Molecular Simulation Study On The Adsorption And Diffusion Of CH₄ On The Surface Of Kaolinite

Clay mineral is an important inorganic component of coal seam,shale layer and tight sandstone layer.It plays a role of cementation and has a large specific surface area.Its interaction mechanism with methane has become a research hotspot in the field of unconventional natural gas exploration and development.Although the adsorption and diffusion of methane in clay minerals and its variation with the influencing factors have been studied in a series of studies,it is still in its infancy,because the above research is limited to macroscopic experiments and theoretical analysis,and the research objects are the smectite and illite,kaolinite and other clay minerals are relatively rare.The thermodynamic and kinetic parameters of methane adsorption heat,adsorption energy,diffusion activation energy and interaction energy on the surface of kaolinite are still unclear.In view of the fact that kaolinite is an unconventional natural gas reservoir,especially the main component of coal-bearing strata?coal top and bottom?,based on the basic principle of molecular simulation,this paper systematically studies the adsorption and diffusion of methane on the surface of kaolinite in order to clarify Coalbed methane,shale gas and tight sandstone gas provide a microscopic theoretical basis for the adsorption and diffusion characteristics of kaolinite-rich rock formations,providing a scientific basis for unconventional natural gas enhanced extraction and parameter optimization.The main conclusions obtained are:?1?Based on the clay mineral slit pores and methane regular tetrahedral structure,considering the crystal structure of kaolinite and the total atomic coordinates of methane molecules,the crystal structure model of kaolinite and the molecular model of methane were established,and the density and pores were The correctness of the model was verified by various angles such as volume,lattice constant,adsorption amount and X-ray powder diffraction.?2?The effects of different pore size and doped kaolinite crystal microstructure on the adsorption and diffusion of methane on the surface of kaolinite were obtained.With the increase of pore size,the adsorption amount of CH₄ on the surface of kaolinite decreases exponentially,while the diffusion coefficient of CH₄ increases exponentially with D=Aexp?7?-?/B?8?+C.The order of CH₄ in undoped,calcium ion doped,magnesium ion doped and adsorbed from large to small is:divalent iron ion doping>magnesium ion doping>calcium ion doping>pure kaolinite,and The order of diffusion coefficient of CH₄ from large to small is:pure kaolinite>calcium ion doping>magnesium ion doping>2valent iron ion doping,and logarithmic law with the increase of doping concentrationis lowered D=0.6505exp?7?-0.665C?8?.?3?The effects of pore pressure,temperature and moisture on the adsorption and diffusion of CH₄ on the surface of kaolinite were analyzed.With the increase of pore pressure,the adsorption amount of CH₄ on the surface of kaolinite first increased and then balanced,which accorded with the Langmuir law,while the self-diffusion coefficient and Fick diffusion coefficient of CH₄ decreased first and then increased.With the increase of temperature,the self-diffusion coefficient and Fick's diffusion coefficient of CH₄ increased linearly and the saturated adsorption capacity and diffusion activation energy of CH₄ decreased linearly.With the increase of water content,the saturated adsorption capacity of CH₄ on the surface of kaolinite,the adsorption heat and the interaction of CH₄-kaolinite decreased linearly,and the expansion ratio of kaolinite increased from 2.19%to 3.14%.The diffusion coefficient of CH₄increases linearly withD_methane=1.6395wt+3.5362.?4?The effects of CO,CO₂ and C₂H₆ on the adsorption and diffusion of CH₄ on the surface of kaolinite were analyzed.Under the condition of single gas,the order of adsorption of CO,CO₂,C₂H₆ and CH₄ on the surface of kaolinite from large to small is:CO>CO₂>C₂H₆>CH₄.Under binary gas conditions,CO,CO₂ and C₂H₆ compete with CH₄ for adsorption.And the interaction between kaolinite and CO,CO₂ and C₂H₆is stronger than that of CH₄,which makes kaolinite to CO,CO₂ and C₂H₆.The adsorption amount is larger than that of CH₄,and the adsorption amount of CH₄ on the surface of kaolinite is not in accordance with the Langmuir model.With the increase of CO,CO₂ and C₂H₆ adsorption,the diffusion coefficient of CH₄ decreases exponentially.?5?The engineering significance of molecular simulation results and the main functions of molecular simulation including gas content prediction,permeability prediction and enhanced extraction method evaluation methods,parameters and implementation process are introduced.Based on the reservoir conditions of the main coal seam in the Qinzhuang block of Minnan,the gas content and CH₄ permeability of kaolinite in the No.3 coal seam of Shanxi Formation and No.15 coal seam of Taiyuan Formation are predicted,and the method of enhanced drainage is evaluated and give an example.Under the reservoir conditions of No.3 coal seam and No.15 coal seam,the adsorption capacity of CH₄on kaolinite surface is 3.88⁴.44m³/t and 5.03⁶.22m³/t,respectively,and the permeability is 0.72md⁰.86md and 0.29 md⁰.33 md,respectively;drilling or slitting pressure relief can reduce rock pore pressure and pore pore pressure,reduce pore pressure can reduce the adsorption energy of kaolinite to methane,facilitate coal gas desorption and extraction;hydraulic fracturing Or hydraulic cutting and other methods will increase the water content of the rock formation.Increasing the water content will help reduce the adsorption energy of kaolinite to methane and improve the desorption characteristics of kaolinite to methane.After the pore pressure of the reservoir drops to 6MPa,the effect of injecting CO₂ to drive CH₄ in the reservoir is the most obvious,this can improve the efficiency of CO₂ cracking and save engineering cost.With the increase of electric field strength,the methane diffusion coefficient increases linearly with D=0.2948E+1.3078.When the electric field strength is 10¹⁰V/m,the diffusion coefficient of methane increases by 239.98%compared with no electric field.The electric field facilitates the diffusion of methane in the kaolinite.