(29f) Insight Into the Physical Asphaltene Precipitation Mechanism by Small-Angle Neutron Scattering

The discovery of slow asphaltene precipitation kinetics has provided a new avenue to probe asphaltene behavior to discover both the physical precipitation mechanism and the factors that control the process. Results published by Maqbool, et al.revealed that even though a mixture of oil and precipitant appeared stable, if monitored for up to several months, asphaltene precipitation could be detected [1]. Small-angle neutron scattering (SANS) results presented at Petrophase XII, revealed that the slow precipitation kinetics are the result of detection limitations associated with standard laboratory practices [2]. When SANS was used on similar systems, the instability could be detected nearly instantly due to the sensitivity of small-angle scattering to nanometer length-scale changes.

A new model has been developed to analyze SANS results of slowly precipitating asphaltene systems that not only fits the scattering results, but also predicts the fraction of insoluble and precipitating asphaltenes within reasonable accuracy. One of the inherent major limitations of small-angle scattering analysis is a lack of validation for the models selected to fit the results. Typically the only evidence for correctness of a particular scattering model is the quality of the fit. However, in this approach we have compared the predictions generated from analysis of the scattering results to the solubility of asphaltenes after we allowed the slow precipitation kinetics to resolve.

According to the new model, the physical mechanism for asphaltene precipitation is that asphaltene fractal aggregates or “clusters” grow in size once a precipitant is added. Results suggest that when the size of the clusters reach a critical value, a fraction precipitate and a geometric population balance has been previously utilized to show that this final stage of precipitation is likely a reaction-limited aggregation process [3]. Based on the model, the scattering observed is due primarily to the interaction of asphaltenes and the physical structure of the clusters. The asphaltenes not precipitating remain as stable clusters and at a size smaller than in the original oil. Soluble asphaltenes at a specific precipitant concentration will be of a more stable fraction and will have weaker interactions. In conclusion, we believe that the success of the model to predict asphaltene solubility strongly supports the described precipitation mechanism and represents an important step toward deciphering the complex behavior of asphaltenes.

[1] T. Maqbool, A. T. Balgoa, and H. S. Fogler, Energy & Fuels, 23, 3681-3686 (2009).

[2] M. P. Hoepfner, C. V. B. Favero and H. S. Fogler, Oral Presentation, Petrophase XII, London, UK, 10-14 Jul. (2011).

[3] T. Maqbool, S. Raha, M. P. Hoepfner, and H. S. Fogler, Energy & Fuels, 25, 1585-1596 (2011).