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NATURAL GAS Source: Library of Congress Natural gas well, Pleasantville, PA in 1907
The history of natural gas		What is natural gas?			How is natural gas formed?				What are methane hydrates?

Energy Use at a Glance:

In 2006, the U.S. consumed about 22 trillion cubic feet of natural gas (21% of the world total), compared to 105 trillion cubic feet worldwide. The U.S. produced about 18 trillion cubic feet compared to about 105 trillion cubic feet worldwide. About 22% of the energy used in the U.S. comes from natural gas. There are about 300,000 miles of natural gas pipelines in the lower 48 states and 425,000 gas producing wells. For more statistics, see http://www.eia.doe.gov/basics/naturalgas_basics.html

The history of natural gas

Early human civilizations, while not understanding its cause, were certainly influenced by natural gas. In what was deemed a mysterious and otherworldly phenomenon, lightning is known to have ignited natural gas seeping from the earth’s surface, where a continuous flame would persist. The most famous of these occurred near 1,000 B.C., on Mt. Parnassus where…

Near 500 B.C. the Chinese finally discovered its potential use. Seeping gas was corralled into bamboo pipelines, and the transported gas was used to boil sea water, thus separating out the salt and producing drinking water. They may have also drilled wells using bamboo poles and primitive bits.

Natural gas was first commercialized in Britain in 1785, where gas produced from coal was used in gas lamps. In modern times, the first well drilled intentionally to obtain natural gas was drilled in 1821 in Fredonia, N.Y. It was 27 feet deep, and was drilled by William A. Hart to get a larger flow of gas than was currently seeping out of the ground. For more history, see http://www.naturalgas.org/overview/history.asp.

In the recent past, natural gas was recovered during the process of extracting petroleum and could not be sold profitably and so was simply burned at the well head (“flaring”). It was dismissed as a useless byproduct of oil production until the second half of the 20^th century. Its value now realized, gas is typically re-injected to the petroleum formation, aiding in petroleum recovery and making it available for future use. Natural gas now supplies 23% of all energy produced in the world.

What is natural gas?

Natural gas is principally methane, CH₄, with some ethane (C₂H₆) and propane (C₃H₈), and impurities such as CO₂, H₂S, and N₂. Natural gas is odorless and colorless; the slightly sour smell that we associate with the gas coming from a stovetop is due to an odorization process (for safety and leak detection) which adds mercaptan compounds to the end-use gas (more precisely, odorization compounds are mixtures of t-butyl mercaptan, isopropyl mercaptan, tetrahydrothiophene, dimethyl sulfide and other sulfur compounds).

· Dry natural gas refers to a purified product that is almost entirely methane.

· Wet natural gas contains compounds other than methane and ethane.

· Sour natural gas contains larger amounts of hydrogen sulphide, which is highly undesirable due to corrosion, and results in SO₂ formation upon combustion.

When we say that natural gas is combustible, we are really just saying that it can be oxidized in a way that releases energy. The simplest form of combustion is for methane, which reacts with oxygen to form carbon dioxide, water, and energy by this reaction:

This is what happens when methane “burns”.

How is natural gas formed?

As a fossil fuel, natural gas is formed from the decaying remains of pre-historic plant and animal life. As with petroleum, most natural gas formation is due to the breakdown of prehistoric marine zooplankton. Zooplankton subsist on a diet of phytoplankton, which, in turn, rely upon the energy of the sun to produce organic matter and energy through photosynthesis. Photosynthesis and the formation of organic matter are described in more detail on the PETROLEUM page.

We can think of gas (or oil or coal for that matter) as organic material that is prevented from complete decay. Typically, it will be found at the top of petroleum reservoirs, where it has been formed by the combined action of methanogenic bacteria (they produce methane while they decompose organic material) and through catagenesis (the thermal decomposition of kerogen). As marine sediments are buried deep within the earth, high temperatures and pressures lead to varying degrees of the completion of catagenesis, which is the process that produces both petroleum and natural gas. Higher temperatures and pressures favor the formation of lighter hydrocarbons (natural gas), and so oil/gas formations that are deeper in the earth tend to have a greater ratio of gas to petroleum. A more detailed explanation of the formation of natural gas and petroleum can be found on the PETROLEUM page.

Following the production of natural gas through the processes of diagenesis and catagenesis, the newly formed natural gas will attempt to migrate to a new location. Because the earth is filled entirely by layers of solid (or at significant depths) molten rock, the gas it contains cannot exist within a self-contained “lake”, but must decide to live within the small fraction of space (or pores) that exist in these rocks. Like the sponge in your kitchen sink (albeit, less spongy and a bit heavier) certain kinds of rock (mainly sandstone and limestone) contain pores large enough and with enough connections to serve as storage and migration sites for gas or oil or water or any other fluid wishing to call them home. Because the light hydrocarbons that comprise natural gas are lighter than water and rock, those that exist within the earth will tend to migrate upwards until they reach the surface, or are trapped by an impermeable layer of rock

Methane is also produced by bacteria (methanogens) that decompose organic matter under anoxic conditions, referred to as biogenic methane. These microorganisms are active in the intestines of most animals, and are responsible for methane release from decomposing landfill waste. In the process of petroleum formation, methane may be formed in this manner during the early stages burial.

A large supply of methane is also present within coal seams, where it is found adsorbed to the structure of the coal; where it was formed by methanogenic bacteria during the decomposition process and also during catagenesis of the forming coal. “Coalbed methane” is found to lack the presence of heavier hydrocarbons (as in hydrocarbon reservoirs) and also sulfur compounds (thus the name “sweet gas”). This methane is in a near liquid state lining the pores of the coal, and is partially released when pressure in the reservoir declines due to the presence of a withdrawal well.

What are methane hydrates?

In the search for fuels that are alternative to petroleum and plentiful, a lot of recent talk has centered on what are called methane hydrates (also called methane clathrates or methane ice). Essentially, these are chunks of ice that are saturated with methane or natural gas (during formation, the gas becomes trapped in the structure of the freezing water). Because of their frozen structure, they are to be found in arctic permafrost regions or at the bottom of the ocean in places where conditions are favorable.

As with natural gas, methane hydrates can be formed through biogenic or thermogenic processes:

In biogenic formation, the methane is produced by biological activity as microorganisms attempt to decompose the remains of marine life (as above, primarily marine phytoplankton and zooplankton). In this case, methane is produced by the anoxic behaviors of methanogenic bacteria.

In thermogenic formation, the gas is formed in the same manner as natural gas…through catagenesis of kerogen. In fact, this may be the same natural gas that was formed above, it just migrates to a region (remember, gas is lighter than earth materials and wants to reach the surface) where the formation of hydrates is favorable.

Through either method of formation (biogenic or thermogenic) the gases, once formed, are thought to migrate (perhaps through geologic faults) and, upon contact with cold sea water, to crystallize into ice-like structures.

Historically, the U.S. has been able to produce roughly the same quantity of natural gas that it needs to supply energy to the country (about 23% of countrywide energy use). However, estimates project an increase of 18% in the amount of natural gas that will be required in 2030. Therefore, it is becoming increasingly important to locate further sources of natural gas, and methane hydrates could fill this need. Hydrates are projected to exist in huge quantities (approximately the amount energy contained in all the world’s other fossil fuels combined…although, as we have yet to develop a successful extraction method, the fraction of this that is accessible remains an open question).

The difficulty arising in the utilization of methane hydrates owes primarily to the development of an efficient extraction process. Beneath the sea floor, hydrates are stable (at these pressures, the ice chunks will remain in their structures up to about 18 degrees Celsius), but once removed, the ice become unstable as pressures decrease and the gas develops a strong desire to escape. Methods for efficient extraction are currently in development.

Did you know?

CATA (Centre Area Transportation Authority) has been operating their city buses on compressed natural gas (CNG) since 1993. The fleet currently operates 44 buses and, in 1995, CATA, the Pennsylvania Department of Environmental Protection and Columbia Gas of Pennsylvania joined in a cooperative effort to build a CNG fueling station at CATA’s administrative facility. In 2001 more than 3.8 million riders used CATA buses through Penn State’s University Park campus. See http://www.catabus.com/accngprog.htm.