Research Of Adsorption-Deformation-Seepage-Diffusion Characteristics Of CO₂/CH₄/N₂ In Coals With Different Coal Ranks

The adsorption,deformation,diffusivity and permeability of coal are the key factors affecting coalbed methane yield and CO₂ geological sequestration.Aiming at the problem that the development of coal-bed methane and the CO₂ sequestration effect in China are not satisfied.This dissertation takes anthracite,coking coal and long flame coal as the research object,The CO₂,CH₄ and N₂ adsorption-deformation-seepage experiments of different rank coals under stress state were carried out.Molecular simulation study on the adsorption behavior of different functional groups on CO₂,CH₄ and N₂、Molecular simulation studies of the adsorption and diffusion behavior of CO₂,CH₄ and N₂ in different coal macromolecule models and the competing adsorption behavior of different ratios of CO₂/CH₄ and CH₄/N₂ binary component gases in different coal macromolecule models.The characteristic laws and mechanisms of adsorption,diffusion,seepage,deformation and gas competitive adsorption of different coal rank coals are revealed,providing theoretical support for the efficient mining of CH₄ in coal seams and the coal seam storage of CO₂.Utilize the coal and rock adsorption-deformation-seepage test platform under multi-physics conditions,The influence of gas properties and coalification degree on the adsorption capacity,deformation amount and permeability of columnar coal samples was studied.The results show that the adsorption capacity and deformation of CO₂ gas adsorbed by the same coal sample are the largest,followed by CH₄,and N₂ is the smallest,and the change trend of permeability is opposite to it.The adsorption capacity of the same gas increases with the increase of the degree of coalification,while the deformation amount shows an opposite trend.The permeability of coking coal is the largest,followed by anthracite,and the smallest in long-flame coal;with the increase of adsorption pressure,the permeability of coal sample first decreases and then increases in the process of gas adsorption.The critical pressure points of CO₂,CH₄ and N₂ gas permeability on the three coal samples were about 2.0 MPa,1.75 MPa and 1.5 MPa,respectively,and the sensitivity coefficients of permeability to pressure showed a trend of rapid decrease followed by a small increase;Based on the adsorption-deformation-seepage change law of coal,an improved dynamic prediction model of adsorption capacity,an improved dynamic prediction model of coal body deformation and an improved dynamic prediction model of permeability were constructed.Using Fourier transform infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）,X-ray diffraction（XRD）and nuclear magnetic resonance(¹³C NMR)experimental methods,The evolution characteristics of parameters such as aliphatic structure,oxygen-containing functional group structure,aromatic structure and crystallite structure of coal during the coalification process were obtained;The coal macromolecular structures of anthracite,coking coal and long-flame coal were constructed,and the ¹³C NMR spectra of the constructed models were in good agreement with the experimentally obtained ¹³C NMR spectra.Fragment models and slit pore models modified with different functional groups were constructed by Materials Studio（MS）software.Based on the fragment model,the electrostatic potential distribution of different functional group fragment models and the adsorption stable configuration and adsorption energy of CO₂,CH₄ and N₂ gases on different functional groups were obtained by molecular simulation.Based on the slit pore model,molecular simulations obtained the changing laws of adsorption capacity,adsorption heat and energy distribution for CO₂,CH₄ and N₂ gas adsorption by the slit pore model with different functional groups.Based on different coal macromolecular models by MS software,the micro pore structure characteristics of different coal macromolecular models were obtained by probe method analysis;Molecular simulations obtained the variation laws of adsorption amount,adsorption heat,energy distribution,diffusion rate and diffusion activation energy of CO₂,CH₄ and N₂ single-component gas molecules adsorbed by different coal macromolecular structures at 293.15-323.15K temperature;Molecular simulations obtained the variation laws of competitive adsorption capacity,density distribution,adsorption selectivity,adsorption heat and energy distribution of different proportions of binary components CO₂/CH₄ and CH₄/N₂ in different coal macromolecular models.