Experimental Research And Influencing Factors Of Supercritical CH₄/CO₂ Adsorption On Shale

The growing interest in shale gas as an important unconventional energy resource during the past decades has led to increased exploration and production activities and has stimulated considerable research interest.Since a large proportion of gas in shale,approximately 20-85%in some U.S.shale basins,is in an adrobed state,knowledge of sorption behavior is required to meet the need of resource assessment and exploitation.In this paper,high pressure sorption isotherms of pure CH₄ and CO₂ at 50,60 and80?and CH₄/CO₂ mixtures at 30 and 80?on shale samples from Sichuan Basin,China,were measured by volumetric method.The multicomponent sorption measurements were conducted on the mixtures with feed gas compositions of 92.03%,75.21%and 46.80%CH₄.The adsorption isotherm of pure CH₄ and CO₂ was fitted by the improved Langmuir,DR and DA equations,and the adsorption model of supercritical pure components was optimized to analyze the influence of mineral content,pore structure,temperature and pressure on the adsorption capacity.The thermodynamic equation was used to calculate the equivalent adsorption heat and limit adsorption heat of pure CH₄ and CO₂,indicating that the adsorption heat of CO₂ is larger and the shale interacts with CO₂ more strongly.The total isothermal adsorption curve of CH₄/CO₂ mixture and the isothermal adsorption curve of each component were fitted by the EL equation and the MPSD-BWR equation,and the influences of temperature,pressure,gas concentration and other factors on the selective adsorption of CH₄ and CO₂ were analyzed.The results show that the adsorption capacity of pure CO₂ is stronger than that of CH₄,and the selected adsorption coefficient is always greater than 2.The improved DA equation is the optimal pure component adsorption model in this paper.The shale with the same TOC content has a stronger adsorption capacity for CO₂ than CH₄.The higher the clay mineral content,the stronger the adsorption capacity of shale sample.Shale with rich micropore development has stronger adsorption capacity for CO₂ than that of CH₄.Temperature fluctuations have a greater impact on CO₂ adsorption.For the adsorption of mixed components,the fitting effect of mpsd-bwr equation is better than that of EL equation,and the separation coefficient of CO₂ in competitive adsorption(1.0?3.0)is less than that of uncompetitive adsorption(3.5?8.5),indicating that the shale absorbs CO₂preferentially,but is inhibited by CH₄ gas.The preferential adsorption of CO₂ will be promoted by increasing the temperature,increasing the CO₂ content in the mixture and increasing the pressure.