Page 32 - Handbook of Natural Gas Transmission and Processing Principles and Practices

1.4. Natural Gas Composition and Classification
Natural gas is a complex mixture of hydrocarbon and nonhydrocarbon constituents and exists as a
gas under atmospheric conditions. Virtually hundreds of different compounds may be present in
natural gas in varying amounts. Even two wells producing from the same reservoir may produce
gases of different composition as the reservoir is depleted.
While natural gas is formed primarily of methane (CH ), it can also include significant quantities
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of ethane (C H ), propane (C H ), butane (C H ), and pentane (C H ) as well as traces of hexane
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(C H ) and heavier hydrocarbons. Many natural gases often contain nitrogen (N ), carbon dioxide
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(CO ), hydrogen sulfide (H S), and other sulfur components such as mercaptans (R–SH), carbonyl
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sulfide (COS), and carbon disulfide (CS ). Trace quantities of argon, hydrogen, and helium may also
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be present. Trace quantities of metallic substances are known to exist in natural gases including
arsenic, selenium, mercury, and uranium.
According to the proportion of hydrocarbons heavier than methane, different types of natural gas
(dry, wet, and condensate) can be considered. Natural gas is considered “dry” when it is almost
pure methane, having had most of the other commonly associated hydrocarbons removed. When
other hydrocarbons are present, the natural gas is “wet,” where it forms a liquid phase during
production under surface conditions. ‘Condensate’ gases have a high content of hydrocarbon
liquids and form a liquid phase in the reservoir during production, during the depletion process.
Natural gases are commonly classified according to their liquid contents as either lean or rich and
according to the sulfur content as either sweet or sour.
The lean and rich terms refer to the amount of potentially recoverable liquids. The term usually
applies to ethane and heavier components but sometimes applies instead to propane and heavier
components (if ethane is not regarded as a valuable liquid component). To quantify the liquid
contents of a natural gas mixture, the industry uses GPM, or gallons of liquids recoverable per 1000
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standard cubic feet (Mscf ) of gas. Lean natural gas has a liquid content less than 2 GPM.
Moderately rich natural gas has between 2 and 5 GPM, and very rich natural gas has greater than
5 GPM (Ewan et al., 1975).
The sweet and sour terms refer to the H S content. Strictly speaking, “sweet” and “sour” refer to
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both acid gases (H S and CO ) but are usually applied to H S alone. A sweet gas contains negligible
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amounts of H S, whereas a sour gas has unacceptable quantities of H S. The terms are relative, but
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generally, sweet means the gas contains less than 4 ppmv of H S. Carbon dioxide can be tolerated
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to much higher levels, say 3–4 mol%, as long as the heating value of the sales gas is satisfactory.

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