Study On The Adsorption Kinetics Of Carbon Dioxide On Shale And The Shale Gas Displacement By Carbon Dioxide

China's shale gas resources are rich,and the efficient exploitation of shale gas can improve the natural gas production and reduce natural gas imports.CO₂-enhanced shale gas recovery(CO₂-ESGR)technology can not only increase shale gas production,but also achieve the underground storage of CO₂ to reduce CO₂ emissions.The adsorption kinetics of CO₂ on shale and the CH₄ adsorbed in a shale displaced by CO₂ have been studied,which is helpful to understand the dynamic mechanism of the CO₂-ESGR process.The adsorption kinetics experiments of CO₂ in different states were carried out by volumetric method on the marine Wufeng shale obtained from Sichuan Basin.The diffusion coefficient was calculated using the unipore model,and the adsorption rate constant was calculated using the pseudo-first-order kinetics model.The experiments of CO₂ displacement shale gas under different temperatures and pressures were carried out on the continental Yanchang shale obtained from Ordos Basin.The dynamic process of CO₂ displacement shale gas in the reservoir was simulated by the Coats-Smith model.The main results are as follows:(1)When the temperature and pressure are near the critical point of CO₂(30.98?,7.38 MPa),the adsorption kinetics curves of CO₂ fluctuate greatly,and the adsorption rate changes greatly with the increase of the temperature and the pressure.The adsorption kinetics curves of CO₂ at other pressures and temperatures can be fitted by the unipore diffusion model and pseudo-first-order kinetics model well.With the increase of temperature,the diffusion coefficient of CO₂ on shale increases while the adsorption rate of CO₂ on shale decreases.with the increase of pressure,the diffusion coefficient and the adsorption rate of CO₂ on shale increase.(2)At the same injection pressure,the diffusion coefficient and mass transfer coefficient(K_m)of CO₂ increase with increasing temperature,CO₂ can easily replace CH₄,resulting in higher shale gas recovery.When the injection pressure is 8.0?10.5MPa and the injection pressure is the same,the ratio of the movable region(F)of supercritical CO₂ of 32?45?is smaller,and the K_m and the dispersion coefficient(K_d)of supercritical CO₂ of 32?45?are larger,the final amount of shale gas replaced is more.(3)The dispersion of CO₂ is composed of the diffusion and the mechanical mixing.When the injection pressure is 5.5 MPa,the pressure is lower,and the dispersion of CO₂is mainly diffusion.When the injection pressure are 7.0?10.5 MPa,the pressures are higher,and the dispersion of CO₂ is mainly mechanical mixing.When the injection pressures are 7.5?8.5 MPa,within the range of 32?45?,as the temperature rises,the proportion of diffusion increases.