Asphaltene Instability Measured by Small-Angle Neutron Scattering

Asphaltenes are defined as the petroleum fraction that precipitates with the addition of a normal alkane; however, it was only recently discovered that there is a strong time-dependency for precipitation in oil/n-alkane mixtures [1]. It was found that at low precipitant concentrations, asphaltenes would eventually precipitate if one simply waited, sometimes up to several months. Asphaltenes were also recently found to deposit before the precipitation could be detected by optical microscopy, further revealing asphaltene instability [2]. Small-angle neutron scattering (SANS) was selected as a technique that could detect subtle changes in the asphaltene size and shape once a precipitant is added. Optical microscopy requires asphaltene aggregates to grow to ~0.5 µm before the precipitation can be detected. With SANS, changes in size or shape of a few nanometers can easily be measured.

Experiments were performed on the CG-2 SANS instrument at Oak Ridge National Laboratory. Oil samples were diluted with heptane at various concentrations, and were allowed to mix for times ranging from less than 1 day and up to 6 months prior to performing the scattering experiments. The goal was to investigate the asphaltene stability over a wide range of precipitant concentrations and to monitor the aggregation process over time. If asphaltenes are found to be stable at low enough precipitant concentrations, the scattering results will indicate that there is no change in the morphology of an asphaltene nanoaggregate and that any change in the scattering would be due to dilution and scattering contrast effects.

It was found that asphaltenes grew in size with the small addition of precipitant, as low as 10 vol. % heptane, for an oil with an instantaneous onset condition of 40 vol. % heptane. The asphaltenes continued to grow over time. Attempting to detect this instability with optical microscopy may not be possible due to the long-term precipitation/aggregation kinetics [1]. Experiments were also performed on a second oil and a model oil consisting of 3 wt. % asphaltenes dispersed in d8-toluene. Additionally, the asphaltenes remaining in solution were much smaller than those that precipitated, suggesting that the largest of the asphaltene nanoaggregates are the most unstable and precipitate first.

These findings question whether asphaltenes are ever truly stable and a small fraction of asphaltenes may precipitate with any slight perturbation. Future models to understand asphaltene stability need to consider this effect and the assumption of equilibrium may not be valid for a wide range of conditions.

[1] T. Maqbool, A. T. Balgoa, and H. S. Fogler, “Revisiting Asphaltene Precipitation from Crude Oils: A Case of Neglected Kinetic Effects,” Energy & Fuels, vol. 23, no. 7, pp. 3681-3686, Jul. 2009.

[2] M. Hoepfner and H. S. Fogler, “Effects of Preciptant Concentration on Asphaltene Deposition,” presented at the 10th Annual International Conference on Petroluem Phase Behavior and Fouling, Rio de Janiero, Brazil, 2009.