The Adsorption And Diffusion Of Methane/Carbon Dioxide In Na-Montmorillonite Use Molecular Simulation

Methane is the main composition of Shale gas, which is an important kind of unconventional gas. Methane exist in clay-shale in the forms of adsorbed、Free and dissolved state, especially the adsorbed state. It is a complex issue to determine the mechanism of adsorption and migration diffusion of methane in shale during in the resource evaluation and exploitation of Shale gas. Clay minerals are the main components of clay-shale, which mainly are montmorillonite, illite, kaolinite and chlorite. Montmorillonite adsorbed methane and carbon dioxide not only has in-depth academic value, but also has major practical significance. Based on the montmorillonite exhibits the strongest adsorption, the largest contribution to the adsorption of methane, it is significant to study montmorillonite adsorpt methane and carbon dioxide not only on the academically but also in terms of the application.In this paper, three methods of molecular mechanics, Monte Carlo and molecular dynamics were combinated to explore the adsorption behavior and relocation diffusion of methane and carbon dioxide of Na-montmorillonite at different burial depths, the results of the calculation analysis results are as follows:(1)With the increase the degree of hydration, the volume of Na-montmorillonite increases, the density decreases, the interlayer water molecules of Na-montmorillonite stability configuration distributed in the interlayer domain more uniform, sodium ions deviated from the central layer to substitution position of isomorphous, the Silicon-oxygen tetrahedron and Aluminum-oxygen octahedral of Na-montmorillonite changed from the rule to irregular polyhedron.(2)The adsorption energy and capacity showed the increases first, then decreases with the increases of burial depth and the degree of hydration of Na-montmorillonite. Methane and carbon dioxide displayed the competitive adsorption in the interlayer of Na-montmorillonite.(3)The effect of carbon dioxide substituted methane is more ideal in shale gas when the burial depth is about2km, while it is ineffectiveness when the burial depth is4km.(4)Methane of Na-montmorillonite adsorption methane configuration distributed on both sides of the center layer symmetrically, while the methane and carbon dioxide of Na-montmorillonite adsorption methane and carbon configuration mainly distributed two sides of the center layer of clay platelets respectively. (5)Temperature of stratum and the self-diffusion coefficient of inter-layer particle increases with the reservoir burial depth increased.(6)Since the presence of carbon dioxide, hydration of interlayer water and the attraction of clay were weakened, the self-diffusion coefficient of inter-layer particle increases, since the diffusion especially methane and sodium coefficient increases.(7)It is reasonable and feasible using carbon dioxide to replace methane in shale reservoirs with montmorillonite.