No abstract

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.

Adsorption isotherms of alkylbenzene-derived amphiphiles were determined experimentally and correlated with their activity as stabilizers of asphaltenes. The isotherms of these amphiphiles indicate an adsorption process in two steps:  in the first step, the amphiphiles are adsorbed through the interactions with the surface of the asphaltene particles; in the second step, the interactions between adsorbed amphiphiles become predominant and the formation of aggregates on the surface begins. A direct correlation was also found between the maximum surface excess concentration of the amphiphile and its activity as stabilizer. This indicates that the aggregation of amphiphiles on the asphaltene surface plays a very important role in the inhibition of the precipitation of asphaltenes.

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.