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Crude oil characterization
Crude oil characterization has long been an area of concern in refining; however, the need to identify the chemical nature of crude has gained importance in upstream operations. Traditionally, this has been done by simply stating the crude oil gravity, but more information is required to understand the oil well enough to estimate the volume in the reservoir and its recoverability.
Regional trends in crude oil composition
During the last 60 years, several correlations have been proposed for determining pressure-volume-temperature (PVT) properties. The most widely used correlations treat the oil and gas phases as a two-component system. Only the pressure, temperature, specific gravity, and relative amount of each component are used to characterize the oil’s PVT properties. Crude oil systems from various oil-producing regions of the world were used in the development of the correlations. These crude oils can exhibit regional trends in chemical composition, placing them into one of the following groups:
Because of the differences in composition, correlations developed from regional samples, predominantly of one chemical base, may not provide satisfactory results when applied to crude oils from other regions.
Classification of hydrocarbons based on structure
Hydrocarbons are classified according to the structure of the molecule. Paraffin hydrocarbons are characterized by open or straight chains joined by single bonds. Examples are:
Isomers of these compounds, which contain branched chains, are also included as paraffins. The first four members of the series are gaseous at room temperature and pressure. Compounds ranging from pentane (C5H12) through heptadecane (C17H36) are liquids, while the heavier members are colorless, wax-like solids. Unsaturated hydrocarbons, which consist of olefins, diolefins, and acetylenes, have double and triple bonds in the molecule. These compounds are highly reactive and are not normally present to any great extent in crude oil. Naphthene hydrocarbons are ringed molecules and are also called cycloparaffins. These compounds, like the paraffins, are saturated and very stable. They make up a second primary constituent of crude oil. Aromatic hydrocarbons are also cyclic but are derivatives of benzene. The rings are characterized by alternating double bonds and, in contrast to olefins, are quite stable, though not as stable as paraffins. Crude oils are complex mixtures of these hydrocarbons. Oils containing primarily paraffin hydrocarbons are called paraffin-based or paraffinic. Traditional examples are Pennsylvania grade crude oils. Naphthenic-based crudes contain a large percentage of cycloparaffins in the heavy components. Examples of this type of crude come from the US midcontinent region. Highly aromatic crudes are less common but are still found around the world. Crude oils tend to be a mixture of paraffins-naphthenes-aromatics, with paraffins and naphthenes the predominant species. Fig. 1, although not complete, shows a distribution of crude oil samples obtained worldwide. Geochemical analyses provided the crude’s chemical nature.
Fig. 1 – Chemical nature of crude oils found worldwide (after Reservoir Fluid Database).
Resins and asphaltenes
Resins and asphaltenes may also be present in crude oil. Resins and asphaltenes are the colored and black components found in oil and are made up of relatively high-molecular-weight, polar, polycyclic, aromatic ring compounds. Pure asphaltenes are nonvolatile, dry, solid, black powders, while resins are heavy liquids or sticky solids with the same volatility as similarly sized hydrocarbons. High-molecular-weight resins tend to be red in color, while lighter resins are less colored. Asphaltenes do not dissolve in crude oil but exist as a colloidal suspension. They are soluble in aromatic compounds such as xylene, but will precipitate in the presence of light paraffinic compounds such as pentane. Resins, on the other hand, are readily soluble in oil.
Full component characterization
No crude oil has ever been completely separated into its individual components, although many components can be identified. Table 2 lists the more important compounds in a sample of Oklahoma crude. A total of 141 compounds were identified in this oil sample that account for 44% of the total crude volume. Despite this complexity, several properties relevant to petroleum engineers can be determined from black oil PVT correlations.
The petroleum industry uses API gravity as the preferred gravity scale, which is related to specific gravity as
Whitson has suggested use of the Watson characterization factor as a means of further characterizing crude oils and components. In 1933, Watson and Nelson introduced a ratio between the mean average boiling point and specific gravity that could be used to indicate the chemical nature of hydrocarbon fractions and, therefore, could be used as a correlative factor. Characterization factors are calculated with
Use of characterization factor
Characterization factors are useful because they remain reasonably constant for chemically similar hydrocarbons. A characterization factor of 12.5 or greater indicates a hydrocarbon compound predominantly paraffinic in nature. Lower values of this factor indicate hydrocarbons with more naphthenic or aromatic components. Highly aromatic hydrocarbons exhibit values of 10.0 or less; therefore, the Watson characterization factor provides a means of determining the paraffinicity of a crude oil. Using work from Riazi and Daubert,  Whitson developed the following relationship in terms of molecular weight and specific gravity.
Table 1 provides values of Watson characterization factors for selected pure components classified as paraffins, naphthenes, or aromatics. The characterization factor values provide insight into their use.
Crude oils typically have characterization factors ranging from 11 to 12.5. Table 2 was derived from assay data available in the public domain. It samples crudes from around the world and can be used to provide insight into PVT behavior on a regional basis.
The properties of the heptanes-plus fraction in the stock tank crude oil are an additional source that can provide insight into the Watson characterization factor. It is important to account for the lighter paraffin components found in the oil to arrive at the characterization factor for the entire crude.
Relationship between crude oil gravity and characterization parameter
Fig. 1 depicts a relationship between crude oil gravity and characterization parameter. While not definitive, it can be observed that lower gravity crudes tend to be more naphthenic, while higher-gravity crudes tend to be more paraffinic.
|γo||=||oil specific gravity|
|γAPI||=||oil API gravity|
|Kw||=||Watson characterization factor, °R1/3|
|Tb||=||mean average boiling point temperature, T, °R|
|Mo||=||oil molecular weight, m, lbm/lbm mol|
- Burcik, E.J. 1957. Properties of Petroleum Reservoir Fluids, Chap. 1. New York: John Wiley & Sons.
- Pirson, S.J. 1977. Oil Reservoir Engineering, 303–304. Huntington, New York: Robert E. Krieger Publishing.
- GeoMark Research. 2003. RFDbase (Reservoir Fluid Database), http://www.RFDbase.com.
- Allen, T.O. and Roberts, A.P. 1982. Production Operations: Well Completions, Workover, and Stimulation, second edition, Vol. 2. Tulsa, Oklahoma: Oil and Gas Consultants International.
- McCain, W.D. Jr. 1990. The Properties of Petroleum Fluids, second edition. Tulsa, Oklahoma: PennWell Publishing Company.
- Whitson, C.H. 1983. Characterizing Hydrocarbon Plus Fractions. SPE J. 23 (4): 683-694. SPE-12233-PA. http://dx.doi.org/10.2118/12233-PA
- Watson, K.M. and Nelson, E.F. 1933. Improved Methods for Approximating Critical and Thermal Properties of Petroleum. Industrial and Engineering Chemistry 25 (1933): 880.
- Watson, K.M., Nelson, E.F., and Murphy, G.B. 1935. Characterization of Petroleum Fractions. Industrial and Engineering Chemistry 7 (1935): 1460–1464.
- Riazi, M.R. and Daubert, T.E. 1980. Simplify Property Predictions. Hydrocarb. Process. 59 (3): 115–116.
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