Molecular Simulations Of Water And Methane Adsorption And Flow In Shale Nanopores

Shale gas reservoirs are characterized by low porosity,low permeability,high temperature and high pressure.Its interior contains many nano scale kerogen pores and clay pores.Shale gas is generated inside the kerogen pores,and a large number of clay pores are not only store part of shale gas,but also flow channels during hydraulic fracturing development of shale gas.Shale formation contains original residual water,and it is invaded by large quantities of water at fracturing stage.Therefore,it is very important for the exploration and development of shale gas to study the adsorption and flow state of gas and water in the shale nanopores.Although there are many experiments related to shale gas or water,but the assumption of continuous medium is insufficient at nanoscale,the experiment can not fully explore the mechanisms of adsorption and flow in the nanoscale pores.As a simulation method,molecular simulation starts with the basic forces between molecules or atoms,and explores microscopic phenomena and mechanisms by building models at molecular or atomic level.In this dissertation,the adsorption,coexistence and flow state of methane gas and water in shale clay pores are studied by molecular simulations(molecular dynamics simulation and Monte Carlo simulation).These clay minerals include kaolinite,illite and montmorillonite.The pore shapes concerned include slit,cylindrical,and rectangular pores.The simulation results are compared with the macroscopic phenomena and experimental data,and the mechanisms of nano scale gas water adsorption and flow are proposed.The main work and innovation of the dissertation are as follows:1)The adsorption of CH4 confined in nanoscale illite slit pores with basal and edge surfaces was investigated by grand canonical Monte Carlo and molecular dynamics simulations.The surface chemistry characteristics of nanopores are key factors in adsorption phenomena.The clay pores in shale formations exhibit basal surface and edge surfaces(mainly as A&C chain and B chain surfaces in illite).Little research regarding CH4 adsorption on clay edge surfaces has been carried out despite their distinct surface chemistries.The adsorbed phase density,adsorption capacity,adsorption energy,isosteric heat of adsorption,and adsorption sites were calculated and analyzed.The simulated adsorption capacity compares favorably with the available experimental data.The results show that the edge surfaces have van der Waals interactions that are weaker than those of the basal surfaces.The adsorption capacity follows the order basal surface>B chain edge surface>A&C chain edge surface.However,the differences of adsorption capacity between these surfaces are small;thus,edge surfaces cannot be ignored in shale formation.Additionally,we confirmed that the adsorbed phase has a thickness of approximately 0.9 nm.The pore size determines the interaction overlap strength on the gas molecules,and the threshold value of the pore size is about 2 nm.The preferential adsorption sites locate differently on edge and basal surfaces.2)The coexisting and competing patterns of water and methane in illite slit and cylindrical nanopores are investigated based on molecular dynamics simulations.In sharp contrast to the prevailing view that water film is ubiquitous in shale formations,we observe that water bridge dominates in the slit pores while water film appears in the other scenarios.The occurrence of water film or water bridge is determined by pore surface chemistry,pore shapes and pore sizes.The electric field generated by the atom charge distribution induces the water bridge phenomenon as evidenced by the molecular orientation and hydrogen bond networks of water.In particular,the water bridge is formed as a consequence of competition between the hydrophilic nature of clay surface and the intrinsic strong and directional electric field induced by the opposite clay surface charges and mobile metal ions.3)The dynamic process of water flooding in kaolinite and illite pores contained with gas was investigated.It was found that water can enter the nano pores of the shale from the fracture and displace the gas.After the gas is expelled out of the pore,the water in the nano pores is difficult to drain back.This explaines the phenomenon during shale gas production that gas production rate is high when the backflow rate of fracturing fluid is low from the microscopic point of view.4)The mixture flow of water and methane inside the kaolinite nanopore is investigated using nonequilibrium molecular dynamics simulations.The kaolinite pore has rectangular cross-sectional shape with inner pore surface area of basal surface and B chain edge surfaces.The results indicate that methane has more affinity with kaolinite surface siloxane oxygen atoms than water molecules.The nanoscale confinement makes the water channel in the cuboid pore exhibit quasi-round shape.The calculated number flux of methane is about two times as large as water,and the slip velocity of water and methane is analyzed.In addition,the water gains much larger viscosity when confined in the kaolinite nanopores,significantly higher than its bulk state viscosity while the viscosity of methane changes seldom.