Molecular Simulation Of Methane Adsorption Performance Of Kerogen Extracted From Shale In Longmaxi Formation

Currently,China possesses rich shale gas resources.The successfully commercial exploitation of shale gas can greatly alleviate the problem of the shortage of energy.The present studies suggest there are many factors which can affect the shale adsorption performance.But the effect of organic matter on the adsorption performance of shale is especially prominent.In order to comprehensively understand and analyze the adsorption properties of shale,the scholars at home and abroad have conducted the physical adsorption experiments and the molecular dynamics simulation to study shale adsorption performance.A lot of researches have made to promote the understanding of shale gas reservoirs.However the molecular dynamics simulation mainly uses the simplified graphite structure to approximately replace the organic structure.The effect of the simplified structure on the the shale adsorption performance needs to be discussed.Taking the kerogen extracted from the shale as the study objects,this article adopts the constructed average molecular structure of kerogen instead of the graphite structure to conduct the molecular simulation method.A series of conclusions on kerogen adsorption properties are drawn as followed:(1)The low pressure nitrogen adsorption experiment finds that the kerogen of Lonmaxi formation mainly contain the micropore and mesopore.(2)XRD shows that the content of aromatic hydrocarbons in shale of Lonmaxi formation was the highest.The isothermal adsorption experiments indicate that the kerogen can adsorb water,and the average structure of the kerogen should exsit the oxygen-containing functional groups.The FTIR experiment indicates that the kerogen is mainly made up of the structures with oxygen atom,the saturated hydrocarbons and the aromatic hydrocarbon.13C-NMR result expresses that the kerogen is mainly made up of aliphatic carbons,aromatic carbons and carbonyl carbons,and aromatic carbon is the main skeleton structure.These information above help construct the kerogen average molecular structure.Combined with the microscopic component experiment,organic carbon test and so on,this structure was verified.(3)In order to reduce the unnecessary errors in the process of calculating the excess adsorption capacity,the helium as probe molecule was selected to calculate the free volume of the model and the PR equation was used to calculate the gas density.When the kerogen adsorbs methane,the energy of the whole system is less than OkJ/mol,which is an exothermic process.The greater the pressure is or the lower the temperature is,the more heat released.The force existing between kerogen and methane is mainly van der Waals force,and the whole system energy is mainly the Van der Waals energy of the non-bonding energy.(4)The total gas content of methane increases rapidly and then increase slowly as the pressure increases,and the slope of the adsorption amount gradually decreases.The absolute adsorption amount increases and finally becomes flat with the increase of pressure,However,when the pressure increases,the excess adsorption amount increases at low pressure and decreases under the high pressure,indicating there exists a maximum excess adsorption capacity.The higher the temperature is,the smaller the total gas content,the absolute adsorption amount and the excess adsorption amount reaches,indicating that the low temperature is favorable to the methane adsorption.Under the same temperature and pressure,the total gas content of methane,the absolute adsorption amount and the excess adsorption amount decreased with the increase of water content,suggesting that water molecules are not conducive to the methane adsorption of kerogen.The competitive adsorption of carbon dioxide and methane also indicates that the adsorption capacity of carbon dioxide is higher than that of methane.In addition,the total gas content of methane increased as the pore radius increased,while the excess adsorption amount decreased with the increase of pore size.When the pore diameter increased,the adsorption capacity of the pore wall to methane decreased,resulting in the decrease of the excess adsorption amount.(5)The radial distribution curves show that the higher the temperature is,the smaller the pore size is or the greater the water content is,the smaller the methane density reaches.Therefore,the high temperature and water content are not conducive to the methane adsorption.The density of carbon dioxide in the model is greater than that of methane,which indicates that the kerogen is more easily adsorbed carbon dioxide.(6)The representative box model represented the micropore rather than the slit structure model or the graphite structure model should be used to comprehensively reflect kerogen adsorption properties,which provides some theoretical guidance for engineering application.