Molecular Dynamics Simulation On Interaction Between Asphaltene And Surfactants In Heavy Crude Oil

Heavy crude oil has the characteristics of high viscosity and low fluidity,so it has great difficulties in mining,transportation and industrial production.In recent years,due to the increasing demand for oil by human being,the reserves of high-quality oil that are easy to extract are becoming less and less.Heavy crude oil reserves are rich and have extremely high mining potential.However,due to the complexity of the composition of the heavy crude oil,besides the common hydrocarbons,it also contains a large amount of asphaltene,resins,and some inorganic salts.It is the presence of these asphaltene and resin molecules that give the heavy oil a very high viscosity.As far as asphaltene is concerned,its composition is extremely complicated,It contains a huge number of asphaltene molecules with different structures.The molecular structure of the asphaltene contained in the different oil deposit is also very diverse.Experimentally,studying the properties of asphaltene from molecular level is time consuming and laborious.With the continuous development of theoretical chemistry and computer technology,molecular dynamics simulation can study the structural characteristics and properties of asphaltene and resin molecules from the molecular level,and explain the reason why heavy crude oil has extremely high viscosity from the molecular level.Therefore,the molecular dynamics method has a good explanatory and complementary effect for the experiments.Based on the light oil model containing four kinds of alkane,two kinds of cyclones and two kinds of aromatic hydrocarbon,in this thesis,we select multiple asphaltene and resins as the heavy component to make up the heavy crude oil model.We try to use molecular dynamics simulation to theoretically study the heavy crude oil system from the molecular level.By using MD simulation,we have study the structure of asphaltene aggregation and the distribution of asphaltene and resins molecules in oil droplets from the molecular level.Then different surfactants are added to the oil droplet systems.By doing this,we attempt to study what have changed in the properties of the oil droplets after the adding of surfactants,and reveal the role of surfactants in the emulsification and demulsification process.The main contents are as follows:(1)Theoretical Study on the Structure of Crude oil and Oil DropletsUnlike light oil with less asphaltene and resins,heavy crude oil contains large amounts of asphaltene and resin molecules.Most of these asphaltene and resins are aromatic hydrocarbons,which can aggregate through face-to-face staking.These aggregations make the heavy oil extremely viscous.It is difficult in experiments to study asphaltene and resins aggregates at the molecular level.In this thesis,the structure of crude oil and oil droplets with a single kind of asphaltene,C5Pe,was studied by molecular dynamics method.Then,based on the complexity and richness of asphaltene and resins in heavy crude oil,the structure and properties of heavy crude oil and oil droplets with comprehensive asphaltene and resins were studied.We hope that our research can provide theoretical assistance and guidance for the experimental study on the high viscosity of heavy crude oil,promote the exploration and exploitation of heavy crude oil.(2)Theoretical Study on the Interaction between Crude Oil Droplets and highly CO2 responsive super-amphiphile-D230H-OAD230H-OA is a kind of carbon dioxide-responsive super amphiphilic molecule based on fatty acid and Jeffamine D230.After being added to the heavy crude oil droplets system,it will dissolve and give out amphiphilic molecule oleate and protonated JefFamine,which is a good surfactant.In the case of adding carbon dioxide,oleic acid anion and protonated Jeffamine are converted to fatty acid molecules and Jeffamine molecules,losing most of amphipathy..In this thesis,molecular dynamics method was used to conduct a simulation system,in which D230H-OA in ionic state and D230-HOA in molecular state were added to the simulation system with heavy crude oil droplets.By comparing the two states of D230H-OA with the two kinds of oil droplets,the difference in properties between these four systems was figured out.We try to explain the role of surfactants in the emulsification and demulsification of heavy crude oil from a molecular level.(3)Theoretical study on the interaction between asphaltene in heavy crude oil and different concentrations of SDSThe aggregates of asphaltene and resin molecules play an important role in stabilizing heavy crude oil.Although many experiments are applied to investigate the complex aggregating structure of asphaltene and resin molecules in heavy crude oil,those micro-structure and properties are still not clear at the molecular level.As another auxiliary tool,molecular dynamics simulation can be used to simulate the behavior of asphaltene and resin in the heavy oil droplet or emulsified oil droplet.In this thesis,we try to study the interaction between asphaltene in oil droplets and SDS molecules.The simulation results showed that:i)asphaltene and resin molecules can form net-like structure in heavy oil through face-to-face or edge-to-face stacking interaction,and the aggregating structure is considered to be the main reason that heavy crude oil has high viscosity,ii)When surfactant molecules were added to the heavy oil phase,the asphaltene molecules moved to the center of emulsified oil droplet from the oil/water interface.The adsorption of surfactant molecules at the interface resulted into the increase of hydrophilic surface area of oil droplet.We think that the changed hydrophilicity of emulsified oil droplet is the key to the viscosity reduction of heavy oil.iii)the steered molecular dynamics simulation can prove that the interaction among asphaltene and resin molecules becomes fragile in emulsified heavy oil droplet,and it indicates that the added surfactant molecules are beneficial to the viscosity reduction in crude oil.