Molecular Simulation Of The Adsorption Behavior Of Shale Gas In Clay

As one of significant unconventional natural gas, shale gas has the advantages of wide distribution, long-life mining and stable gas production. However, the low permeability of shale and complex of shale gas occurrence not only lead to the difficulty in gas migration and diffusion, but hinder the exploitation of shale gas. The adsorption and diffusion mechanism of shale gas in shale component were investigated theoreticially, so as to provide theoretical basis for shale gas exploitation.The interaction between methane and shale component was investigated from thermodynamics and dynamics aspects. The adsorption thermodynamics of methane onto different kerogens were researched by density functional theory to obtain active adsorption site. The specie and region of the interactions between methane and kerogens were further determined by reduced density gradient. The methane diffusion kinetics in different kerogens and montmorillonte hydrate were studied through molecular dynamics. In order to depict low permeability of shale reservoir, the diffusion coefficients of methane in kerogen and montmorillonte were also analyzed. The main conclusions were presented as following.1. The results of density functional theory and reduced density gradient show that the active adsorption site of kerogen locates above the carbon skeleton. Methane adsorbs above carbon atom of kerogen with hydrogen tripod oriented to the surface of kerogen. Hydrogen atoms of methane are attracted by delocalized electrons, while the delocalized electrons repulse carbon atom of methane. The adsorption energy between methane and kerogen calculated at M06-2x-D3/Jun-cc-pVDZ level is around 14 kJ-mol"1, which belongs to van der Waals interaction.2. The results of molecular dynamic simulation show that diffusion coefficient of methane decreases with the increasing of kerogen maturity. Diffusivity of methane in type II kerogen is 1.927×10-4 m2·s1 at COMPASS force field, while in type ? kerogen the diffusion coefficient is 1.578×10-4 m2·s-1.In other word, methane is difficult to diffuse in type ? kerogen. Meanwhile, methane mainly adsorbs on interlayer of montmorillonite hydrate. Water forms cluster on the surface of silicon-oxygen tetrahedron layer. Diffusion coefficients of methane in different montmorillonite hydrate are 3.25×10-5 m2·s-1 and 8.10×10-5 m2·s-1 respectively.