Main occurrence form of methane in coal:Micropore filling

The form of methane storage in coal is the characterization of its adsorption capacity and the basis of methane diffusion and seepage studies.Therefore,clarifying the form of methane storage in coal is of great sig-nificance to coal mine gas extraction and disaster prevention.Based on low pressure N2 adsorption and low pressure CO2 adsorption experiments,the quantitative analyses were performed on investigating microscopic pore structures of six coal samples with different metamorphisms.A methodology for quantitatively characterizing the methane adsorption capacity of coal samples was proposed according to the investigation on the optimal adsorption equilibrium of methane in coal based on the theory of micropore filling and monolayer adsorption.The reliability of the theoretical results was then verified by the optimal methane adsorption experiments and the characteristics of methane occurrence in the pore structure of different scales in coal reservoirs and the control factors of its adsorption capacity were obtained.The study reveals that the methane adsorption capacity of the 0.38-1.50 nm pores in coal is mainly restricted by the micropore volume,as for the pores exceeding 1.5 nm,the methane ad-sorption capacity is controlled by the pores’external surface area.The volume of micropores of the six samples is 0.028-0.065 cm3/g,contributing 66.1%-96.6% of the total pore volume,and the specific surface area of the mi-cropores of the six samples is 75.4-197.3 m2/g,contributing 82.8%-98.2% of the total specific surface area.The adsorption capacity calculated through the micropore filling methodology,the external surface area of micropores and theoretical optimal adsorption is 14.50-32.27 cm3/g,0.01-7.31 cm3/g and 16.13-33.35 cm3/g,respectively.The optimal adsorption capacity obtained through the experiments is 13.30-35.75 cm3/g,which displays a good correlation with the theoretical calculations.The evidence above indicates that the methane in coal is mainly stored through the form of micropore filling.Five out of the six investigated samples’ methane adsorption amount through micropore filling exceed 91% of the optimal adsorption capacity,where the methane adsorption in the 0.38-0.76 nm microporous structure accounts for 41.27%-55.32% of the total amount.As an exception the methane adsorption amount through micropore filling of XQ sample contributes 72.73% of the optimal adsorption capacity due to its adsorption on the external surface,where the methane adsorption in the 0.38-0.76 nm microporous structure accounts for 37.51% of the total amount.