Asphaltenes are aromatic hydrocarbons and defined as a solubility class as the n-heptane-insoluble, toluene-soluble fraction of a crude oil or carbonaceous material. They are always present in crude oils and influence the oil properties. Phase changes, viscosity, and interfacial properties of crude oils are strongly affected by asphaltenes. Problems arise when asphaltenes are exposed to changes in temperatures, pressure, or composition, and they become insoluble in the oil. When asphaltenes precipitate, they can deposit onto the walls of the pipe, inhibiting the flow of oil and can end up blocking the pipe entirely. Although, the negative impact of asphaltenes to the oil industries is well known, however, the exact mechanism by which asphaltene flocculation and aggregation occurs is still not fully understood.   

         Over the last decade methods have been developed to characterize and model the mechanisms of asphaltene flocculation, aggregation and precipitation. [1, and references listed therein].  To date, there have been TEM analyses of asphaltenes that have impacted petrochemical research activities [2].  However, the disadvantage is that the asphaltene sample may be altered as a consequence of sample preparation.  With the development of commercially available liquid cell holders for in situ TEM there is now the opportunity of direct observations of the oil emulsion system at the nm scale in their natural environment.

         Initial in situ TEM experiments of asphaltene formation and aggregation were conducted in a FEI Talos F200X TEM operated at 200 keV using the Protochips Poseidon P210 analytical liquid cell holder.  A light crude oil with a nominal asphaltene content of 3.7% was mixed with heptane to initiate flocculation of the asphaltenes in the liquid in situ TEM cell. Our first results indicate that the aggregation process is driven by the initial formation of 10-20 nm spherical colloids. These colloids cluster to flocculates in a range of several tens to hundreds of nanometers in the oil-heptane emulsion (Figure 1). The flocculation sequence is in good agreement with the proposed Yen model [1]. Further asphaltene flocculation experiments from different crude oils and their morphology evolution will be compared and discussed. In addition, opportunities and limitations for using in situ liquid cell holders for studying asphaltene flocculation in an analytical TEM will be described.

References:

[1] O.C. Mullins, Energy & Fuels, 24, (2010), p. 2179-2207.

[2] L. Goual et al, Langmuir, 30, (2014), p. 5394-5403.

Acknowledgement:

The authors would like to acknowledge the funding and technical support from BP through the BP International Centre for Advanced Materials (BP-ICAM), which made this research possible.

Figures:

Figure 1. STEM-HAADF images of asphaltene flocculates in oil + heptane emulsion in an in situ liquid cell TEM holder.

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