Molecular Dynamic Simulation Study Of The Interaction Between Montmorillonite And Methane/carbon Dioxide Brine Fluid

Clay minerals are abundantly distributed in earth surface such as weathering crust and sedimentary rocks.They are characterized for large specific surface area and strong adsorption capacity.The montmorillonite in clay minerals has the strongest cation exchange capacity,which make them the prominent adsorbent material for heavy metals,organic pollutants and nuclear waste.Under geological conditions,adsorption of methane or carbon dioxide on the surface of montmorillonite could affect the fluid properties.The adsorption will further impact the gas accumulation,migration processes in the oil or gas reservoir,and also influence security of geological sequestration for CO2.A large number of experiments have been employed to study the interaction between clay minerals and fluids,but the microscopic structure and dynamic properties of fluid confined in montmorillonite nanopore are still lacking,molecular simulation(MD)can make up some experiment defects.In this thesis,based on MD,we investigated the interaction between montmorillonite and brine fluids frequently appeared under geological conditions.Three salty fluids are involved in this study:salt solution,methane fluid and CO2 fluid.The results show that:(1)The cation exchange capacity between two-layer hydrated sodium montmorillonite and different cations such as K+,Mg2+,Ca2+,Ba2+ is related to the cation hydration capacity.The order of cation exchange capacity is Ba2+>Ca2+>K+>Mg2+.Equivalent charge amounts of K+ and Ca2+ have the similar free energy change when they undergo cation exchanges with montmorillonite.(2)The diffusion coefficients of cations in the interlayer of montmorillonite are slowed down,the cation exchange of Ba2+ is more stable than those of K+,Ca2+ and Mg2+.In addition,when cation exchange reaches dynamic equilibrium,the diffusion of Ba2+ in the interlayer is the slowest.The distribution and migration of methane fluids vary a lot when confined in nanopores of montmorillonite.(1)The amount of methane adsorbed in the 0.6 nm interlayer of montmorillonite is quite small,and methane tends to form clusters in the interparticle mesopore.(2)A small amount of methane is adsorbed on the montmorillonite surface when interlayer pore is as large as 6 nm,and the increase in water content reduces the adsorption amount of methane.(3)The migration speed of methane fluid is closely related to the methane content,while the speed difference between different surfaces of montmorillonite is inconspicuous.Studies on the interaction of montmorillonite surface with CO2fluids show that:(1)The amount of CO2 dissolved in the two layers of hydrated montmorillonite is larger than that in the interparticle mesopore.(2)The surface of montmorillonite significantly reduced the diffusion properties of CO2 fluid.The inhibition of CO2 fluid diffusion in the interlayer is more evident than that in the interparticle mesopore.In this study,we explore the microstructure and mobility of cations,methane and CO2 fluids near the surface of montmorillonite.This will help to understand the adsorption,enrichment and migration processes of methane in nanopores in mud shale reservoirs,and at the same time better assess the geological storage of CO2.