I.

Introduction

Natural Gas, flammable gaseous mixture consisting mostly of hydrocarbons (chemical compounds that contain carbon and hydrogen). Along with coal and petroleum, natural gas is a fossil fuel. Natural gas may contain as much as 85 percent methane (CH₄) and about 10 percent ethane (C₂H₆), and also contains smaller amounts of propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂), and other alkanes. Natural gas, which is usually found together with petroleum deposits in Earth’s crust, is extracted and refined into fuels that provide approximately 25 percent of the world energy supply.

Natural gas contains small amounts of impurities, including carbon dioxide (CO₂), hydrogen sulfide (H₂S), and nitrogen (N₂). Because these impurities can detract from the heating value and properties of natural gas, they are often removed during the refining process and used as commercial by-products.

II.

Uses

Natural gas is used both as a fuel and as a raw material in the manufacture of chemicals. As a residential fuel, it is burned in furnaces, water heaters, cooking stoves, and clothes dryers. As an industrial fuel, it is burned in kilns (special furnaces) used to bake bricks and ceramic tiles and to produce cement. Natural gas is also used for generating steam in water boilers and as a source of heat in glass making and food processing.

Natural gas serves as a raw material for creating petrochemicals, which are chemicals that are specifically derived from natural gas or petroleum. In turn, petrochemicals are used as a base product for making fertilizers, detergents, pharmaceuticals, plastics, and numerous other goods.

III.

History

Humans have used natural gas for centuries. Historical records show that natural gas was burned as fuel in China as early as ad 250. In the 17th century, natural gas was used for heating and lighting in northern Italy. In the United States, it was first discovered in Fredonia, New York, in 1821.

As its name implies, natural gas emerges from the ground in gas form, which is difficult to transport and store. As a result, natural gas has historically been used in areas close to the gas reservoirs. As the petroleum industry grew in the 19th and 20th centuries, natural gas that was recovered with petroleum from underground reservoirs was typically treated as a waste by-product and was often burned off at the well. Today, natural gas is transported through extensive networks of pipelines or is liquefied and transported by ship.

IV.

Formation

Natural gas is formed from plankton, water-dwelling microorganisms that include algae and protozoans. As these microorganisms died and accumulated on the ocean floors, they were slowly buried and the remains were compressed under layers of sediment. Over millions of years, the pressure and heat exerted by the overlying sediments chemically transformed this organic material into natural gas.

Because petroleum and natural gas are formed by similar natural processes, these two hydrocarbons are often found together in underground reservoirs. After gradually forming in Earth’s crust, petroleum and natural gas slowly flow into the tiny holes of nearby porous rocks that serve as reservoirs. Because this porous reservoir rock is often filled with water, the petroleum and natural gas, both of which are lighter than water and less dense than the surrounding rock, migrate upward through the crust, sometimes for long distances. Eventually, some of these upward-moving hydrocarbons become trapped by an impermeable (nonporous) layer of rock, known as the cap rock. Natural gas is lighter than petroleum, so it forms a layer over the petroleum. This layer is called a gas cap.

Coal beds contain appreciable quantities of methane, the principal component of natural gas. In coal deposits, methane is often dispersed throughout the pores and fractures of the coal bed. This type of natural gas is often referred to as coal-bed methane.

V.

Occurrence

Natural gas has been discovered on all continents except Antarctica. World natural gas reserves total approximately 150 trillion cu m (5.3 quadrillion cu ft). The world's largest natural gas reserves, totaling 48 trillion cu m (1.7 quadrillion cu ft), are located in Russia. The second-largest reserves, 50 trillion cu m (1.9 quadrillion cu ft), are found in the Middle East. Vast deposits are also located in other parts of Asia, in Africa, and in Australia.

Natural gas reserves in the United States total 5 trillion cu m (177 trillion cu ft). By rank according to total reserves in each state, large natural gas fields have been discovered in Texas, in the Gulf of Mexico off the coast of Louisiana, in Oklahoma, in New Mexico, in Wyoming, and in the Prudhoe Bay field of the North Slope of Alaska. In Canada, natural gas reserves total 1.7 trillion cu m (61 trillion cu ft). Most of Canada’s natural gas is in Alberta.

VI.

Classification

Natural gas is an odorless gas that is classified according to its composition. Dry gas has a very high methane content, while wet gas contains considerable amounts of hydrocarbons of higher molecular weight known as alkanes, which include ethane, propane, and butane. Residue gas is the gas remaining (mostly methane) after the alkanes have been extracted from wet gas. Sour gas contains high concentrations of hydrogen sulfide (a colorless, poisonous gas with the odor of rotten eggs).

The nonhydrocarbon constituents of natural gas are classified as diluents and contaminants. Diluents include gases such as nitrogen, carbon dioxide, and water vapor. Contaminants include hydrogen sulfide and other sulfur compounds. The diluents are noncombustible gases that reduce the heating value of the gas. The contaminants can damage production and transportation equipment. If burned, the contaminants can also cause environmental problems such as air pollution and acid rain. Acid rain forms when sulfur compounds in natural gas and other fossil fuels, such as coal, are burned and react with atmospheric moisture to form sulfuric acid (H2SO4). This acidic moisture falls to the earth as precipitation that can damage crops and forests, as well as lakes, streams, and rivers.

VII.

Extraction

To locate deposits of natural gas, exploration geologists search for geologic regions containing the ingredients necessary for the formation of natural gas: organic-rich source rock, burial conditions sufficiently severe to generate natural gas from organic material, and rock formations that can trap hydrocarbons.

When a geologic formation that may hold natural gas is identified, usually but not always in a sedimentary basin, wells are drilled into the formation. If a well goes into porous rock containing a significant reserve of natural gas, pressure within the porous rock may force the natural gas up to the surface. Typically, the pressure eventually declines until the natural gas must be pumped to the surface.

VIII.

Processing

Once natural gas has been extracted from the ground, it is usually transported by pipeline to a refinery, where it is processed.

Natural gas is processed in an extraction unit to remove the nonhydrocarbon compounds, especially hydrogen sulfide and carbon dioxide. Two processes used for this purpose are absorption and adsorption.

Absorption uses a liquid that absorbs the natural gas and impurities and disperses them throughout its volume. In a process known as chemisorption, the impurities react with the absorbing liquid. The natural gas can then be stripped from the absorbent, while the impurities remain in the liquid. Common absorbing liquids are water, aqueous amine solutions, and sodium carbonate.

Adsorption is a process that concentrates the natural gas on the surface of a solid or a liquid in order to remove impurities. A substance commonly used for this purpose is carbon, which has a large surface area per unit mass. For example, sulfur compounds in natural gas collect on a carbon adsorbing surface. The sulfur compounds are then combined with hydrogen and oxygen to form sulfuric acid (H₂SO₄), which can be removed.

After the impurities have been removed in the extraction unit, the natural gas is transported to a processing plant, where compounds such as ethane, propane, butane, and other substances are separated and removed for different uses. For example, ethane, propane, and butane are used extensively in the petrochemical industry.

IX.

Transportation and Storage

After being processed, natural gas is transported through pipelines to communities and other markets. As gas moves through a pipeline, the friction of the gas on the pipe walls slows the gas flow. As a result, compression stations placed along the route keep the pipeline sufficiently pressurized to keep the gas moving.

Once natural gas reaches its market, gas companies often store it so that enough remains available for peak customer demand. During cold weather, for example, the amount of natural gas used for heating often exceeds the amount of gas that can be piped from processing plants. As a result, gas companies store natural gas in large storage tanks (each with a capacity of up to 50,000 cu m/1,766,000 cu ft) or underground in porous rock formations. In many cases the underground storage areas are abandoned coal mines or empty oil wells. When gas stored underground is needed, it is pumped back to the surface.

Natural gas can also be shipped and stored as a liquid. It turns into a liquid at -160° C (-256° F). Natural gas occupies 600 times less volume as a liquid than as a gas. Liquefied natural gas is transported primarily by ship.