Study On CO₂ Replacement CH₄ Effect With Medium And High Metamorphic Grade Of Coals

In order to study the effect of CO₂ replacement CH₄ in coal with medium and high metamorphism degree,three coal samples with medium and high metamorphism such as gas-fat coal,lean coal and anthracite coal were selected for experiment.On the basis of studying the adsorption laws of three kinds of coal samples on the single component gas of CO₂ and CH₄,initial CH₄ adsorption equilibrium pressure conditions of 0.75MPa and 1.3MPa were set up,and the study of CO₂ replacement CH₄ was carried out.The effect of CO₂ replacement CH₄ and its laws along with the degree of metamorphism under different initial CH₄ adsorption equilibrium pressures were obtained.The experimental and scientific basis for the determination of the reasonable gas injection pressure of coal seams with different gas contents and the engineering application of efficient and safe gas injection and displacement enhanced extraction.The experimental coal samples were selected from the gas-fat coal from Pingdingshan No.8 Coal Mine,the lean coal from Guizhou Liulong Coal Mine and the anthracite coal from Yonghong Coal Mine,Jincheng,Shanxi Province.The adsorption constants a of CO₂ for them were 26.70m³/t,40.65m³/t and 54.29m³/t and the adsorption constants a of CH₄ are 21.75 m³/t,23.95 m³/t and 38.64 m³/t.The results show that for the same coal samples with the same degree of metamorphism,as the amount of injected CO₂ increases,the amount of CO₂replacement CH₄ and the replacement rate of CH₄ generally increase linearly.The replacement efficiency of CO₂ decreases slightly with the increase of equilibrium pressure after gas injection.And after gas injection equilibrium,the amount of CO₂adsorbed in the coal sample tank is always more than the amount of CH₄ replaced.Ratio of replacement to adsorption is less than 1.The results show that for coal samples with different degrees of metamorphism,the higher the degree of metamorphism of coal samples,the larger the amount of CO₂replacement CH₄,that is?Q_WY>?Q_SM>?Q_QF,But the replacement rate of CO₂replacement CH₄ showed the opposite law,that isd_rWY<d_rSM<?d_rQF.CoMParing the experimental results at initial pressures of 0.75 MPa and 1.3 MPa,the results show that the amount of CO₂ replacement CH₄ reduce with the increase of CH₄ initial equilibrium pressure,Indicating that the higher the gas pressure in the coal seam,the more difficult to replacement CH₄ from coal.However,the efficiency of CO₂replacement CH₄ when the CH₄ adsorption equilibrium pressure of 1.3MPa is greater than CH₄ adsorption equilibrium pressure of 0.75MPa.The analysis shows that the mechanism of gas injection and replacement CH₄mainly includes dilution and diffusion of injected gas and competitive adsorption between gases.The causes of the difference and regularity of gas injection experiment results mainly include the pore structure is different and adsorption potential is different of coal samples with different metamorphic degrees.