Crude oil characterization has long been an area

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 twocomponent 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: Paraffinic Napthenic Aromatic 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: Methane Ethane Propane Decane

Fig. 1 – Chemical nature of crude oils found worldwide

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.