Effects of asphaltene concentration and asphaltene agglomeration on viscosity

Asphalenes are regarded as the most polar and heaviest components in crude oil system. Their precipitation and agglomeration cause problems in oil recovery, production, and transportation. Asphaltenes are directly responsible for determining the viscosity of crude oil. In this work, the effects of asphaltene concentration and asphaltene agglomeration on viscosity were investigated. Toluene and heptane were used as asphaltene solvent and precipitant. The viscosities of asphaltenes-toluene solutions with various asphaltene concentrations were measured to investigate the impact of asphaltene concentration on viscosity. The results indicate that the viscosity of asphaltene-toluene solution exponentially increases with the asphaltene concentration. Elevated temperature causes viscosity reduction. The effect of temperature on viscosity reduction is more significant at higher asphaltene concentration. The dependence of viscosity on temperature can be well described by the Arrhenius equation, by considering the significance of activation energy. On the other hand, the asphaltene agglomerates form and grow with the increase of heptane concentration in crude oil-heptane-toluene mixture. A sharp viscosity rise of crude oil-heptane-toluene mixture was observed beyond heptane threshold concentration of 60 vol%. This is mainly resulted from the significant formation and growth of asphaltene agglomerates in asphaltene suspensions. The interactions between micro-sized asphaltene agglomerate particles could significantly affect the viscosity of liquid-solid system. The particle properties of asphaltene agglomerates including size, size distribution, number and shape factor were analyzed by microscopic images of the synthetic diluted oil. These contribute to the understanding of the effect of particle properties of asphaltene agglomerates on the viscosity of oil suspension.