The role of shearing effect in the evolution of the microscopic behavior of wax crystals

Abstract

The microstructure and dynamic behaviors of wax crystals in waxy crude oil are the fundamental reasons for a series of physical phenomena in the process of its transportation. Thus, to investigate the role of the shearing effect in the evolution of the microscopic behavior of wax crystals, herein, a new in situ microscopic observation method was developed to obtain the microstructure images of wax crystals and the maximum entropy threshold segmentation method was used for the quantitative analysis of the microscopic behaviors of wax crystals in a shear flow. The results showed that when the shear rate was less than 1 s⁻¹, shear dominated the aggregation of wax crystals, the average diameter increased, and the number of wax crystals decreased. Between 1 s⁻¹ and 50 s⁻¹, the effects of shear on the aggregation and dispersion of wax crystals occurred simultaneously, and both dominated alternately. When the shear rate was greater than 50 s⁻¹, the shear always dominated the dispersion of wax crystals, the average diameter decreased, and the number of wax crystals increased. However, in all cases, the proportion of small-size wax crystals was always high. The perimeter-based fractal dimension, D_pf, was used to quantitatively describe the morphological complexity of wax crystals, and it was found that the average fractal dimension between 1 s⁻¹ and 50 s⁻¹ fluctuated to some extent. This implies that shearing does not significantly change the morphology of individual wax crystals, and most morphological complexity is attributed to the wax aggregates formed by the mutual aggregation of wax crystals. Furthermore, the aggregates had a certain deformation under shear, which also played an important role in the morphological complexity. Finally, based on the mathematical model of the population balance model, the population balance equation, which is suitable for characterizing the microscopic behavior of wax crystals, was established to predict the change in the number of wax crystals caused by the microscopic behavior such as collision, fragmentation and aggregation. This is reasonable for further probing the multi-size behavior characteristics of waxy crude oil.