Effect of small organic molecule in mid-metamorphism coal on gas adsorption and flow characteristics

Two bituminous coals were extracted with tetrahydrofuran (THF) solvent at 50 ℃ and atmospheric pressure by microwave-assisted extraction. The experiments of methane sorption isotherms, desorption and radial seepage for raw coals and their residues were carried out. The results indicate that the ability to adsorb methane is reduced after extraction of the small organic molecule, and the methane desorption rate is improved. Pore volume and average pore diameter of raw coal is higher than that of the residual coal,while the specific surface area and micropore volume are reduced. After solvent extraction, the desorption velocity and desorption quantity are higher that of raw coal because the resistances of surface mass transfer and diffusion are reduced. Permeability of residues is higher than that of raw coal under the same seepage condition. The gas adsorption is lowered, the internal and external diffusion resistance of methane is decreased, and the gas flow channel in coal is expanded. These behaviors are caused by the change of coal pore structure after dissolving some small organic molecule in coal. Therefore, solvent extraction can change the storage and transportation of coal seam features, which provides the basis for improving the permeability of the coal reservoir with chemical methods.