Summary
Tremendous strides have been made recently in asphaltene science. Many
advanced analytical techniques have been applied recently to asphaltenes,
elucidating many asphaltene properties. The inability of certain techniques to
provide correct asphaltene parameters has also been clarified. Longstanding
controversies have been resolved. For example, molecular structural issues of
asphaltenes have been resolved; in particular, asphaltene molecular weight is
now known. The primary aggregation threshold has recently been established by a
variety of techniques. Characterization of asphaltene interfacial activity has
advanced considerably. The hierarchy of asphaltene aggregation has emerged into
a fairly comprehensive picture, essentially in accord with the Yen model with
the additional inclusion of certain constraints. Crude oil and asphaltene
science is now poised to develop proper structure-function relations that are
the defining objective of the new field: petroleomics. The purpose of this
paper is to review these developments in order to present a more clear and
accessible picture of asphaltenes, especially considering that the asphaltene
literature is a bit opaque.
Introduction
The asphaltenes are a very important class of compounds in crude oils
(Chilingarian and Yen 1978; Bunger and Li 1981; Sheu and Mullins 1995; Mullins
and Sheu 1998; Mullins et al. 2007c). The asphaltenes represent a complex
mixture of compounds and are defined by their solubility characteristics, not
by a specific chemical classification. A common (laboratory) definition of
asphaltenes is that they are toluene soluble, n-heptane insoluble. Other light
alkanes are sometimes used to isolate asphaltenes. This solubility
classification is very useful for crude oils because it captures the most
aromatic portion of crude oil. As we will see, this solubility defintion also
captures those molecular components of asphaltene that aggregate. Other
carbonaceous materials such as coal do possess an asphaltene fraction, but that
often will not correspond to the most aromatic fraction.
Petroleum asphaltenes, the subject of this paper, can undergo phase
transitions that are an impediment in the production of crude oil. Fig.
1 shows a picture of an asphaltene deposit in a pipeline; obviously,
asphaltene deposition is detrimental to the production of oil. Immediately it
becomes evident that different operational definitions apply for the term
asphaltene in the field vs. the lab. Indeed, the field deposit is very enriched
in n-heptane-insoluble, toluene-soluble materials, but this field asphaltene
deposit is not identically the standard laboratory solubility class. It is
common knowledge that a pressure drop on certain live crude oils (containing
dissolved gas) can cause asphaltene flocculation, the first step in creating
deposits that are seen in Fig. 1. Highly compressible, very undersaturated
crude oils are most susceptible to asphaltene deposition problems with a
pressure drop (de Boer et al. 1995). In depressurization flocculation, the
character of the asphaltene flocs is dependent on the extent of pressure drop,
suggesting some variations in the corresponding chemical composition (Hammami
et al. 2000; Joshi et al. 2001). Comingling different oils can result in
asphaltene precipitation that can resemble solvent precipitation.
Asphaltenes are hydrogen-deficient compared to alkanes; thus, either
hydrogen must be added or coke removed in crude oil refining to generate
transportation fuels. Thus, asphaltene content lowers the economic value of
crude oil. Increasing asphaltene content is associated with dramatically
increasing viscosity, especially at room temperature; again, this is of
operational concern. The strong temperature dependence of viscosity of
asphaltic materials is one of their important properties that make them useful
for paving and coating; application of asphaltic materials is facile at
moderately high temperatures, while desired rheological properties are obtained
at ambient temperatures.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
14 July 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
10 October 2007
- Manuscript approved:
11 October 2007
- Version of record:
20 March 2008
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