Waxy Gels with Asphaltenes 1: Characterization of Precipitation, Gelation, Yield Stress, and Morphology

We examine the effects of asphaltenes upon the crystallization behavior of a model waxy oil. Yield stress measurements on the model waxy oils with asphaltenes isolated from Shengli crude oil showed that both the relative amount of wax to asphaltenes and the aggregation state of the asphaltenes affected the crystallization properties of the wax. At very low asphaltene concentrations and high wax concentrations, the yield stress of the waxy gel is not significantly affected. At higher asphaltene concentrations, the asphaltenes significantly degraded the microscopic structure of the wax network and drastically reduced the yield stress. There is a threshold ratio of ∼100 paraffin/asphaltene molecules for such behavior. Asphaltenes produced large decreases in yield stress when they were highly aggregated. Oscillatory testing showed that in such cases asphaltene−asphaltene interactions contributed to the gel strength, in addition to the wax platelet interactions. Asphaltenes increased the wax precipitation temperature at high concentrations when large aggregates were present. However, at lower concentrations where the asphaltenes were less aggregated they suppressed precipitation. The aliphatic nature of the Shengli asphaltenes is an important determinant of the observed decrease in precipitation temperature and yield stress.