Molecular Dynamics Simulation Of The Interfacial Properties Of Methane-brine And Oil-water-anti-agglomerants Systems

Under the dual requirements of economic growth and green development,natural gas hydrate(NGH)has become a new type of energy that is mainly developed and utilized in China.At the same time,it is very important to prevent the formation and aggregation of NGH in oil and gas pipelines to ensure the safety of transportation.Methane-brine and oil-water interfaces are ubiquitous in the natural gas industry.The determination of their interfacial properties is of great significance to the exploitation and prevention of NGH.In this study,a series of molecular dynamics(MD)simulations are performed for methane-brine systems under the conditions of 263.15 K?283.15 K,1 MPa?10 MPa,and the ion mass fraction is 0 wt%?25 wt%.The properties such as interfacial tension(IFT)?density profile,relative adsorption,charge profile,interfacial thickness are calculated.The interfacial characteristics of the systems are analyzed at the molecular level.Our results show that with the increase of pressure and ion concentration,the IFT between methane and water increases.From the density profile and the snapshot of the system,it can be observed that methane molecules accumulate and ions deplete at the interfacial layer.The influence of temperature,pressure and ion concentration on the interfacial density of methane is measured by the relative density.Moreover,methane is positively adsorbed at the Gibbs dividing surface,and the relative adsorption is larger at high pressure and low temperature conditions.However,the relative adsorption of ions is negative,and the negative adsorption of Na~+is higher than that of Cl~-due to the effect of ion hydration.The“10-90”method is employed to evaluate the interfacial thickness,and the results show that the interfacial thickness increases with the rise of pressure and ion concentration,but the effect of temperature on it may depends on the pressure.In addition,the charge distribution at the interface is significantly different from that in the bulk phase,and the presence of ions leads to a more violently fluctuate of the charge profile.In the simulation of oil-water-anti-agglomerants system,the temperature and pressure are273.15 K and 5 MPa,respectively,and the interfacial coverage(IC)of anti-agglomerants(AAs)is from 90 A~2/molecule to 240 A~2/molecule.It is found that the addition of quaternary-ammonium AAs stabilize the structure of the interfacial layer,lower the total energy of the system,and effectively reduced the IFT between oil and water.The adsorption of AAs changes the density distribution of water and oil molecules at the interface,which is more evident at high concentration of AAs.Meanwhile,the AAs increase the interfacial thickness of the oil-water system and separate the oil and water molecules.Moreover,the molecular distribution of AAs at the interface,the interaction between hydrophilic groups and water,and the interaction between hydrophobic groups and the light components of oil phase are related to the concentration of AAs.Our results also show that if the concentration of the AAs is too large,the interfacial properties will change significantly,and the stability of the interface decreases.Based on the results of MD simulations,the interfacial features of methane-brine and oil-water-AAs systems are quantitatively analyzed,the microstructure of the interfacial layers are defined.The effects of temperature,pressure,and ion concentration on gas-water interface and AAs concentration on oil-water interface are elucidated.The conclusions of this paper are helpful to further understand the interaction between methane-water-ions and oil-water-AAs,and provide guidance for hydrate exploitation and control,as well as the design and development of new type of AAs.