Classification of Crude Oil and Natural Gas

Crude oil is a mixture of hydrocarbons with some non-hydrocarbons.  Different areas of the world produce crude with different proportions of the five main hydrocarbon series. The character of the crude oil is determined by the proportion of paraffinic components relative to cyclo-paraffinic components.

Hence there are:

  • light crudes (large proportion of C5-C8 hydrocarbons),
  • Heavy crudes (naphthenic hydrocarbons),
  • Waxy crudes (very long chain paraffins).

Different crude have different proportions of each of the hydrocarbons and therefore different cuts or products can be expected with different specification when the crude is processed. (Hence, this would also lead to different in pricing and export duty payable for the said crude oil, determined by host government or it’s appointed regulators per crude oil name)

Paraffinic & Napthenic Crude

  • Paraffinic crudes are normally fairly low specific gravity crudes and are often light (like straw) in colour. They can easily and quickly be refined.
  • Naphthenic crudes generally have a higher specific gravity than paraffinic crudes and are darker (like tar, brown to black) in colour. Naphthenic crudes are not as easily fractionated as paraffinic crudes and as such give a greater percentage of residues.

Crude Distillation

The initial product range from a crude distillation unit is based on the difference in boiling points of the products.  Crude oil has a very low initial boiling point with a final boiling point that could exceed one thousand degree Fahrenheit.  Crude distillation results in products that have broad ranges in their boiling points.

Impurities

Impurities in the crude are also an important factor.  Most crudes contain some sulphur and acid gases that are toxic and corrosive, making refining more difficult and expensive.

Natural gas is a mixture of gases taken from a reservoir, or, gas which is removed from produced crude (solution gas).  The main constituent of natural gas is methane with various lesser proportions of ethane, propane, butane, pentane, hexane and heptane.  Hydrocarbons from other series such as Alkenes, Benzenes and Naphthenes may also be present as traces.

Methane and ethane cannot be condensed at the pressures and temperatures which are experienced at the wellhead and will always appear as gases.  Impurities often occur in natural gas and in some instances, in large quantities.

The main impurities are hydrogen sulphide, carbon dioxide, nitrogen and Helium.

Water is associated with the production of both crude oil and natural gas.

Aliphatic Hydrocarbons

Aliphatic hydrocarbons are acyclic and alicyclic hydrocarbon compounds found naturally in, or derived from crude oil and natural gas.

 Acyclic compounds form open chains and the most common forms in natural gas are:

  1. methane
  2. ethane
  3. propane
  4. butane
  5. pentane
  6. hexane

Alicyclic compounds are those compounds whose atoms are arranged in a single ring without double or triple bonds.  These are heavier components and are not normally associated with natural gas.

Condensate

Condensate is a very light hydrocarbon fraction in a liquid form and is associated with natural gas production but is not produced from all gas reservoirs.

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Categories: Oil & Gas

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