| Shale gas is an important unconventional natural gas resource with abundant reserves and wide distribution.And CH4,as the main component,is often stored in shale in adsorbed,free and dissolved states,especially in the adsorbed state.In the development and evaluation of shale gas resources,it is of great significance to understand the adsorption and diffusion mechanisms of shale gas to predict its reserves in the reservoir,and shale gas displacement by CO2 is very important to improve shale gas recovery and complete CO2 geological storage.In this paper,the adsorption and diffusion behaviors of CH4 and CH4/CO2 binary mixture components in quartz pores are studied by means of grand canonical Monte Carlo combined with molecular dynamics（GCMC-MD）methods.The effects of temperature,pressure,pore size,water content and CO2 mole fraction on the adsorption and diffusion characteristics of CH4 in quartz pores are analyzed.The results show that the adsorption of CH4 in pores follows the Langmuir isothermal adsorption model,and the adsorption process is divided into rapid adsorption and equilibrium adsorption.The adsorption amount of CH4 increases with the increase of pressure and pore size,but decreases with the increase of temperature and water content.The spatial distribution of CH4 molecules is axisymmetric along the pore center.In the small pore diameter,there is a density peak only at the pore wall,while with the increase of pore diameter and pressure,there is a secondary density peak in the pore center.The diffusion coefficient increases with the increase of pore diameter and temperature,and decreases with the increase of pressure and water content.For the binary mixed CH4/CO2 system,the energy at the adsorption site of CH4 is always higher than that of CO2,and the energy of the adsorption site is higher in large pore size,CH4diffuses more easily than CO2.The maximum equivalent heat of adsorption for both CH4 and CO2 is less than 42 k J/mol,and the adsorption is typical physical adsorption.In addition,the increase of pressure and CO2 content can promote the increase of the equivalent heat of adsorption.Based on the change of selective adsorption coefficient,the injection of high concentration of CO2 at smaller pore size and lower pressure is more favorable to CH4replacement,but not conducive to the adsorption of CO2.Therefore,in the actual CO2sequestration enhanced gas recovery（CS-EGR）project,the adsorption amount and adsorption selectivity should be considered comprehensively to determine the optimal operating conditions. |
