The phase stability of crude oil is dependent on a multitude of factors, including temperature, pressure, and component fractions, especially long chain paraffin and polar asphaltene fractions. Paraffins precipitate out of the crude oil during pipeline transportation due to solubility limits, and form paraffin-oil gel deposits on the pipe walls. The presence of asphaltenes in crude oil is postulated to affect the formation of these paraffin gels. To quantify this effect, a controlled stress rheometer was used to study the gelation temperature and the yield stress of a model paraffinoil system. It was observed that the addition of asphaltenes in small proportions (∼0.1 wt %) resulted in a significant decrease both in the gelation temperature and the yield stress of the model system, indicating that the presence of asphaltenes hinders the gelation mechanism. Addition of higher amounts of asphaltenes resulted in macroscopic phase separation: a deposit consisting of asphaltenes and paraffins separated out of the liquid. The effects of operating conditions and the asphaltene polarity on the gelation process were also studied. Polarized light microscopy and nuclear magnetic resonance spectroscopy were used to obtain insights into the rheometric results.

Structural and compositional characterization of asphaltenes that were extracted from unstable crude oils, stable crude oils, and organic solid deposits was performed to elucidate their similarities and differences. A fractionation technique that divided the asphaltenes into different subfractions, based on polarity, was used to characterize these asphaltene samples. The parameters affecting the stability of these asphaltene subfractions were elucidated. The asphaltenes that were extracted from unstable crude oils and from solid deposits contained substantially greater portions of the higher polar fractions and have a higher polarity, compared to the asphaltenes obtained from crude oils with no asphaltene stability problems in the field. The dielectric constant, solubility, and flocculation experiments showed that these higher-polarity fractions have a greater tendency to aggregate and are more difficult to remediate. These results suggested that the presence of a certain type of asphalteneparticularly, a high-polarity asphaltenehas a key role in the stability of asphaltene in crude oils.

The goal of this work is to provide characterization and a unified framework for understanding asphaltenes in crude oil. A fractionation technique that divides an asphaltene sample into different components based on polarity was developed. The morphology of the fractions varied from dense, shiny black particles that displayed a crystalline microstructure under SEM examination (the most polar fraction) to porous, dull-brown powders of a completely amorphous nature (the least polar fraction). Extensive studies using gel permeation chromatography, FTIR spectroscopy, and elemental analysis revealed no apparent structural differences between various fractions. However, substantially different dissolution characteristics were displayed by the fractions in a differential reactor with both toluene and an amphiphile/alkane micellar solvent. Fractions of higher polarity displayed lower dissolution rate constants and dissolved to a lesser extent than the lower polarity fractions. Analysis of asphaltene samples from eight different crude oils indicate that the fractionation technique may be a powerful tool for predicting the dissolution rate constant of an asphaltene sample and subsequently its difficulty of remediation. Further analysis of the fractions using ICP-90 and X-ray flourescence suggests that heteroatom content (especially metals such as iron, nickel, vanadium, aluminum, and nonmetals such as chlorine) plays a major role in determining the high polarity of asphaltenes. Treatment of the highest polarity fraction with the salt form of ethylenediamine tetraacetic acid (EDTA; a powerful metal-chelating agent) significantly increased dissolution rates when dodecyl benzene sulfonic acid (DBSA) was used in a heptane solution, suggesting that metal content may have a direct effect on dissolution rates. A slight increase in dissolution rate was observed with toluene as the solvent.