Adsorption isotherms of two native resins on two different asphaltene surfaces were obtained using the UV spectrophotometric technique. The shape of the curves obtained can be attributed both to multilayer adsorption and to penetration of resins in the microporous structure of the asphaltenes. The relationship between adsorption behavior and activity of the native resins as asphaltene stabilizers indicates that their effectiveness is related to their adsorbed amount on asphaltene particles and also to their capacity to dissolve asphaltenes. The results obtained support a model for asphaltene stabilization where the resins are incorporated into the bulk asphaltenes helping them to diffuse in the solvent. Significant differences in the behavior of native resins and alkylbenzene-derived amphiphiles as asphaltene stabilizers were observed. At the same equilibrium concentration, the adsorbed amount of native resins is lower than the adsorbed amount of amphiphiles. However, the native resins exhibit a higher asphaltene dissolution power than amphiphiles and a comparable effectiveness as asphaltene stabilizers.

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.

Amphiphiles are frequently used to prevent asphaltene precipitation in reservoir rocks and wellbore tubing. It is supposed that these substances can stabilize the asphaltenes forming a steric stabilization layer around them. In this study, adsorption isotherms of two amphiphiles (nonylphenol and nonylphenolic resin) and a native resin on asphaltene particles are obtained in order to analyze the relationship between adsorption and effectiveness of these compounds as asphaltene stabilizers. The comparison of the three isotherms reveals a significant difference between the adsorption behavior of the amphiphiles and the resin fraction in terms of the shape of the isotherm. Nonylphenol and nonylphenolic resin adsorption isotherms show an LS-shape that can be explained using an adsorption mechanism in two steps. The adsorption isotherm of the native resin can be explained by penetration of the micropores of the asphaltene particles by resin molecules. There were found significant differences in the activity of the studied compounds as asphaltene precipitation inhibitors that can be linked to the adsorption behavior of these species. In particular, the native resins seem to show an activity stabilization mechanism different than that found for the other species studied.

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