in this study cannot represent the coal samples in all places. The follow-up work can select coal samples from different places for the same coal rank for the quantitative analysis of the pore structure, which is convenient for the optimization of CBM surface development blocks. Conceptualization, J.S. and Y.Y. ; methodology, J.S. and Y.Y. ; software, J.S. and Y.Y. ; formal analysis, J.S. and Y.Y. ; writing--original draft preparation, J.S. and Y.Y. ; writing--review and editing, J.S., Y.Y. and Y.L. ; supervision, Y.L.