Molecular Dynamics Simulation Of Mechanism Of Asphaltene Complexation On Solid Surface

Asphaltenes are mixture of a series of macromolecular compound.They are closely related to the activity of various types of rock mineral surfaces.A series of behaviors of asphaltenes on the interface between rock minerals and organic solvents,such as flocculation and precipitation,control and affect the flow,sedimentation,enrichment and migration of crude oil.The poor fluidity of asphaltenes,the property of aggregation and precipitation,and the stable deposition structure make them regarded as the main problems restricting the recovery of remaining oil in the petroleum industry.It can be seen that the interaction mechanism between asphaltenes and mineral surface plays an extremely important role in the field of earth science and petroleum.Therefore,it is necessary to quantitatively understand the adsorption and deposition mechanism of asphaltenes on solid surfaces,which is not only beneficial to fundamentally understand the activity characteristics of asphaltenes in the process of oil extraction,but also for the development of effective techniques to relieve asphaltene precipitation.In recent decades,scholars have used a variety of methods,such as viscosity testing and transmission electron microscopy,to explore the aggregation characteristics of many asphaltenes with different structures in solution and the adsorption and deposition characteristics on different types of solid surfaces.However,due to the complexity of asphaltene structure and the diversity of mineral surfaces in porous media,experimental methods cannot accurately detect the mechanism of interaction between asphaltenes and solid surface.The degree of understanding of the reaction is far from enough,and many questions have not been answered,including: the microstructure of the asphaltene deposition,the growth mode of the sediment,and the mechanism of action of the inhibitor.The solution of these key problems is closely related to the prediction of asphaltene deposition behavior and the effect of deposition inhibitors.As an indispensable supplement to experimental research,molecular simulation has played an important role in the research of asphaltenes,especially in the study of the exploration of interface processes,molecular simulation methods have been successfully applied.In this paper,the coarse-grained molecular dynamics method is used to systematically study the deposition mechanism of asphaltenes on solid surfaces by calculating the structural characteristics and free energy information of asphaltenes on highly oriented pyrolytic graphite surfaces.In addition,the structural characteristics of asphaltene deposition after the action of surfactants were calculated,and the mechanism of interaction between asphaltenes and surfactants was obtained.1.By calculating the deposition structure and desorption free energy of asphaltene on the solid surface,and comparing the quantitative difference between the asphaltene aggregation process in the solution and the asphaltene deposition process on the surface,this study revealed the micro-complexation mechanism of asphaltenes on the solid surface :(1)The free energy of desorption of asphaltenes in vacancies is much greater than that in other places,so vacancies are the most preferred complexation sites in the deposition process;when continuously deposited to the third asphaltene,the free energy of desorption is basically the same as the free energy of dissociation of asphaltene dimer,indicating that the probabilities that asphaltene continues to deposit and form aggregation in solution at this time are similar.(2)The surface reduces the degree of asphaltene aggregation,accelerates the stage evolution of the aggregation process,and enriches asphaltenes in nearby areas.The deposition rate and aggregation degree on the heptane-solid interface are higher than that in toluene-solid interface.(3)The deposition process of asphaltenes can be divided into 4 stages: ?.Asphaltenes cover the surface;?.Nanoaggregates are formed on the surface;?.Nanoaggregates on the surface and nanoaggregates in solution form clusters through edge stacking and Tshaped stacking;?.Aggregates on surface and aggregates in solution contact with each other through fat side chains.The temperature does not change the asphaltene deposition process,but only has a certain effect on the aggregation level of aggregates at various stages.2.This article analyzes the effect of surfactants on the deposition mechanism of asphaltenes on solid surfaces,compares the difference between the coarse-grained molecular dynamics simulation trajectory of asphaltene aggregation and deposition process before and after surfactant addition,and quantitatively analyzes the difference in the effects between alkyl polyethylene glycol(C12E2)and sodium dodecyl sulfate(SDS): alkyl polyethylene glycol can desorb asphaltenes,while sodium dodecyl sulfate can cause partition between the asphaltenes deposited on the surface and the asphaltenes in the solution.Under high temperature conditions,the effect of alkyl polyethylene glycol fluctuates,and the effect of sodium dodecyl sulfate decreases slightly.In summary,this paper comprehensively studies the deposition process of asphaltenes on solid surface at the molecular scale,and explores the continuous reaction process of asphaltenes on the surface.During the process,the structure and free energy information obtained help to reveal the micro-mechanism of the deposition process and the mechanism of surfactants to inhibit asphaltene aggregation and deposition.The findings in this paper lay a physical foundation for understanding asphaltene migration and precipitation processes in crude oil,and provide methods and theoretical models for the development of new surfactants.