Another important alkane source is natural gas, which is mostly methane. There are also significant biological sources of methane that could someday be exploited commercially. For example, methane is produced by the action of certain anaerobic bacteria (bacteria that function without oxygen) on decaying organic matter. During the immature or biological stage of petroleum formation, biogenic methane (often called marsh gas) is produced due to the decomposition of organic material by the action of anaerobic microbes. These microorganisms cannot tolerate traces of oxygen and are also inhibited by high concentrations of dissolved sulfate.
Consequently, biogenic gas generation is confined to specific environments that include poorly drained swamps and bays, some lake bottoms, and marine environments beneath the zone of active sulfate reduction. Gas of predominantly biogenic origin is thought to constitute more than 20 percent of the world’s gas reserves. It is conceivable that the biological production of methane could become a source of natural gas in the future.
Natural gas is frequently found with or near accumulations of oil because the same geologic conditions are favourable for generating both gaseous and liquid hydrocarbons. Most of the constituents of natural gas come from organic matter (the remains of land and aquatic plants) in sediments that have been transformed through geologic processes over extremely long periods. The gas that eventually resulted from these natural processes has become entrapped in reservoirs distributed in the Earth’s strata. Natural gas is often found in close association with crude oil. In fact, in many instances, the pressure of natural gas exerted upon the subterranean oil reservoir provides the drive to force oil up to the surface. Such “associated” gas is often considered to be the gaseous phase of crude oil and usually contains some light liquids-hence the term wet gas.
However, there are also instances of “dry gas reservoirs that are not connected with any known source of liquid Petroleum.
Natural gas is the term applied to the large quantities of gas associated with or unassociated with liquid Petroleum. Natural gas is often dissolved in oil at the high pressures existing in a reservoir, and it can also be present as a gas cap above the oil. Such natural gas is known as associated gas. Some reservoirs contain gas and no oil. This gas is termed nonassociated gas.
The first discoveries of natural gas seeps were made in Iran between 6000 and 2000 BC. Many early writers described the natural petroleum seeps in the Middle East, especially in the Baku region of what is now Azerbaijan. The gas seeps, probably first ignited by lightning, provided the fuel for the “eternal fires” of the fire-worshiping religion of the ancient Persians. The use of natural gas was mentioned in China in about 900 BC. In China in 211 BC, the first known well was drilled for natural gas to reported depths of 150 metres (500 feet). The Chinese drilled their wells with bamboo poles and primitive percussion bits to search for gas in Late Triassic limestones (more than 208,000,000 years old) in an anticline west of modern Chungking. The gas was burned to dry the rock salt found interbedded in the limestone. Eventually, wells were drilled to depths approaching 1,000 metres, and more than 1,100 wells had been drilled into the anticline by 1900.
Natural gas was unknown in Europe until its discovery in England in 1659, and even then, it did not come into wide use. Instead, gas obtained from carbonized coal (known as town gas) became the primary fuel for illuminating streets and houses throughout Europe from 1790 onwards. In North America, the first commercial application of Petroleum products was utilizing natural gas from a shallow well in Fredonia, New York, in 1821. The gas was distributed through a small-bore lead pipe to consumers for lighting and cooking.
The composition of natural gas varies with its source. When a well is drilled through the protective cap, the gas is released and gushes forth, carrying some of the more volatile liquids (low-boiling Petroleum). The gas from gas deposits flows to the surface of the Earth through natural outlets (say springs). It is also found dissolved in natural Petroleum and is given out when the oil comes to the surface. Some wells produce nothing else but gas. In the 1950s, natural gas replaced coal as the primary energy source for domestic and industrial heating in many parts of the United States. Natural gas components are mostly saturated light paraffins such as methane, ethane, and propane that exist in the gaseous phase, depending on the pressure in the reservoir. When pentane and heavier compounds coexist, they are usually found as liquids. Often natural gases contain substantial quantities of hydrogen sulfide or other organic sulfur compounds.
When a natural gas reserve contains substantial amounts of ethane and the higher paraffinic compounds, these are usually extracted at the production site and produced as natural gas liquids (NGL). The NGLS can be separated into fractions, ranging from the heaviest condensates (butanes, pentanes, and hexanes) through LPG (essentially propane and butane) to ethane. This source of light hydrocarbons is especially prominent in the United States, where natural gas processing provides a significant portion of the ethane feedstock for olefin manufacture and the LPG for heating and commercial purposes.
The gas emerging from gas fields is a mixture of mostly methane with small amounts of ethane, propane, butane, nitrogen and carbon dioxide. On Earth, methane is the major component of natural gas. The tentative composition of a typical sample of natural gas is as follows:
| CH4 | C₂H6, C3H8, C4H10 | CO2 | N2 |
|---|---|---|---|
| 85% | 13% | 0.48% | 1.92% |
Methane is a significant component of the atmospheres of Jupiter, Saturn, Uranus and Neptune. Recently, methane has also been detected in interstellar space far from the Earth (106 km) in a celestial body that emits radio waves in the constellation Orion.
The proportion of ingredients of the gas varies with the source and consists chiefly of six alkanes, the percentage of each decreasing with increasing molecular weight. C₂-C4 gases are present in 20% in the petroleum field. Propane and higher alkanes may be absorbed in high boiling oil at ambient temperature and purified by low-temperature fractional distillation. Ethane may be absorbed similarly at -50°C. H₂S and CO₂ are scrubbed out with aqueous mono- or diethanolamine, and water is removed with hygroscopic diethylene glycol.
Several nonhydrocarbon gases also are found in natural gas mixtures. Nitrogen and carbon dioxide are noncombustible and may be found in substantial proportions. Nitrogen is inert, but if present in significant amounts, it reduces the heating value of the mixture; it must therefore be removed before the gas is suitable for the commercial market. Carbon dioxide is removed to raise the heating value, reduce volume, and sustain even combustion properties. Hydrogen sulfide is generally removed by treatment with ethanol amine in a process similar to that used in petroleum refining.
Commercial natural gas stripped of NGL and sold for heating purposes usually contains 85 to 90 percent methane and the remainder mainly nitrogen and ethane. It usually has a heating value of approximately 40 mega joules per cubic metre (about 9,300 kilocalories per cubic metre, or about 1,050 British thermal units per standard cubic foot of gas). While sulfur compounds are removed in processing, a minute quantity of a noxious mercaptan odorant is always added to commercial natural gas to ensure the rapid detection of any leakage that may occur in transport or use.
LNG is produced by cooling natural gas below its boiling point, -162°C (-259° F), and is stored in double-walled cryogenic containers at or slightly above atmospheric pressure. Liquefied natural gas occupies only about 1/600 of the volume of the gas. It is a colourless, highly inflammable gaseous hydrocarbon consisting primarily of methane and ethane. It is a type of Petroleum commonly associated with crude oil and can be converted back to its gaseous form by simply raising the temperature. LNG is more practical than liquefied petroleum gas or other liquid gases, particularly for use in large volumes, because it has the same chemical composition as natural gas. This fact and the growing demand for natural gas have stimulated LNG production. Moreover LNG technology has made it possible to utilize natural gas from remote areas of the world where it previously had no commercial use and was flared (burned). Special tankers transport LNG from such countries as Algeria, Borneo, and Indonesia to markets in Europe, Japan, and the United States.
Besides methane and ethane, two volatile hydrocarbons gaseous under normal atmospheric conditions at the surface of a well, natural gas may also contain heavier hydrocarbons such as propane, butane, pentane, and hexane, some of which condense at the surface and hence are known as natural-gas liquids. Other frequently occurring along with the gaseous hydrocarbons include carbon dioxide, hydrogen, hydrogen sulfide, nitrogen, helium, and argon.
In the early days, natural gas was considered a hazard in the petroleum industry as it formed explosive mixtures with air, was taken away from the source and burnt. The production and utilization of gas started at the beginning of the 20th century in the USA. It was not before 1950 that USSR and other countries did so. In India, the gas industry is in its infancy and is largely derived from the cracking units of petroleum refineries and the gas fields at Cambay (Gujarat). As compared to 172 million cubic meters of gas produced in 1961, India produced 9855 million cubic meters in 1980. In petroleum countries like USA and USSR, the gas is stored in large steel reservoirs and distributed through pipelines as fuel for domestic stoves or boilers. Liquefied natural gas is now being transported from North Africa to France and Great Britain by tankers. In Britain, the ‘coal gas’ supply for domestic use has been almost replaced by the ‘oil gas’ supply.
A liquid mixture of propane and butane can be obtained from natural gas or refinery gases at room temperature under a pressure of 3-5 atmospheres. This is stored and distributed in 40-100-litre capacity steel cylinders, which can be directly connected to domestic gas stoves. The fuel gases themselves are odourless, but to warn against leakage, a very small amount of foul-smelling substance is added. In India, bottled gas’ is being made available for domestic consumption in large towns under the trade names ‘Indane’ and ‘Burshane.’ Using liquid gas in this way obviates the need for gas pipes, which would be both costly and cumbersome. In the USA and USSR, natural gas is the starting material for making valuable ‘Petrochemicals.’ Thus natural gas provides the chemical industry with methane primarily for conversion to synthesis gas. Ethane and propane are for steam cracking to olefins, and butane is oxidized to maleic anhydride and isomerizes to isobutene.
Although natural gas has been used since ancient times, it did not become an essential energy source until the 1930s, when improved pipeline technology allowed natural gas to compete with town gas, produced from coal, in the United States. Since that time, natural gas has been exploited increasingly as a residential and industrial fuel and as a chemical feedstock, particularly since the oil crises of the 1970s. Discoveries of major natural-gas fields in Western Europe, Russia, North Africa and the Middle East have contributed to this trend. The average production of natural gas liquids in the United States is nearly 38 barrels per 1,000,000 cubic feet of produced gas.
