Lean gases burn with a low air-to-gas ratio and display a colorless to blue or yellow flame, whereas rich gases require comparatively higher amounts of air for combustion and burn with an orange flame. In addition, operators may intentionally add odorants, tracers (such as helium), or other components.ĭry, or lean, natural gas systems have high concentrations of the lighter hydrocarbons (methane and ethane), while wet, or rich, gas systems have higher concentrations of the intermediate-weight hydrocarbons. These constituents may occur naturally in gas reservoirs, or they may enter the system as contaminants during production, processing, and transportation. Natural gas systems can also contain non-hydrocarbon constituents, including hydrogen sulfide (H2S), carbon dioxide (CO2), nitrogen (N2), and water vapor. These components are commonly referred to as pentanes-plus, condensate, natural gasoline, and natural gas liquids (NGL). The intermediate-weight hydrocarbons (pentane through decane) exist as volatile liquids at atmospheric conditions. These are the primary components of liquefied petroleum gas, or LPG. Propane (C3H8), butane (n-C4H10 and i-C4H10), and heavier hydrocarbons may be extracted from the gas system and liquefied for transportation and storage.
Methane and ethane exist as gases at atmospheric conditions.
Natural gas systems are composed primarily of the lighter alkane series of hydrocarbons, with methane (CH4) and ethane (C2H6) comprising 80% to 90% of the volume of a typical mixture. Phase diagrams illustrate the phase that a particular substance will take under specified conditions of pressure, temperature, and volume.