Molecular Dynamics Simulation Reveals Unique Rheological and Viscosity-Temperature Properties of Karamay Heavy Crude Oil

Heavy oil, with high viscosity and complex compositions, often faces a series of challenges in the process of its exploitation and utilization. There are huge amounts of compositions with different molecular structures in heavy oil, and it is a great challenge to clarify the influence of the microstructure of heavy oil on the rheological and viscosity-temperature properties. Based on the experimental data, such as acid value, element content, functional group type of the heavy oil, and so on, we constructed six types of heavy oils from three regions to reveal the unique properties of Karamay heavy oil. Molecular dynamics simulation was used to study the effects of the compositions and the aggregation structure on the rheological and viscosity-temperature properties of heavy oil. The results show that the rheological properties of Karamay heavy oil with low asphaltene concentration are dominated by molecular conformation, and heavy oil with molecules that are not easily stretched has a higher viscosity. When the concentration of asphaltene is high, the nanoaggregates formed by asphaltene molecules and their variation with shear rate affect the viscosity. Compared with the aggregates by the island structure asphaltene molecules, the larger nanoaggregate size of continental structure due to stronger pi-pi interaction leads to the increase in viscosity. However, the viscosity decreases faster with the increase in shear rate, which is attributed to the more orderly aggregate structure of polycondensed aromatic rings that can be easily pulled along the velocity plane. In addition, we found that metal ions, as a bridge, can interact with molecules containing different heteroatoms, forming larger clusters and increasing the viscosity of Karamay heavy oil. For the viscosity-temperature properties, the more intense thermal movement of molecules with the increased temperature weakens the interaction between the aromatic rings and decreases the viscosity of heavy oils.