Study On The Microscopic Mechanism Of Adsorption And Diffusion Of C7 Shale Gas Reservoir

Shale gas,an important unconventional energy,plays an important role in the oil&gas exploration and development as well as in national strategic energy reserves.Adsorption and diffusion have significant influence on the occurrence and migration of shale gas.It is necessary to study the macro-micro mechanism of shale gas adsorption and diffusion in different scales.This paper has combined the laboratory study and theoretical calculation,and the foregoing problems have been studied from micro and macro perspectives furtherly:on the one hand,we measured the diffusion coefficients of methane in shale on real reservoir core and analyzed the effect of pressure,water saturation and permeability on diffusion coefficient.On the other hand,after systematical evaluation of the existing multicomponent adsorption models,one adsorption model applicable to shale gas reservoir was recommended.At last,we researched the adsorption and diffusion of shale gas on clay mineral(montmorillonite)and organic(kerogen)in the micro-nano scale by molecular simulation.The research is financially supported by the National Natural Science Foundation of China(51204141).On the basis of systematic research,the following conclusions can be drawn:(1)The pressure has obviously inhibiting effect on the diffusion coefficient.Diffusion coefficient decreases with the increasing pressure,and increases with the increasing permeability.The water occupies the pore space of shale,which hinders the migration path of gas.Therefore,the diffusion coefficient decreases linearly with the increase of water content.(2)The Langmuir,Langmuir-Freundlich and Toth adsorption models can be used to describe the adsorption of single component in shale reservoirs;the revised Langmuir,extended Langmuir,and LRC adsorption models can applied to study the adsorption of the binary components gas;the extended Langmuir and LRC adsorption models fit the ternary components gas best.(3)Microscopic adsorption rules:the adsorption capacity of shale gas increases with the increase of pressure or diameter.The isosteric heats of adsorption decreases first and then increases with the increasing pressure.As pore size increases,multi-layer adsorption presents,and thickness of each layer is approximately equal to the molecular kinetic diameter of the adsorbed gas.The effect of water on the gas adsorption is considered furtherly.The adsorption capacity of fluid is the descending order:water>carbon dioxide>methane>nitrogen.(4)Microscopic diffusion rules:the diffusion coefficient decreases with the increase of pressure.When the pressure is small,the decrease magnitude is relatively large.For anisotropy in the organic pore,diffusion coefficient is not the same in the different directions.The diffusion coefficient in horizontal direction(x,y direction)is larger than that in the vertical direction(z direction),which indicates that horizontal direction is the main direction of gas diffusion in organic matter.The diffusion capacity of fluid is the following descending order:nitrogen>methane>carbon dioxide.(5)Compared the radial distribution function of fluid molecules with the atoms in the organic matter,the interaction between atoms S,O and N in the kerogen with carbon dioxide molecule are much larger than that with methane and nitrogen molecule.So adsorption capacity of carbon dioxide outperforms that of methane and nitrogen.Based on the selective permeation study,selective adsorption of the gas mixture increases with the increasing molar ratio of carbon dioxide,while the selective diffusion has no significant change.This indicates that the permeability of gas mixture is mainly influenced by the adsorption rather than diffusion.