Structural response characteristics and adsorption mechanism of anthracite coal under supercritical CO2
China is rich in deep coalbed methane(CBM)resources.Injecting CO2 into deep coal seams can not only enhance CBM recovery factor,but also realize CO2 geological storage(CO2-ECBM).Normally,CO2 existing its supercritical state in deep coal seam and significantly affects the adsorption capacity of coal,but the structure response characteristics and adsorption mechanism of coal under supercritical CO2 effect is not clear.Taking the No.3 coal seam of Permian Shanxi Formation in Chengzhuang Mine of Jincheng,Shanxi Province as the research object,this paper experimentally studies the high-pressure adsorbability of anthracite coal to supercritical CO2.Then,the response characteristics of chemical structures and pore structures in anthracite coal caused by the high-pressure adsorption of supercritical CO2 are analyzed by means of Fourier-transformed infrared spectrum(FTIR),X-ray diffraction photoelectron spectroscopy(XPS)and specific surface area(BET)measurements.Finally,the high-pressure adsorption behaviors and mechanisms of anthracite coal to supercritical CO2 are revealed.And the following research results are obtained.First,there is a mutation point mutation point in supercritical CO2 adsorption under high pressure.Under 35℃,the mutation point is close to the critical pressure(8 MPa),where the adsorption capacity is the smallest.Second,supercritical CO2 exhibits the ability to graft aromatic rings with functional groups,break ether-oxygen bonds and hydroxyl hydrogen bonds and remove methyl groups from the fat structure,which can provide more adsorption sites for CO2 and subsequently enhance the adsorption capacity on the surface of anthracite coal.Third,supercritical CO2 alters the pore structures and fractal characteristics of anthracite mesopores.After adsorption,the average pore diameter,pore volume,specific surface area and fractal dimension present an increasing trend,and the pore roughness increases.Thus,the reconstruction of pore structure is characterized by pore increase,pore enlargement and pore roughening,which can provide more adsorption space and enhance the adsorption capacity of anthracite coal.Fourth,when CO2 is injected into deep coal reservoirs,priority should be given to the reservoirs with high porosity and permeability.The environment of the target coal reservoir should be away from CO2 critical point as far as possible to ensure the maximum adsorption capacity of the reservoir to CO2.In conclusion,the research results provide an important theoretical basis for the assessment of CO2 injectability and storage potential in deep coal seams,and are of important and realistic significance to efficient CBM development.
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