The influence of different factors on the asphaltene stability in four crude oils was evaluated. Compositional studies and structural characterization of resins and asphaltenes were carried out in order to study a possible relationship between these properties and the asphaltene deposition behavior. Low hydrogen-to-carbon ratios, high aromaticities, and high condensation of aromatic rings were the main characteristics of the asphaltenes from unstable crude oils. According to these results, the stability behavior of the crude oils studied was strongly influenced by the structural characteristics of their asphaltenes. Since the self-aggregation of asphaltenes is a first step in the formation of precipitated asphaltene particles, this study also evaluates the formation of asphaltene aggregates as a function of the structural characteristics of the asphaltenes. The self-aggregation behavior was studied on the basis of surface tension measurements. Critical micelle concentrations were determined in different solvents. Using these results, it was possible to establish a relationship between self-association and structural characteristics through the calculation of the Flory parameter. On the basis of these findings, different characteristic patterns were identified depending on the origin of the asphaltenes. A new correlation has been found to describe the critical micelle concentrations of the asphaltene solutions.
Fundamental physicochemical factors involved in asphaltene precipitation under ambient conditions are studied in an effort to correlate them with the precipitation behavior observed in the oilfields. Asphaltene stabilization was evaluated by flocculation onset determination in titration experiments. The reversibility of asphaltene precipitation at ambient conditions was also explored with the proposed titration technique. It was found that the flocculation onsets of recombined long distillation residua are usually lower than the ones of the original samples.This finding suggests an irreversible nature of the process; this irreversibility is particularly observed for crude oils deemed stable from their field production records. Physicochemical characterization of crude oil hydrocarbon group-types was carried out and their influence on asphaltene stability was evaluated. Stable crude oils are characterized by possessing asphaltenes with lower density and lower aromaticity, compared with asphaltenes from unstable crude oils. Their resins show a higher stabilizing activity than the ones from unstable crude oils, and their maltenes also exhibit higher asphaltene stabilization effectiveness. Flocculation onsets of stable crude oils are higher than the flocculation onsets of unstable oils, at the same asphaltene content in the sample to be titrated. On the basis of this finding, a simple method is proposed for the evaluation of the possible risk of asphaltene precipitation in the oilfields.
The electrophoretic mobilities of asphaltenes in n-heptane and ethanol were determined. The asphaltenes studied show a positive charge in both solvents. However, the magnitude of the charge is considerably lower in n-heptane than in ethanol. It is concluded that although the electrostatic forces are presented in both solvents, the van der Waals' atractive forces are the main responsable for the flocculation of asphaltene particles. The addition of a well known asphaltene stabilizer, dodecyl benzene sulphonic acid (DBSA), decreases the electrophoretic mobility of asphaltene particles in ethanol until a constant positive value is reached. This means that the neutralization of the positive charges of the asphaltene particles is not complete, probably due to the limited adsorption of the DBSA on asphaltene surface and also, to the presence of charges inside the asphaltene particles that are not accessible to the DBSA.
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