Molecular polydispersity improves prediction of asphaltene aggregation

Asphaltene flocculation and deposition from petroleum fluids may cause severe formation damage and flow assurance challenges. Flocculation occurs when asphaltene aggregates are no longer colloidally stable in solution. Although extensive experimental and modeling studies have been performed on asphaltene aggregation, some of the mechanisms and parameters governing this phenomenon (such as molecular structure and polydispersity) are still not fully understood. In this study, a polydisperse mixture of asphaltene molecules varying in size, aromaticity and functionality was studied by atomistic molecular dynamics simulations. Aggregation in systems spanning 105 to 1005 molecules was simulated for approximately one microsecond under ambient conditions and a concentration of 5 wt% in heptane. Simulation trajectories revealed two stages of aggregation: nanoaggregation and clustering, in agreement with the Yen-Mullins hierarchy. Distinctions between nanoaggregates and clusters were observed by various analyses including trajectory visualization, fractal dimension, and cluster size analysis. Different roles for molecules varying in size and aromaticity were observed in the aggregation process: molecules low in molecular weight and/or aromaticity tended to cap larger aggregates, prohibiting further accumulation and limiting nanoaggregate size. Comparisons to simulations of monodisperse systems containing only one type of molecules, either highly hydrogen bonding or non‑hydrogen bonding, accentuated these findings. This study demonstrates the importance of polydispersity on asphaltene aggregation and provides a lower limit of approximately 375 molecules in such a mixture to represent the two stages of aggregation.