Comparative Study On Adsorption And Desorption-induced Coal Deformation Characteristics Of Supercritical CO₂ And CH₄

Coal bed methane is a kind of unconventional gas which is mainly stored in coal seam and adjacent rock strata with adsorption state and dissociation state,and its main component is CH₄.Rich resources in China,as a kind of high efficient clean energy,its development and utilization of the resources to resolve boundary less gas pattern and reduce greenhouse gas CO₂ emissions is of great significance.The adsorption and desorption of CO₂ and CH₄is the important research content of coal bed gas extraction and the basic research of the application of supercritical CO₂ displacement coal bed methane（ECBM）.Therefore,based on the xishan Du Er ping coal mine as the research object,using the taiyuan university of technology,independent research and development equipment in different temperature and the constant volume under stress（36 MPa）on three types of gas of coal is（He、CH₄ and CO₂）adsorption desorption properties and the mechanical properties of coal in the process of adsorption desorption change has carried on the detailed research,the main research results are as follows:（1）Under the same temperature and different pore pressure（8MPa,9MPa,10MPa）,the adsorption tests on He,CH₄ and SC-CO₂ of coal samples with a size of 50×100mm are carried out.The adsorption capacity of the three gases increases with the increase of injection pressure,and the increase of He and CH₄adsorption capacity gradually decreases,and the SC-CO₂ adsorption capacity is higher in 9MPa.With the same injection pressure,at different temperatures（40℃,60℃,80℃）,the amount of He gas injection decreased slightly with the temperature,and the adsorption capacity of CH₄ and SC-CO₂ decreased with the temperature,especially at 80℃.（2）Under different temperature and pressure conditions,the desorption rate of He gas is between 94.0%-97.0%,and its size is not affected by temperature and pressure.At the same temperature,the desorption rate of CH₄ and CO₂ is basically unchanged with the increase of injection pressure;at the same injection pressure,when the temperature rises from 40℃to 80℃,the CH₄desorption rate is linearly increased,and CO₂ at temperature from 40℃.When the temperature rose to 60℃,the desorption rate increased slightly,and the desorption rate increased significantly at 80℃.（3）Under the same injection pressure,the volume strain of coal body caused by injection of He,CH₄ and SC-CO₂ increases first and then decreases with the temperature.When the temperature rises from 40 to 60,the volume strain of coal body caused by He is the largest,the increase of SC-CO₂ is the least,the CH₄ increase is between two,and the volume of coal caused by He should be caused by the temperature from 60 to 80.The minimum drop is the largest drop in SC-CO₂ and the decrease in CH₄ is between the two.This indicates that SC-CO₂ is more affected by temperature than CH₄.During the desorption process,the residual strain of He first increased and then decreased slightly with the increase of temperature,while the residual strain of CH₄ and SC-CO₂ decreased with the increase of temperature.（4）The modulus of elasticity of coal body gradually decreases with the increase of temperature,but the decrease gradually slows down,that is,the ability of coal rock to resist deformation and damage is decreasing;the Poisson’s ratio of coal body decreases with the temperature first,and the increase of Poisson’s ratio at 80℃degrees is due to the change of the internal structure of the coal body.（5）Under the same temperature condition,the adsorption stress of CH₄increases slightly with the injection pressure,and it is very weak.The adsorption stress of SC-CO₂ increases with the injection pressure,especially when 9MPa rises to 10MPa.Under the same injection pressure,the adsorption stress decrease of CH₄ at 40℃to 60℃is larger.The decrease of adsorption stress decreases at 60℃to 80℃,while the adsorption stress of SC-CO₂’s adsorption stress is less than that of 60℃at 40℃,and the decrease of adsorption stress decreases at 60 C to 80℃.（6）At 60℃,10MPa,the maximum SC-CO₂/CH₄ adsorption stress ratio is3.58.The reason is that it is beneficial to improve the fluidity of CH₄ at 60℃,while SC-CO₂ still has strong adsorbability at 60℃,and the adsorption stress of SC-CO₂ has a large increase when 9MPa rises to 10MPa.Based on the above results,the injection of 60℃and 10MPa SC-CO₂ in the CO₂-ECBM process can more effectively displace the CH₄ from the coal and increase the production of CBM.The research results in this paper enrich the mechanism research of coal adsorption and desorption of SC-CO₂ and CH₄,which is of great significance for the development of coalbed methane resources in China.