THERMODYNAMIC MICELLIZATION MODEL FOR ASPHALTENE AGGREGATION AND PRECIPITATION IN PETROLEUM FLUIDS

A thermodynamic framework is proposed to describe the structure of asp haltene micelles and precipitation in crude. The asphaltene micelle is assumed to consist of an asphaltene core surrounded by a solvated she ll. The shell contains both resin and all the other species except asp haltenes. The theoretical framework consists of the standard Gibbs fre e energy of micellar formation and the Gibbs free energies of the petr oleum liquid and the precipitated phase. The standard Gibbs free energ y of the micellar formation is modeled as the sum of various contribut ions including the association between asphaltene molecules in the cor e, the deformation of asphaltene and resin molecules in the micelle, a nd the adsorption of resin molecules onto the micellar core. The Gibbs free energy of the petroleum liquid phase includes the contributions from the standard state, mixing, and interactions among the species. T he precipitated phase is assumed to be an ideal solid mixture of aspha ltenes and resin. The direct minimization of the Gibbs free energy of the liquid/ solid system is invoked to calculate the micellar structur e and composition in crude and the amount and composition of the preci pitated phase. The predicted results reveal that the addition of dilue nts n-C-5 to n-C-10 leads to asphaltene precipitation; whereas diluent s such as C-2 or C-3 give rise to precipitation of nearly all resin an d asphaltenes from crude. These predictions are in agreement with labo ratory measurements. The predicted micellar size increases with diluti on ratio. This prediction is also in agreement with measured data.