CH₄ And CO₂ Adsorption Characteristics Of Low Rank Coal In The Southeastern Of The Junggar Basin

Studying the adsorption characteristics of CH₄and CO₂in low-rank coal reservoirs is an important basis for the development of CO₂-enhanced coalbed methane mining technology(ECBM).This study took the low-rank coal reservoirs in the southeastern margin of the Junggar Basin in Xinjiang as the research object,and collected the 43 and45 coal seams(JG₄₃,JG₄₅)of Jiangou Coal Mine,Liuhuanggou Coal Mine(LHG),Hutubi Coal Mine(HTB)of the southern margin of the Junggar Basin,and Jiangjunmiao No.2 Coal Mine(JJM)of the eastern margin of the Junggar Basin as the carriers.Based on data collection,field research,experimental testing,mathematical model application,theoretical analysis,the adsorption characteristics and differences of CH₄and CO₂in low-rank equilibrium water coal samples were systematically studied,and the following main understandings were obtained:(1)There is a great difference in coal composition among coal reservoirs in the southeastern margin of the Junggar Basin.From west to east,the moisture content of coal decreases first and then increases,and the ash content increases first and then decreases.The volatile content of all coal samples in southern Junggar has no obvious rule,but the volatile content of coal samples from Xigou Coal Mine in southern margin of the Junggar Basin to the eastern margin of the Junggar Basin shows a downward trend,and the fixed carbon content shows a decreasing trend.The distribution of vitrinite mass fraction in coal is inverted“U”shape,inertinite mass fraction is positive“U”shape,and exinite mass fraction has no obvious distribution rule.The metamorphism intensity of each coal rock shows an"M"type distribution.The compositions of coal samples are dominated by organic microscopic composition,and vitrinite is dominated.(2)In the equilibrium water isothermal adsorption test,the maximum adsorption amount of CO₂is greater than that of CH₄in different coal samples at each stage,and the absolute value of CO₂entropy change?S₂of all coal samples is greater than that of CH₄entropy change?S₁,the absolute value of CO₂entropy change of all coal samples is greater than that of CH₄,indicating that the low-rank coal reservoirs have a stronger adsorption capacity for CO₂than that of CH₄.The effective diffusion coefficient(D_e)and adsorption rate(K)of CH₄and CO₂in each stage decreased exponentially with time.Before 4000 s,the D_eand K in each stage differed greatly,and gradually became the same after 4000 s.The D_eand K of CH₄increased gradually with the increase of pressure,while the D_eand K of CO₂were significantly different under the influence of pressure.The D_eand K of CH₄and CO₂in each coal sample is obviously constrained by pressure.(3)The Langmuir,BET,Freundlich,D-R,and D-A models were used to characterize the adsorption laws of CH₄and CO₂in low-rank equilibrium water coal samples,found that the Langmuir model was the best fit for the experimental results of CH₄and CO₂,followed by D-A model,D-R model and Freundlich model,while BET model was not suitable for describing the adsorption law of CH₄and CO₂in the equilibrium water coal samples.(4)The relationship between organic microscopic components and industrial components with the adsorption amount of CH₄and CO₂are different from the results of previous studies,which may be related to the adsorption of a certain amount of water by coal under equilibrium water conditions.The mineral components(except sulphide minerals)have the same constraining ability to the diffusion and adsorption process of CH₄and CO₂.The coal metamorphism has a more significant impact on the diffusion and adsorption process of CO₂.The main reason for the differential adsorption of CH₄and CO₂is the difference in mass fractions of vitrinite and inertinite in each coal sample.(5)Based on the maximum adsorption capacity of CH₄and CO₂,selectivity coefficient(?)and Henry's constant(K_H)of each coal sample,It is considered that ECBM technology has a greater potential in LHG,JG₄₃,JG₄₅and HTB low-rank coal reservoirs,while ECBM technology has less potential in JJM coal reservoirs.By comparing and analyzing the adsorption characteristics of CH₄and CO₂from coal samples with different coal rank,found that CO₂-ECBM technology has better potential in low rank coal reservoir.