Study On The Characterization And Variation Of Pore And Fracture Structure Of Typical Scale Coal

Different scale coals have different pore-fracture networks,and the pore-fracture structure parameters will inevitably affect the mechanism of gas adsorption and migration.In this paper,several experimental methods for characterizing pore fissure structure,such as adsorption method,mercury intrusion method and visual CT,were combined with fractal theory to study the pore-fracture characteristics and differences of coal with various scales.The scale of the selected coal is<0.1mm,0.1-0.2mm,0.2-0.25mm,0.25-0.5mm,0.5-1mm,1-3mm,3-6mm,6-8mm,10*10mm and 25*50mm.The pore and fracture distribution characteristics of coal at larger scale can be revealed by correlating the pore and fracture structural characteristics of coal at different scales such as pulverized coal,coal particle and coal block.The experimental results show that:The micropores of both coal samples increased significantly with the decrease of particle size and then tend to be stable.The peak values were 0.25-0.5mm and 0.5-1mm respectively.The pore characteristics of micropores,small pores and some mesopores were measured by low temperature N₂adsorption method.The BET specific surface area and pore volume of the two coal samples increased with the decrease of particle size,and the increase range was divided into three stages:the first stage（0.5-1mm to 6-8mm）,the second stage（0.2-0.25mm to 0.25-0.5mm）and the third stage（<0.1mm to 0.1-0.2mm）.When the coal particle size is less than 0.1mm,it increases significantly.For pores greater than 100nm.The porosity of coal increases with the decrease of particle size.The size of the mesopore in Jiulishan coal mine does not change obviously,while the mesopore in Zhujiao coal increases significantly with the decrease of the size.Because the mesopore in anthracite is relatively undeveloped,while the mesopore in coking coal is relatively developed,the particle size has a prominent effect on the relatively large pores in coal.Both macropores and visible pores increased with the decrease of particle size,indicating that the influence of particle size on macropores and visible pores was more common.The effect of particle size on the pore structure of coal is mainly because the pores filled with minerals,the narrow pores in the pore neck or the closed pores leak out with the decrease of particle size.The pore structure of coal is characterized quantitatively by FHH fractal theory,D₁represents the complexity of the pore surface,and D₂represents the heterogeneity of the pore structure.It is found that for the anthracite coal with more developed micropores,the FHH fractal model is not applicable in the low-pressure region,and the fractal dimension D₂has no obvious change with the decrease of the particle size,and only increases significantly when the particle size is less than 0.2 mm.As the particle size decreases,the fractal dimension D₁of the main coking coal gradually increases.The D₂of coal particles smaller than 1 mm is basically unchanged,and the D₂of coal particles larger than 1 mm is basically unchanged.The former D₂is smaller than the latter.Industrial micro-CT technology is used to scan coal bodies of different scales,and 3D digital coal cores can be obtained using 3D reconstruction technology.The fracture structure of the coal body can be truly characterized by the image.The fracture rate of coal sample with small scale is larger,which indicates that the scale has a certain influence on the fracture of coal.The pore fissure structure of the multi-scale coal body is the physical basis of the gas migration in the coal seam.The study of the variation law of the pore fissure of the multi-scale coal body laid the foundation for the subsequent research on gas migration.