Effect of resin/asphaltene ratio on the rheological behavior of asphaltene solutions in a de-asphalted oil and p-xylene: A theoretical–experimental approach

Abstract This study focuses on understanding the effect of the resin/asphaltene mass ratio (R/A) on the rheological behavior of asphaltene solutions. To this aim, two cases were evaluated experimentally and theoretically. The first case comprised asphaltenes dissolved in a de-asphalted oil (DAO) at asphaltene concentrations of 2, 10, and 16.5% of mass fraction, with R/A ratios of 3, 6, and 31 being obtained. The second case consisted of asphaltenes dissolved in p-xylene, at an asphaltene mass fraction of 1 and 18%, and adding resins to obtain low R/A ratios (0, 0.125, 0.5, 1, and 1.5). The viscosities measured experimentally were reproduced by molecular dynamics (MD) simulations, using the Green-Kubo method and non-equilibrium MD (NEMD) for estimating the viscosity of solutions with Newtonian and non-Newtonian behavior, respectively. In all cases, the experimental measurements were accurately reproduced by MD. Further, the asphaltene-asphaltene and asphaltene-resin coordination number (CN), the asphaltene-asphaltene and asphaltene-solvent interaction energies, the radius of gyration, and the volumetric fraction for asphaltene aggregates were calculated. Theoretical results evidence that the asphaltene molecules dissolved in DAO exhibited repulsive interaction energies between them, whereby the asphaltene is mainly dispersed for R/A ratios higher than 3 in DAO. This result indicates that the viscosity of the heavy oil is not only dependent on the asphaltene aggregation, but that the resin plays a key role in the rheological behavior of this type of crude oil. This latter suggestion was evidenced in the viscosity measurements due to the DAO viscosity (1776 cP) being considerably higher compared to the individual viscosity of the other compounds of the mixture (i.e., saturates and aromatics), which were at least two orders of magnitude lower. In the experimental evaluations of rheological behavior, varying the resin amount in asphaltene dissolved in p-xylene, a minimum viscosity point was obtained for a specific R/A ratio. The MD results indicated that the resins inhibit the asphaltene aggregation, reducing the volumetric fraction and, therefore, the viscosity decreased. Understanding the physical principles of the effect of the resin on the asphaltenes' aggregation, from an atomistic point of view, is a valuable contribution for designing inhibitors that can be used for the specific molecular characteristics of any crude oil.