Effect of the dispersion degree of asphaltene on wax deposition in crude oil under static conditions

• The theory of wax deposition is developed.
• The dispersed asphaltenes (< 2 μm) are favorable for wax deposition.
• The aggregated asphaltenes (> 2 μm) can depress the process of wax deposition.
• The dispersed asphaltenes cannot hinder wax diffusion.
• The component of wax deposits is a function of the dispersion degree of asphaltene.

This paper summarizes the experimental work done on the wax deposition in a waxy crude oil, considering the impact of the dispersion degree of asphaltene. Asphaltene from Venezuela residue was added into the waxy crude oil at several concentrations, and the dispersion degree of asphaltene was determined by the size distribution of asphaltene measured with microscopic observation and software analysis. Based on the size distribution of asphaltene, it shows two types of asphaltenes: dispersed asphaltenes (< 2 μm) and aggregated asphaltenes (> 2 μm). By using a differential scanning calorimeter (DSC) measurements and coldfinger experiments, it is shown that the dispersed asphaltenes can inhibit the wax precipitation, and the concentration gradient of wax molecules between the bulk and deposit surface is increased, favoring wax deposition. The aggregated asphaltenes can provide crystal cores, promote wax molecules to precipitate in advance, and decrease the concentration gradient of wax molecules, depressing the wax deposition. Additionally, it is found that the dispersed asphaltenes play little role in hindering wax molecule diffusion towards the deposition surface. Furthermore, the idea that the n-paraffin component in deposits is a function of the dispersion degree of asphaltene is supported by the high-temperature gas chromatography (HTGC) measurements.