Asphaltenes are Molecule substances that are found in crude oil, along with resins, aromatic hydrocarbons, and saturates (i.e. saturated hydrocarbons such as ).[Mullins, O. C. et al. (eds.) (2007) Asphaltenes, Heavy Oils and Petroleomics, Springer, New York.]
Composition
Asphaltenes consist primarily of
carbon,
hydrogen,
nitrogen,
oxygen, and
sulfur, as well as trace amounts of
vanadium and
nickel. The C:H ratio is approximately 1:1.2, depending on the asphaltene source. Asphaltenes are defined operationally as the n-
heptane ()-insoluble,
toluene ()-soluble component of a
material such as crude oil,
bitumen, or
coal. Asphaltenes have been shown to have a distribution of
molecular masses in the range of 400 u to 1500 u, but the average and maximum values are difficult to determine due to aggregation of the molecules in solution.
Analysis
The molecular structure of asphaltenes is difficult to determine because the molecules tend to stick together in solution.
These materials are extremely complex mixtures containing hundreds or even thousands of individual chemical species. Asphaltenes do not have a specific chemical formula: individual molecules can vary in the number of atoms contained in the structure, and the average chemical formula can depend on the source. Although they have been subjected to modern analytical methods, including SARA, mass spectrometry, electron paramagnetic resonance and nuclear magnetic resonance, the exact molecular structures are difficult to determine. Given this limitation, asphaltenes are composed mainly of polyaromatic carbon ring units with
oxygen,
nitrogen, and
sulfur heteroatoms, combined with trace amounts of heavy metals, particularly chelated vanadium and nickel, and aliphatic side chains of various lengths.
[Asomaning, S. (1997). Heat exchanger fouling by petroleum asphaltenes. Ph.D. Thesis, University of British Columbia] Many asphaltenes from crude oils around the world contain similar ring units, as well as polar and non-polar groups, which are linked together to make highly diverse large molecules.
[G.A. Mansoori, (2009). Int. J. Oil, Gas and Coal Technology 2 141.]
Asphaltene after heating[J.H. Pacheco-Sánchez, and G.A.Mansoori, (2013) Revista Mexicana de Física 59, 584-593.] have been subdivided as: nonvolatile (heterocyclic N and S species), and, volatile (paraffin + olefins, benzenes, naphthalenes, phenanthrenes, several others). Speight[J.G. Speight, (1994). in the book Asphaltenes and Asphalts, 1, Developments in Petroleum Science, 40 edited by Yen T. F. and G. V. Chilingarian, (Elsevier Science, New York ). Chapter: Chemical and physical studies of petroleum asphaltenes] reports a simplified representation of the separation of petroleum into the following six major fractions: volatile saturates, volatile aromatics, nonvolatile saturates, nonvolatile aromatics, resins and asphaltenes. He also reports arbitrarily defined physical boundaries for petroleum using carbon-number and boiling point.
Geochemistry
Asphaltenes are today widely recognised as dispersed, chemically altered fragments of
kerogen, which migrated out of the
source rock for the oil, during oil catagenesis. Asphaltenes had been thought to be held in solution in oil by resins (similar structure and chemistry, but smaller), but recent data shows that this is incorrect. Indeed, it has recently been suggested that asphaltenes are nanocolloidally suspended in crude oil and in toluene solutions of sufficient concentrations. In any event, for low surface tension liquids, such as alkanes and toluene,
surfactants are not necessary to maintain nanocolloidal suspensions of asphaltenes.
The nickel to vanadium ratio of asphaltenes reflect the pH and Eh conditions of the paleo-depositional environment of the source rock for oil (Lewan, 1980;1984), and this ratio is, therefore, in use in the petroleum industry for oil-oil correlation and for identification of potential source rocks for oil exploration.
Occurrence
Heavy oils,
oil sands, bitumen and biodegraded oils (as bacteria cannot assimilate asphaltenes, but readily consume saturated hydrocarbons and certain aromatic hydrocarbon isomers – enzymatically controlled) contain much higher proportions of asphaltenes than do medium-
API gravity oils or light oils. Condensates are virtually devoid of asphaltenes.
Measurement
Because the ratio of electron spins per gram is constant for a particular species of asphaltene
then the quantity of asphaltene in an oil can be determined by measuring its paramagnetic signature (EPR). Measuring the EPR signature of the oil at the wellhead as the oil is produced then gives a direct indication of whether the amount of asphaltene is changing (e.g. because of precipitation or sloughing in the tubing below).
In addition, asphaltene aggregation, precipitation or deposition can sometimes be predicted by modeling
Production problems
Asphaltenes impart high viscosity to crude oils, negatively impacting production. Furthermore, the variable asphaltene concentration in crude oils within individual reservoirs creates a myriad of production problems.
Heat exchanger fouling
Asphaltenes, a significant contributor to fouling in the heat exchangers of the crude oil distillation preheat train, are present within micelles in crude oil. These micelles can be broken down by reacting with paraffins at high temperatures. Once the protective micelle is removed, polar asphaltenes aggregate and are transported to the tube walls, where they adhere and form a foulant layer.
Asphaltene removal
Chemical treatments for removing asphaltene include:
-
Solvents
-
Dispersants/solvents
-
Oil/dispersants/solvents
The dispersant/solvent approach is used for removing asphaltenes from formation minerals. Continuous treating may be required to inhibit asphaltene deposition in the tubing. Batch treatments are common for dehydration equipment and tank bottoms. There are also asphaltene precipitation inhibitors that can be used by continuous treatment or squeeze treatments.[ Understanding paraffin and asphaltene problems in oil and gas wells , Petroleum Technology Transfer Council, South Midcontinent Region, July 16, 2003 Workshop in Smackover, Arkansas at the Arkansas Natural Resources Museum]
See also
External links
