The modelling of the behaviour of crude oils is particularly challenging due to inherent uncertainty concerning the chemical characterization of the system, and also due to the diverse length and time scales involved. This work focuses on the particular problem of the characterization of the asphaltene aggregation process. Here we describe a large scale Molecular Dynamics simulation of model asphaltene molecules in a synthetic discrete crude oil at conditions which mimic oil reservoirs.
Coarse-grained (CG) models, in which beads representing multiple atoms are used, significantly reduce the number of particles and calculations involved in the simulation and allow access to large system sizes and exploration of relatively long times. We use here the SAFT-γ Mie approach [1,2,3], where the parameters describing the intermolecular forces are obtained from fitting the properties of molecules and molecular segments to macroscopic thermophysical properties, and produce CG models of the asphaltenes, resins and different components (aromatics, waxes, non-condensables, etc.) of a typical live oil. The use of CG modelling, of the type described here, offer significant advantages over conventional all-atom force fields without compromising on the accuracy of the results, and the SAFT models have been benchmarked against selected atomistic MD simulations.
Complex systems composing of realistic mixtures of solvents, resins and multiple asphaltenes in wide ranges of pressures and temperatures are explored using CG models in large-scale parallel MD simulations, in which more than 120,000 molecules (roughly equivalent to 2M atoms) were involved. Similarly, time scales of fractions of μs were explored. It was observed that only at these larger scales can one fully appreciate the effect of clustering and aggregation in the system. These results suggest that most of the reports in the literature, based on atomistic modelling of systems an order of magnitude smaller, are riddled by system size effects and by observation of un-equilibrated states.
Results suggest ways in which the polydispersity and complexity of the crude help maintain the larger asphaltene molecules from segregating and precipitating. The effects of the concentrations of aromatics and saturated compounds on asphaltene aggregation were investigated, and the bubble points were determined.
References
[1] Müller, E. A., Jackson, G. (2014). Force-Field Parameters from the SAFT-γ Equation of State for Use in Coarse-Grained Molecular Simulations. Annu. Rev. Chem. and Bio. Eng., 5(1), 405–427.
[2] Jiménez-Serratos G. et al. Coarse-graining polycyclic aromatic hydrocarbons using SAFT-γ force fields: application to molecular simulations of asphaltenes, Petrophase 2017
[3] Herdes, C., Totton, T. S., Müller, E. A. (2015). Coarse grained force field for the molecular simulation of natural gases and condensates. Fluid Phase Equilibria, 406, 91–100.
