(428g) Multiscale Molecular Dynamics Simulations of Asphaltenes in Crude Oils Based on the SAFT-? Mie Force Field

The modelling of crude oil behaviour 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 framework that spans multiple time, size and complexity scales to provide a richer description of the problem, ending with a large scale Molecular Dynamics simulation of model asphaltene molecules in a synthetic discrete crude oil at conditions which mimic oil reservoirs.

At the bottom of the size/complexity scale is an experimentally-based description of plausible molecular structures of asphaltenes and resins; the Quantitative Molecular Representation (QMR) approach [1] was used to generate structures based on experimental data from crude oils. These structures were optimised to select a small subset of molecules that give the best match with the experimental data input, consisting of elemental analysis parameters, and both 1H and 13C NMR-spectroscopy parameters.

The behaviour of the model QMR-generated asphaltene structures in heptane, toluene and heptol solvents was then determined using a fully atomistic description employing the OPLS-AA force field in classical molecular dynamics (MD) simulations. The simulations shed light on the incipient clustering mechanisms and the size and distribution of asphaltenes in simple solvents. While simulations at this level of detail are enlightening, it has been shown [2] that no level of current computational resources is sufficient to cover the real times required to represent the clustering process, hence the need to recourse to a lower level of fidelity.

At the next level of scale, coarse-grained (CG) models were built, in which beads representing multiple atoms were used, significantly reducing the number of particles and calculations involved in the simulation. We used here the SAFT-γ Mie approach [3,4], where the parameters describing the intermolecular forces are obtained from fitting the properties of molecules and molecular segments to macroscopic thermophysical properties, and produced CG models of the asphaltenes, resins and different components (aromatics, waxes, non-condensables, etc.) of a typical live oil. The output of selected atomistic MD simulations was used to validate the robustness of the CG models.

Complex systems composing of realistic mixtures of solvents, resins and multiple asphaltenes in wide ranges of pressures and temperatures were then explored using CG models in large-scale parallel MD simulations, in which more than 120,000 molecules or roughly 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 suggested that the polydispersity and complexity of the crude helped 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] Neurock, M.; Nigam, A.; Trauth, D.; Klein, M. T. (1994). Chem. Eng. Sci., 49, 4153−77 . for a recent application see Boek, E.S., Yakovlev, D.S., Headen, T. (2009). Energy & Fuels 23, 1209-1219.

[2] Headen, T. F. et al. (2017). Energy & Fuels, 31, 1108–1125.

[3] Müller, E. A., Jackson, G. (2014). Annu. Rev. Chem. Biomolec. Eng., 5, 405-427

[4] Herdes, C., Totton, T. S., Müller, E. A. (2015). Fluid Phase Equilibria, 406, 91–100.