Probing the interaction between asphaltene-wax and its effects on the crystallization behavior of waxes in heavy oil via molecular dynamics simulation

Abstract

High content of asphaltenes and waxes leads to the high pour point and the poor flowability of heavy oil, which is adverse to its efficient development and its transportation in pipe. Understanding the interaction mechanism between asphaltene-wax is crucial to solve these problems, but it is still unclear. In this paper, molecular dynamics simulation was used to investigate the interaction between asphaltene-wax and its effects on the crystallization behavior of waxes in heavy oil. Results show that molecules in pure wax are arranged in a paralleled geometry. But wax molecules in heavy oil, which are close to the surface of asphaltene aggregates, are bent and arranged irregularly. When the mass fraction of asphaltenes in asphaltene-wax system (ω_asp) is 0–25 wt%, the attraction among wax molecules decreases and the bend degree of wax molecules increases with the increase of ω_asp. The ω_asp increases from 0 to 25 wt%, and the attraction between asphaltene-wax is stronger than that among waxes. This causes that the wax precipitation point changes from 353 to 333 K. While the ω_asp increases to 50 wt%, wax molecules are more dispersed owing to the steric hindrance of asphaltene aggregates, and the interaction among wax molecules transforms from attraction to repulsion. It causes that the ordered crystal structure of waxes can't be formed at normal temperature. Simultaneously, the asphaltene, with the higher molecular weight or the more hetero atoms, has more obvious inhibition to the formation of wax crystals. Besides, resins also have an obvious inhibition on the wax crystal due to the formation of asphaltene-resin aggregates with a larger radius. Our results reveal the interaction mechanism between asphaltene-wax, and provide useful guidelines for the development of heavy oil.