| Petroleum is an important fossil energy source that plays a decisive role in economic development and is also closely related to people’s daily lives.In recent years,the consumption of petroleum has been continuously increasing,and most of the extracted resources are light petroleum resources.In the remaining petroleum resources,heavy oil resources account for a large proportion and have great potential for exploitation.Heavy oil has a high viscosity and complex composition,mainly consisting of asphaltene,resin,and hydrocarbons.With the indepth study of scientific researchers,it is found that the complex molecular structure of asphaltene and resin and the interaction between molecules make heavy oil have a high viscosity.Therefore,it is necessary to have a deeper understanding of asphaltene and resin at the molecular level for efficient extraction of heavy oil.It will take a lot of time and experimental cost to study asphaltene and resin at the molecular level through daily experiments.With the continuous development of computer,molecular simulation has become a common research method.In order to study the average molecular structure of asphaltene and resin in target heavy oil and the effect of their aggregation behavior on the viscosity of heavy oil,this paper uses molecular dynamics simulation method to study them.This method not only has a certain guiding role in the experiment,but also shows the molecular structure and the aggregation mode between molecules at the micro level.The specific research content is divided into the following three parts:(1)Based on the experimental test reports of two kinds of target heavy oil,the asphaltene and resin molecules of two kinds of heavy oil were studied,and the average structural parameters were calculated by the improved Brown-Ladner(B-L)method.Then,the number of rings,the length of chains and their condensation methods were determined according to the modified average structural parameters.Then,different kinds of molecules are simulated and calculated,and finally the average molecular configuration of asphaltene and resin of the target heavy oil is obtained.(2)In order to verify whether the simulation parameters and calculation procedure set in the previous chapter are suitable for asphaltene and resin molecules of target heavy oil with relatively large molecular weight,molecular dynamics simulation is carried out on four kinds of polycyclic aromatic hydrocarbons as model compounds,and the difference between the statistical density and the literature value is small,which shows that the average molecular configuration of asphaltene and resin of target heavy oil obtained in the previous chapter is reliable.Because the molecular structure of asphaltene and resin is complex,the average molecular configuration calculated by simulation can’t represent its real molecular structure.Therefore,based on the initial average molecular structure of asphaltene and resin of target heavy oil,the most possible molecular structure of target heavy oil is finally obtained by changing the length of side chain,constructing a periodic simulation box and counting its binding energy.(3)Then,based on the most probable molecular structure of the target heavy oil,light crude oil(saturated hydrocarbons,aromatic hydrocarbons,cycloalkanes)is added,and the heavy oil phase model is constructed.Molecular dynamics method were used for simulation analysis,and the viscosity of two kinds of heavy oil with different mass fractions was counted by periodic perturbation method(PPM).The research shows that the accumulation form of resin molecules in Xinjiang P609-32 heavy oil is mainly "side to face".In Zheng 364-XN363 heavy oil,the accumulation form of resin molecules is mainly "face to face".In the two kinds of heavy oil,it is the aggregates formed between resin molecules,resin molecules and light crude oil that lead to the stability of heavy oil system and the increase of viscosity,At the same time,it also provides some theoretical guidance for efficient extraction of heavy oil. |
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