Simulation Study On The Dynamic Adsorption Characteristics Of CO₂ And CH₄ In Shale

The technology of CCUS（carbon capture,utilization and storage）has great potential to alleviate the greenhouse effect.CO₂-ESGR（CO₂sequestration and enhanced shale gas recovery）is a crucial component of CCUS.The dynamic adsorption characteristic of CO₂and CH₄displacement by CO₂in the organic pore of shale can reflect the dynamic adsorption mechanism,which is of great significance for CO₂-ESGR and can provide a theoretical reference for practical applications.The impact of different factors on the final adsorption efficiency and results have been analyzed in previous researches.However,less attention has been paid to the dynamic adsorption process.In this study,a shale organic pore model composed of 1nm and 4nm pores connected by 8nm pore was established.The effects of pore size and pressure on the dynamic diffusion and adsorption processes of CO₂and CH₄displacement by CO₂in organic pores were investigated by MD（molecular dynamics）simulation method.The effect of pore size and pressure on the dynamic diffusion and adsorption processes of CO₂and CH₄displacement by CO₂in organic pores was investigated.Firstly,the dynamic diffusion and adsorption processes of single-component CO₂at four pressure points of 8.57,11.03,15.67 and 19.47 MPa under 323 K were simulated.The software PACKMOL and Moltemplate were used to the initial model building,the molecular dynamics software LAMMPS（Large-scale Atomic/Molecular Massively Parallel Simulator）was chosen to simulate the dynamic process,and the software VMD（Visual Molecular Dynamics）was chosen to visualize the simulation process.The effects of pressure and pore size on pressure variation,density distribution,diffusion amounts and rates,and adsorption amounts and rates were investigated.The microscopic characterization of the CO₂dynamic adsorption process is revealed.It was found that different pore sizes lead to different density distributions.There is no free zone in the micropore with pore size of 1 nm,which is due to a combination of pore effects and overlapping effects.However,due to pore size limitations,pores larger than 1 nm are more conducive to realistic adsorption and sequestration.In addition,it is beneficial for higher adsorption rate and stability with increasing pressure,which correspondingly improves the adsorption performance.On the basis of the above pore model,the dynamic process of CH₄adsorbed in the shale being displaced by CO₂under four CO₂injection pressures of 8.57,11.03,15.67 and 19.47MPa at 323 K was simulated.The effects of pore size and pressure magnitude on density distribution,the peak and offset of density,and the process amounts of diffusion and adsorption during dynamic adsorption were clarified.The results show that the process of CH₄replacement by CO₂is divided into the CH₄self-resolution phase,the replacement of residual CH₄by CO₂and the stabilization phase.There is not only displacement but also coexistence adsorption of CO₂in place of CH₄,which is due to the fact that the values of the peak density offsets of CO₂and CH₄in the 1 nm pore are close to the difference between the molecular dynamics diameters of the two gases.The diffusion and desorption of CH₄is less affected by pressure and pore size,and the effect of pressure and pore size on the diffusion and adsorption of CO₂is more significant.The high pressure enhances the adsorption capacity between the pore wall and CO₂,which can promote the effect of CH₄replacement by CO₂.In summary,the larger mesopores and high pressures are conducive to the replacement of CH₄by CO₂and the geological sequestration of CO₂.