## 5 Conclusions  
1. Using mercury intrusion porosimetry, liquid nitrogen adsorption, and low-temperature carbon dioxide adsorption methods to comprehensively characterize the full pore size of deep coal samples of different coal grades, the pore volume distribution relationship of each coal sample was found to be macropore\(\rightarrow\)mesopore\(\rightarrow\)micropore, indicating that the connectivity of macropores is relatively good. The specific surface area of micropores and mesopores is the main component, whereas the specific surface area of macropores is relatively small. The development characteristics of micropores and microcracks control the adsorption performance and development potential of deep gas. 2. The influence of metamorphic degree on the pore structure of deep coal was analyzed. The micropore volume and specific surface area both decreased with the increase in volatile matter, highlighting a nonlinear decreasing trend. The metamorphic process of coal promoted the development of micropores. The pore volume and specific surface area of mesopores and macropores first decreased and then increased with the increase in volatile matter, and the curve exhibited a U-shaped change, with the lowest values appearing at approximately 23% of volatile matter. Marked differences in the micropore, mesopore, and macropore structures of coal were observed with different degrees of metamorphism, which are controlled by the degree of metamorphism. 3. The fractal dimensions of the pores in each sample were calculated based on the sponge, FHH, and density function models. The HYS sample (lean coal) with the highest degree of metamorphism exhibited the smallest fractal dimension D\({}_{1}\), the largest D\({}_{2}\), and the smallest D\({}_{3}\), among which the pore complexity was the highest. The XJE sample (gas coal) with the lowest degree of metamorphism had the highest fractal dimension D\({}_{1}\), the lowest D\({}_{2}\), and the highest D\({}_{3}\), and the distribution of mesopores and throats was relatively regular. The fractal dimension D\({}_{1}\) value based on the sponge model exhibited an inverted U-shaped change with the increase in volatile matter, first increasing and then decreasing. The fractal dimension D\({}_{2}\) value based on the FHH model decreased nonlinearly with the increase in volatile matter, and the higher the degree of metamorphism, the more complex the mesoporous  
Figure 10: Relationship between volatile matter and fractal dimension. structure and the larger the fractal dimension. The fractal dimension D\({}_{3}\) value based on the density function model increased nonlinearly with the increase in volatile matter, and the degree of metamorphism intensified, resulting in the development of a more regular microporous structure. Although the degree of metamorphism had different effects on the fractal dimension, they all exhibited certain regular changes, indicating that the degree of metamorphism has a controlling effect on the evolution of the pore fractal dimension. Therefore, further research on the pore structure and fractal characteristics of all coal types is needed to comprehensively and accurately determine the controlling effect of metamorphic degree on pore size distribution. * 4. China's fv deep coal seams. The findings can contribute to the future of deep coal and gas coal mining technology, as well as coal and gas outburst prevention and control engineering. Conceptualization, Q.L. and Y.W. ; methodology, Q.L. ; hsoftware, L.Q. ; validation, Q.L. and Y.W. ; formal analysis, Q.L. and Y.W. ; investigation, L.Q. ; writing--original draft preparation, Q.L. ; writing--review and editing, Y.W. ; funding acquisition, Q.L. All authors have read and agreed to the published version of the manuscript. This research was supported by the Key Laboratory of Rock Mechanics and Geohazards of Zhejiang Province (grant No. ZJRMG-2023-01). Not applicable. The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.