The Effect Of Asphaltenes Polarity On The Gelation Behavior Of Waxy Oil

The interaction between asphaltenes and paraffins is an important factor that influences rheological behavior of waxy crude oil.Generally,the asphaltene is defined as a solution class that precipitates in heptanes but dissolves in toluene.The changes in nature for the various components of asphaltene were not highlighted in this definition（e.g.polarity）.How the difference in the chemical nature of asphaltenes influences the rheological properties of wax oil is rarely mentioned in the literature.In this study,four kinds of asphaltenes with gradually decreasing polarity(F_33/67、F_25/75、F_20/80、F_10/90)were separated from Venezuela vacuum residue asphaltene using the solvent made of dichloromethane and n-pentane at four different volume ratios.The average molecular structure parameters were determined by gel permeation chromatography,nuclear magnetic resonance and elemental analysis.The effect of polarity of asphaltenes on asphaltenes dispersion degree was investigated by optical microscopy and its influence on wax crystallization and gelation properties of waxy oil were studied by differential scanning calorimetry and rheological measurement.The results show that the asphaltenes behave as a wax molecular precipitation inhibitor,which can delay the process of gelation under a certain asphaltene concentration（0.2wt%）,the asphaltenes with lower polarity were better dispersed in model oil and have higher specific surface area,which is able to produce a stronger steric effect,resulting in more effective wax crystallization and gelation process inhibition.In addition,the effects of different polarity of asphaltenes on waxy oil gel structure strength and wax crystal morphology were investigated.The experimental results show that the asphaltenes can enter the wax crystal three-dimensional network structure,and become one of the“weak points”,weakening the gel network structure strength,reducing the ability of wax crystal inclusion of liquid oil,and making the wax crystal particle from the needle into an ellipsoid;the asphaltenes with lower polarity were better dispersed in model oil and have higher specific surface area,thus it can participate in the formation process of wax crystal three-dimensional network structure more easily,which can weaken the wax oil gel structure strength in a large extent.