Microscopic Pore Structure and Coalbed Methane Desorption Law in Low-Permeability Coal Seams
In order to study the relationship between microscopic pore structure and gas desorption law in low-per-meability coal seams,the gas content,porosity,permeability,and pore size distribution of coal samples from No.8 coal seam of Yishang slope in the Ordos Basin were determined by using experimental methods such as direct obser-vation method,nuclear magnetic resonance(NMR)method,and liquid nitrogen adsorption method.The results show that the coalbed methane desorption characteristics are related to the pore size distribution,pore morphology,pore connectivity,specific surface area of the coal,and coal rock composition,etc;the pore characteristics and gas desorption laws of the 2 sets of coal seams less than 10 m apart in the study area have large differences.Among them,the microporous morphology of the glance coal was dominated by cylindrical pores open at one end and ink-bottle pores,with the percentage of micropores and adsorbed pores being 39.2％,and the microcracks in the coal body were relatively developed,but the connectivity between the micropores and mesopores was poor,and the mi-croporous morphology of the dull coal was dominated by cylindrical pores open at one end,with the percentage of micropores and adsorbed pores being 33.2％,the microcracks were undeveloped,and the connectivity between dif-ferent pores was poor.The gas desorption of glance coal has a large output in the early stage but decays quickly in the late stage,and the gas output has obvious paroxysmal characteristics;the gas desorption of dull coal has a small output in the early stage,which is about 1/2 of that of the glance coal,and the decrease of the output in the late stage is relatively flat.The pore structure,pore type,permeability,stress sensitivity,and types of fracture fillers shall be fully considered in the development strategy for low-permeability coal seams.On the basis of fine charac-terization of the pore structure,a reasonable development plan should be formulated in accordance with the charac-teristics of different desorption periods of coalbed methane and in conjunction with geological and development con-ditions.This study has important guiding significance for understanding the mechanism of coalbed methane gas pro-duction and its controlling factors,and improving the mining efficiency of coalbed methane.
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