Precipitation and characterization of petroleum asphaltenes

The goal of this work is to obtain a better understanding of the properties of petroleum asphaltenes and their stability in term of asphaltene precipitating tendency. The measurements of the refractive index of crude oils were utilized to study the asphaltene tendency to precipitate from crude oil. The precipitation of asphaltene was induced by addition of n-heptane. The deviation of the refractive index behavior from the linearity can be used to detect the onset of asphaltene precipitation. The refractive indices of crude oils were found to have linear correlation with the amount of asphaltenes in crude oils. The precipitation tendency of asphaltenes was observed to be independent of the total amount of asphaltenes in crude oil but depend on the properties of asphaltenes.; Asphaltene samples extracted from unstable crude oils, stable crude oils, and field solid deposits were characterized in order to elucidate the factors governing stability of asphaltenes. A fractionation technique that divided the asphaltenes into different subfractions based on polarity was used to characterize these asphaltene samples. The elemental analysis and Fourier Transform Infrared Spectroscopy (FTIR) showed no significant difference elemental compositions or functional groups of these asphaltene samples. The only chemical difference observed was the amount of metal in the asphaltenes. The unstable asphaltenes were observed to be more polar and exhibit a greater tendency to self-aggreagate than the stable asphaltenes. The further characterization on the asphaltene polar subfractions showed substantial differences in the dissolution characteristics between the polar subfractions. The dissolution rate of asphaltene subfractions in both aromatics/alkanes and amphiphile/alkanes solvents was found to obey a first order shrinking core model. The fractions with a higher polarity have a lower dissolution rate constant and dissolve to a lesser extent than the lower polarity fractions. The solubility parameter theory was utilized to determine the molecular weight of unfractionated asphaltenes and of its polar subfractions. The average molecular weight of asphaltene was found to be in a range of 300--1500 g/mol. The asphaltenes with a higher polarity have a higher average molecular weight.