Thermodynamic Micellization Model for Asphaltene Aggregation and Precipitation in Petroleum Fluids

Abstract

Summary A thermodynamic framework is proposed to describe the structure of asphaltene micelles and precipitation in crude. The asphaltene micelle is assumed to consist of an asphaltene core surrounded by a solvated shell. The shell contains both resin and all the other species except asphaltenes. The theoretical framework consists of the standard Gibbs free energy of micellar formation and the Gibbs free energies ofthe petroleum liquid and the precipitated phase. The standard Gibbs free energy of the micellar formation is modeled as the sum of various contributions including the association between asphaltene molecules in the core, the deformation of asphaltene and resin molecules in the micelle, and the adsorption of resin molecules onto the micellar core. The Gibbs free energy ofthe petroleum liquid phase includes the contributions from the standard state, mixing, and interactions among the species. The precipitated phase is assumed to be an ideal solid mixture of asphaltenes and resin. The direct minimization of the Gibbs free energy of the liquid/solid system is invoked to calculate the micellar structure and composition in crude and the amount and composition of the precipitated phase. The predicted results reveal that the addition of diluents n-C5 to n-C10 leads to asphaltene precipitation; whereas diluents such as C2 or C3 give rise to precipitation of nearly all resin and asphaltenes from crude. These predictions are in agreement with laboratory measurements. The predicted micellar size increases with dilution ratio. This prediction is also in agreement with measured data.