Asphaltenes and resins are the polar and saturates and aromatics are the nonpolar fractions of the crude oil. The mutual interaction within crude oil fractions results in different overall polarity. With the onset asphaltene precipitation, the overall polarity starts to change drastically and this change affects the asphaltene stability more. This study investigates the crude oil fractions polarity and their individual impact on asphaltene precipitation.
Two crude oil samples with different asphaltene content, API gravity, and viscosity were divided into their Saturates, Aromatics, Resins, and Asphaltenes (SARA) fractions. The crude oils and their SARA fractions were characterized by Fourier Transform InfraRed (FTIR) spectroscopy. The polarity of crude oils and their SARA fractions were determined through dielectric constant measurements by in-house-built capacitance.
The polarity of the individual fractions and bulk crude oil samples were analyzed together to understand how the mutual interaction of crude oil fractions affects the asphaltene stability. The overall polarity of the crude oil is the key to asphaltene stability. Resins and asphaltenes are the polar components of crude oil, thus, resins to asphaltenes ratio affects the overall stability of the asphaltenes. Asphaltenes are soluble in aromatic solvents and insoluble in normal alkanes, thus, while the increase in the saturates fraction in crude oil decreases the asphaltene stability, the increase in the aromatics fraction in crude oil reestablishes the stabilization. The solvent power of saturates and aromatics fractions are controlled by the impurities in saturates and aromatics fractions. Because while saturates and aromatics are known as nonpolar fractions of crude oils, the impurity content of those fractions results in polar sides in both saturates and aromatics fractions. The polar side of those fractions makes the interaction with asphaltenes more pronounced and affect the stability of asphaltenes considerably.
The holistic understanding of the asphaltene stability is achieved by analyzing the polarity of asphaltenes alone and within crude oil. These results are very useful in preventing the asphaltene precipitation and modelling its stability.