Dynamic Study Of The Displacement Of Adsorbed CH₄ On Shales By Injecting CO₂

Shale gas is an important unconventional natural gas resource.Shale gas resource is rich and widely distributed in our country,and the recoverable reserve of shale gas resource in our country is the highest over the world.The development of shale gas resource in our country will make up for the relatively lacking supply of conventional natural gas.Injecting CO₂ into shale reservoir can provide dual benefits of enhanced shale gas recovery and geological sequestration of CO₂,which is called the technology of CO₂-enhanced shale gas recovery?CO₂-ESGR?.Based on the stronger adsorption capacity of CO₂ on shale surface,injected CO₂ can displace adsorbed CH₄ out of micropores.The study of the dynamic process of the competition adsorption of CO₂ and CH₄ on shale is meaningful for the understanding of the kinetic mechanism of CO₂-ESGR.In this paper,the marine Wufeng?Wf?shale sampled from Sichuan basin and the continental Yanchang?Yc?shale sampled from Ordos basin were chosen as research object.The adsorption kinetic experiments of CO₂ and CH₄ on shale under different temperatures and pressures were conducted by volumetric method.The diffusion coefficients,diffusion activation energies and adsorption rate constants of CO₂ and CH₄on shale were calculated by corresponding models.Experimental results showed that the adsorption kinetic processes of CO₂ and CH₄ on two shale samples could be divived into two stages:the early stage of rapid absorption and adsorption equilibrium stage.The order of magnitudes of the diffusion coefficients of CO₂ and CH₄ on shale were10^-11 m²/s.The diffusion coefficients of CO₂ and CH₄ on shale particles were larger than those on actual field shale gas reservoir,and they were smaller than those on clay minerals and kerogen.The diffusion coefficients of CO₂ and CH₄ on shale increased with the increase of pressure.Increasing temperature could also improve the diffusion coefficients.The diffusion activation energy of CO₂ was lower than that of CH₄ on shale.The diffusion coefficient of CO₂ was higher than that of CH₄,which was beneficial for the rapid diffusion of CO₂ into micropores to displace adsorbed CH₄.Pseudo-first order kinetics model could calculate the adsorption rate contants of CO₂ and CH₄ on two shale samples.Gas adsorption rate increased with the increase of pressure.Increasing temperature reduced the gas adsorption rate.The dynamic process of CO₂ injection into shale gas reservoir preadsorbed by CH₄was investiged by fixed bed experiment and theoretical simulation research.By comparing the experiment results of the breakthrough processes of CO₂ and CH₄ and the displacement of adsorbed CH₄ by injecting CO₂,it was found that the adsorbed CH₄in reservoir could decrease the dynamic adsorption amount and adsorption rate of CO₂significantly.Simulated results indicated that one-dimensional advection-dispersion?AD?equation could describe the migration process of CO₂ in shale reservoir under lower pressure.The competition adsorption of injected CO₂ and preadsorbed CH₄resulted in the gradual breakrough curve,the decrease in the interstitial velocity,the increase in the dispersion coefficient and the widening of transition zone length of CO₂during the displacement process.Under higher pressure,there was an obvious tailing phenomenon of the breakthrough curves of CO₂.With the increase of CO₂ injection pressure,the breakthrough time was shorted,and the tailing phenomenon became more serious.Coats-Smith dispersion-capacitance model was more suited to describe CO₂ migration process in shale reservoir under higher pressure.The existence of stagnant region was the main reason for the appearance of tailing phenomenon of CO₂ breakthrough curves.Stagnant region was the obvious feature for the transport of CO₂ under higher pressure.With increasing CO₂ injection pressure,the dispersion of CO₂ was enhanced,and the recovery of CH₄ product(R_pipeline-CH4)was reduced evidently.The recovery of ultimate CH₄(R_ultimate-CH4)and the storage amount of CO₂(V_storage-CO2)were improved with increasing CO₂ injection pressure.Owing to the adsorption capacity,adsorption rate and diffusion coefficient of CO₂ on shale were larger than those of CH₄,the nature of the displacement of adsorbed CH₄ by injecting CO₂ was the rapid diffusion of CO₂into micropores to compete with CH₄ for adsorbed sites,which caused the desorption of CH₄ from shale surface and the movement of CH₄ to gas producing well.Compared the experiment results of the displacement of adsorbed CH₄ by injecting CO₂,N₂ and the mixture gas of CO₂ and N₂,it could find that the injected CO₂functioned as displacing gas to displace preadsorbed CH₄,and the injected N₂functioned as purge gas to sweep CH₄ out of reservoir.When taking the mixture gas of CO₂ and N₂ as injected gas,the“roll up”phenomenon of N₂ breakthrough curves was emerged.The breathrough time of N₂ was shorted,and R_pipeline-CH4 was decreased with the increase of N₂ concentration in mixture gas.When taking CO₂ as injected gas,R_pipeline-CH4 and V_storage-CO2 were the highest.The effect of enhanced shale gas recovery by injecting CO₂ was the best.R_pipeline-CH4 increased with increasing reservoir temperature.Reducing reservoir voidage could also improve R_pipeline-CH4.Compared with the means of the depletion of reservoir pressure,the means of CO₂dynamic injection could increase the recovery of CH₄ in the adsorbed state(R_adsorbed-CH4),R_pipeline-CH4 and R_ultimate-CH4.When extracting shale gas,it was better if CO₂ was injected into reservoir after taking the means of the depletion of reservoir pressure to reduce reservoir pressure,which was benefical to shale gas recovery and CO₂ geological sequestration.