Molecular Simulation Of Shale Gas Adsorption And Diffusion

Shale gas is widely distributed around the world,and has great potential for development as a kind of unconventional energy.Shale gas reservoir is a continuous natural gas reservoir which is rich in organic and product gas continuously.The main composition of shale matrix is clay minerals and kerogen.The adsorption and diffusion of shale gas in minerals and kerogen is still in perfect.This thesis mainly establishes clay mineral and kerogen models,and optimizes the cell structure before the simulation.The GCMC and MD methods are employed to analyze CH₄/CO₂ adsorption and diffusion by considering the effect of different temperature,pressure and burial depth.Firstly,this thesis establishes an infinite space model,analyzing the diffusion CH₄/CO₂in the conditions that has no other force except the mutual acting force between gas molecules,which is used to compare with the shale matrix.The results show that the diffusion of CH4 is higher than CO₂.When CO₂ and CH₄ are mixed,the diffusion of CH₄ is decreased but the diffusion of CO₂ is increased.Both the diffusion of CH₄ and CO₂ increases with the temperature and decreases firstly with the pressure,then keeps stable.Then,this thesis establishes montmorillonite model to analyze CH₄/CO₂/H₂O adsorption and diffusion.The adsorption to CO₂ in montmorillonite is stronger than to CH₄.When CO₂is put in,the adsorption quantity decreases while the diffusion coefficient increases.When H₂O is put in,both the adsorption quantity and the diffusion coefficient are decreases.The biggest adsorption quantity and the diffusion coefficient of CH₄ are found at 3-4 km.Finally,this thesis establishes kerogen II-A and kerogen II-D models to analyze CH₄/CO₂ adsorption and diffusion.Similarly,the existence of CO₂ decreases the adsorption quantity and increases the diffusion coefficient.The adsorption quantity of CH₄ in kerogen II-A is bigger than that in kerogen II-D,which is showed that the organic maturity is higher,the adsorption quantity is higher too.By comparing the models above,we can find that the force between shale matrix and CH₄/CO₂ stronger than the mutual acting force between gas molecules,and has important effects to the CH₄/CO₂ adsorption and diffusion.The adsorption quantity and diffusion coefficient in montmorillonite are slightly higher than that in kerogen.The biggest adsorption quantity and the diffusion coefficient of CH₄ in montmorilloniteand and kerogen are found at3-4 km.