Asphaltenes
Encyclopedia
Asphaltenes are molecular substances that are found in crude oil, along with resins, aromatic hydrocarbon
s, and alkane
s (i.e., saturated hydrocarbons). The word "asphaltene" was coined by Boussingault
in 1837 when he noticed that the distillation
residue of some bitumens had asphalt
-like properties. Asphaltenes in the form of distillation products from oil refineries are used as "tar-mats" on roads.
, 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 carbonaceous
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, with an average being around 750 u.
FT-ICR MS, APPI, APCI FIMS, LDI) and many different diffusion techniques (time-resolved fluorescence depolarization (TRFD), fluorescence correlation spectroscopy
(FCS), Taylor dispersion
). This result deviates substantially from previous conventional wisdom, the old and incorrect view is promulgated on many popular web sites putatively illustrating asphaltene molecular structures
. Asphaltene molecular weight is ~750 Da with 500 - 1000 FWHM. Aggregation of asphaltenes at very low concentrations (in toluene) led to aggregate weights being misinterpreted as molecular weights with techniques such as VPO or GPC. The chemical structure is difficult to ascertain, due to the complex nature of the asphaltenes, but has been studied by all available techniques including X-ray, elemental, and pyrolysis GC-FID-GC-MS. However, it is undisputed that the asphaltenes are composed mainly of polyaromatic carbon i.e. polycondensed aromatic benzene units with oxygen
, nitrogen
, and sulfur
, (NSO-compounds) combined with minor amounts of a series of heavy metals, particularly vanadium and nickel which occur in porphyrin structures.
Asphaltene molecular architecture has also been controversial. The TRFD rotation diffusion measurements have indicated there is predominantly one PAH per asphaltene molecule. Asphaltene rotational diffusion measurements show that small PAH chromophores (blue fluorescing) are in small asphaltene molecules while big PAH chromophores (red fluorescing) are in big molecules. This implies that there is only one fused polycyclic aromatic hydrocarbon
(PAH) ring system per molecule. Very recent fragmentation studies by FT ICR-MS and by L2MS (two-color laser mass spectrometry) strongly support this 'island' molecular architecture as shown by TRFD and refuting the 'archipelago' molecular architecture.
Nanocolloidal structure: it has been shown that asphaltenes have two distinct nanocolloidal structures. The island molecule architecture, with attractive forces in the molecule interior (PAH)and steric repulsion from alkane peripheral groups gives rise to nanocolloids with aggregation numbers less than 10. Methods to show these structures in clude SANS, SAXS, high-Q ultrasonics, NMR, AC-conductivity, DC-conductivity, centrifugation. These structures are also observed in oil reservoirs with extensive vertical offset (where gravitational effects are evident). In addition to these 'primary' nanoaggrgeates, CLUSTERS of nanoaggregates can also form as seen by a variety of techniques.
, 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 contents 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).
, and biodegraded oils (as bacteria can not assimilate asphalten[e]s, 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 oil
s. Condensates are virtually devoid of asphaltenes.
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.
Aromatic hydrocarbon
An aromatic hydrocarbon or arene is a hydrocarbon with alternating double and single bonds between carbon atoms. The term 'aromatic' was assigned before the physical mechanism determining aromaticity was discovered, and was derived from the fact that many of the compounds have a sweet scent...
s, and alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
s (i.e., saturated hydrocarbons). The word "asphaltene" was coined by Boussingault
Jean Baptiste Boussingault
Jean-Baptiste Joseph Dieudonné Boussingault was a French chemist who made significant contributions to agricultural science, petroleum science and metallurgy.-Biography:...
in 1837 when he noticed that the distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
residue of some bitumens had asphalt
Asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
-like properties. Asphaltenes in the form of distillation products from oil refineries are used as "tar-mats" on roads.
Composition
Asphaltenes consist primarily of carbonCarbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, as well as trace amounts of vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
. The C:H ratio is approximately 1:1.2, depending on the asphaltene source. Asphaltenes are defined operationally as the n-heptane
Heptane
n-Heptane is the straight-chain alkane with the chemical formula H3C5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale...
-insoluble, toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
-soluble component of a carbonaceous
Carbonaceous
Carbonaceous is the defining attribute of a substance rich in carbon. Particularly, carbonaceous hydrocarbons are very unsaturated, high-molecular-weight hydrocarbons, having an elevated carbon:hydrogen ratio....
material such as crude oil, bitumen, or coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
. Asphaltenes have been shown to have a distribution of molecular masses in the range of 400 u
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
to 1500 u, with an average being around 750 u.
Analysis
All techniques are now roughly in accord, including many different mass spectral methods (ESIElectrospray ionization
Electrospray ionization is a technique used in mass spectrometry to produce ions. It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized...
FT-ICR MS, APPI, APCI FIMS, LDI) and many different diffusion techniques (time-resolved fluorescence depolarization (TRFD), fluorescence correlation spectroscopy
Fluorescence correlation spectroscopy
Fluorescence correlation spectroscopy is a correlation analysis of fluctuation of the fluorescence intensity. The analysis provides parameters of the physics under the fluctuations. One of the interesting applications of this is an analysis of the concentration fluctuations of fluorescent...
(FCS), Taylor dispersion
Taylor dispersion
Taylor dispersion is an effect in fluid mechanics in which a shear flow can increase the effective diffusivity of a species. Essentially, the shear acts to smear out the concentration distribution in the direction of the flow, enhancing the rate at which it spreads in that direction. The effect is...
). This result deviates substantially from previous conventional wisdom, the old and incorrect view is promulgated on many popular web sites putatively illustrating asphaltene molecular structures
Molecular geometry
Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.- Molecular geometry determination...
. Asphaltene molecular weight is ~750 Da with 500 - 1000 FWHM. Aggregation of asphaltenes at very low concentrations (in toluene) led to aggregate weights being misinterpreted as molecular weights with techniques such as VPO or GPC. The chemical structure is difficult to ascertain, due to the complex nature of the asphaltenes, but has been studied by all available techniques including X-ray, elemental, and pyrolysis GC-FID-GC-MS. However, it is undisputed that the asphaltenes are composed mainly of polyaromatic carbon i.e. polycondensed aromatic benzene units with oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, (NSO-compounds) combined with minor amounts of a series of heavy metals, particularly vanadium and nickel which occur in porphyrin structures.
Asphaltene molecular architecture has also been controversial. The TRFD rotation diffusion measurements have indicated there is predominantly one PAH per asphaltene molecule. Asphaltene rotational diffusion measurements show that small PAH chromophores (blue fluorescing) are in small asphaltene molecules while big PAH chromophores (red fluorescing) are in big molecules. This implies that there is only one fused polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...
(PAH) ring system per molecule. Very recent fragmentation studies by FT ICR-MS and by L2MS (two-color laser mass spectrometry) strongly support this 'island' molecular architecture as shown by TRFD and refuting the 'archipelago' molecular architecture.
Nanocolloidal structure: it has been shown that asphaltenes have two distinct nanocolloidal structures. The island molecule architecture, with attractive forces in the molecule interior (PAH)and steric repulsion from alkane peripheral groups gives rise to nanocolloids with aggregation numbers less than 10. Methods to show these structures in clude SANS, SAXS, high-Q ultrasonics, NMR, AC-conductivity, DC-conductivity, centrifugation. These structures are also observed in oil reservoirs with extensive vertical offset (where gravitational effects are evident). In addition to these 'primary' nanoaggrgeates, CLUSTERS of nanoaggregates can also form as seen by a variety of techniques.
Geology
Asphaltenes are today widely recognised as soluble, chemically altered fragments of kerogenKerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...
, which migrated out of the source rock
Source rock
In petroleum geology, source rock refers to rocks from which hydrocarbons have been generated or are capable of being generated. They form one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been deposited in a variety of environments including...
for the oil, during oil catagenesis
Catagenesis (geology)
Catagenesis is a term used in petroleum geology to describe the cracking process which results in the conversion of organic kerogens into hydrocarbons.-Theoretical reaction:...
. 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 contents of asphaltenes reflect the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
and Eh
Reduction potential
Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Reduction potential is measured in volts , or millivolts...
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, tar sandsTar sands
Bituminous sands, colloquially known as oil sands or tar sands, are a type of unconventional petroleum deposit. The sands contain naturally occurring mixtures of sand, clay, water, and a dense and extremely viscous form of petroleum technically referred to as bitumen...
, and biodegraded oils (as bacteria can not assimilate asphalten[e]s, but readily consume saturated hydrocarbons and certain aromatic hydrocarbon isomers - enzymatically controlled) contain much higher proportions of asphaltenes than do medium-API
API gravity
The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water. If its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks...
oils or light oil
Light crude oil
Light crude oil is liquid petroleum that has a low density and flows freely at room temperature. It has a low viscosity, low specific gravity and high API gravity due to the presence of a high proportion of light hydrocarbon fractions. It generally has a low wax content...
s. Condensates are virtually devoid of asphaltenes.
Production problems
They are of particular interest to the petroleum industry because of their depositional effect in production equipment, such as tubulars in oil wells. In addition, asphaltenes impart high viscosity to crude oils, negatively impacting production. The variable asphaltene concentration in crude oils within individual reservoirs creates a myriad of production problems.Asphaltene removal
Chemical treatments for removing asphaltene include:- solvents
- dispersant/ 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.
External links
- Article regarding asphaltene fouling by Irwin A. Wiehe
- Asphaltene Aggregation from Crude Oils and Model Systems Studied by High-Pressure NIR Spectroscopy (Source : American Chemical SocietyAmerican Chemical SocietyThe American Chemical Society is a scientific society based in the United States that supports scientific inquiry in the field of chemistry. Founded in 1876 at New York University, the ACS currently has more than 161,000 members at all degree-levels and in all fields of chemistry, chemical...
)