Asphaltenes are molecular substances that are found in

crude oil Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...

, along with resins, aromatic hydrocarbons, and saturates (i.e. saturated hydrocarbons such as

alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...

s). The word "asphaltene" was coined by

Boussingault Jean-Baptiste Joseph Dieudonné Boussingault (2 February 1801 – 11 May 1887) was a French chemist who made significant contributions to agricultural science, petroleum science and metallurgy. Biography Jean-Baptiste Boussingault – an agri ...

in 1837 when he noticed that the distillation residue of some

bitumen Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...

s had asphalt-like properties. Asphaltenes in the form of asphalt or bitumen products from oil refineries are used as paving materials on roads, shingles for roofs, and waterproof coatings on building foundations.

Composition

Asphaltenes consist primarily of carbon (C^4-), hydrogen (H⁺), nitrogen (N), oxygen (O^2-), and

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

(S^2-), as well as trace amounts of

vanadium Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pas ...

(V) and nickel (Ni). 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 carbonaceous material such as crude oil,

, or coal. Asphaltenes have been shown to have a distribution of

molecular masses The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quanti ...

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 the well known SARA analysis b
TLC-FID SARA Method

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

, 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

heteroatoms In chemistry, a heteroatom () is, strictly, any atom that is not carbon or hydrogen. Organic chemistry In practice, the term is usually used more specifically to indicate that non-carbon atoms have replaced carbon in the backbone of the molecula ...

, combined with trace amounts of heavy metals, particularly chelated vanadium and nickel, and aliphatic side chains of various lengths. 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. Asphaltene after heating have been subdivided as: nonvolatile (heterocyclic N and S species), and, volatile (paraffin + olefins, benzenes, naphthalenes, phenanthrenes, several others). Speight 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 (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 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 or machine learning methods and can be measured in the laboratory using imaging methods or filtration.

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 are known to be one of the largest causes of fouling in the heat exchangers of the crude oil distillation preheat train. They are present within micelles in crude oil, which can be broken down by reaction with paraffins under high temperature. Once the protective micelle has been removed polar asphaltenes agglomerate and are transported to the tube walls, where they can stick 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

References

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External links

An in-depth article on asphaltenes from OilfieldWiki.com, the oilfield encyclopediaArticle regarding asphaltene fouling by Irwin A. WieheAsphaltene Aggregation from Crude Oils and Model Systems Studied by High-Pressure NIR Spectroscopy
(Source : American Chemical Society)
A comprehensive website about asphaltene and its role in petroleum fouling
by Prof.

GA Mansoori Gholam Ali Mansoori (born in 1943), G. Ali Mansoori also known as "GA Mansoori" is an Iranian-American scientist known for his research within energy, nanotechnology and thermodynamics. He is a professor at the Departments of Bioengineering, Chemi ...

at the Univ. of Illinois at Chicago * Petroleum production Asphalt