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Natural gas, crude oil, and coal are collectively known as hydrocarbons. Also called petroleum compounds, hydrocarbons are made up of the elements hydrogen and carbon, plus impurities. A wide variety of distinctly different hydrocarbon compounds, each with a different proportion of these two main elements, is encompassed within the general terms natural gas and crude oil. The lower the number of carbon molecules, the lighter the compound, and the more likely the hydrocarbon will be found in the gaseous phase. Crude oils contain longer chains of carbon molecules and are heavier than gas; they are more likely to be found in liquid phase. Coal is usually found in the solid phase and contains even longer chains of carbon molecules. As a strict definition, natural gas consists of hydrocarbons that remain in the gas phase (not condensable into liquids) at 20°C and atmospheric pressure, conditions considered to be standard temperature and pressure (STP). This effectively limits the definition to components with four or fewer carbon molecules: methane (C1H4), ethane (C2H6), propane (C3H8), and butane (C4H10). Hydrocarbons with more carbon molecules are liquid at standard conditions but may exist in gaseous phase in the reservoir. A more practical definition of natural gas (see figure below) includes the C5+ components that are produced with natural gas. Pentane (C5H12) begins the series that includes condensates. Natural gas definitions do not include components heavier than hexadecane (C16H34) that are produced and found as liquid or solid waxy compounds. These may be considered compounds in the crude oil family.
Methane is the main component of natural gas, usually accounting for 70%–90% of the total volume produced. If gas contains more than 95% methane, it is sometimes termed dry or lean gas, and it will produce few, if any, liquids when brought to the surface. Gas containing less than 95% methane and more than 5% of heavier hydrocarbon molecules (ethane, propane, and butane) is sometimes called rich gas or wet gas. This gas usually produces hydrocarbon liquids during production. Methane is the most common component transported by pipelines and converted to liquefied natural gas (LNG). LNG is the liquid product produced by cooling methane to –161.5°C. Methane may also be converted to liquid fuels through gas-to-liquids (GTL) processes. Methane is the main component of natural gas that power stations and industrial and residential users consume. Liquefied petroleum gas (LPG) refers specifically to propane and butane when they are stored, transported, and marketed in pressurized containers. Natural gas liquids (NGLs) include components that remain gaseous at both reservoir and surface conditions. These include ethane, propane, and butane, and components that exist with the gas in the reservoir but become liquid on the surface, such as condensates and natural gasoline. Condensates are low-density liquid mixtures of pentanes and other heavier hydrocarbons. Natural gas can also contain nonhydrocarbon components such as carbon dioxide (CO2), hydrogen sulfide (H2S), hydrogen, nitrogen, helium, and argon. All of these impurities, especially the first two, CO2 and H2S, must be removed from the natural gas stream prior to sale. Gases with high levels of H2S are also called sour gas, referring to the sour smell of sulfur. Conversely, gases with low levels of H2S are termed sweet gases and can be directly sold to consumers. Sour gases usually require treatment to remove sulfur prior to sale. |
