Natural Gas
From Thermal-FluidsPedia
The same factors that promote the formation of petroleum also help in the formation of natural gas. It is therefore expected that natural gas is found at locations near oil fields, often in association with petroleum (thus its name associated or dissolved gas). It is liberated when oil is brought to the surface in the same manner that carbon dioxide gas is liberated when someone opens a carbonated drink. If the gas diff uses through the pores of sedimentary rocks and accumulates in a reservoir other than the oil reservoir, the gas is called non-associated gas. Up to only a few decades ago, most natural gas discovered was flared off or reinjected into the ground, as it was considered to be of little use. Its use increased during the latter half of the twentieth century, as it proved to be a convenient and relatively clean fuel to be burned in gaseous burners for space heating and cooking (Figure 1).
Natural gas is primarily methane (CH4), but a certain percentage of heavier hydrocarbons such as ethane (C2H6), propane (C3H8), butane (C4H10), and a small amount of pentane (C5H12) and hexane (C6H14) are also present. At normal temperature and pressure, pentane and hexane are liquid, but in elevated temperatures and pressures underground, they become gas and fl ow out with other gases. Commercial natural gas is primarily a mixture of methane and ethane. Th e propane and butane are liquefied and sold separately as liquefied petroleum gas (LPG) or “bottled” gas. For a long time, many producer countries used to burn natural gas at the well. Th is was very wasteful and caused huge environmental problems. Because of its lower density, it costs about four times as much to transport natural gas through pipelines as it does for crude oil; these gases are liquefied and shipped to their destinations as liquefied natural gas (LNG) in huge cryogenic containers aboard large tankers and ocean liners.
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Reserves, and Resources
The total amount of available natural gas is shown in Table 1. Roughly two-third of all natural gas reserves lie in Russia and the Middle East (Table 2). Th e United States with 193 trillion cubic feet has a little more than 3% of the world’s natural gas reserves (1).
Because natural gas is relatively clean, its rate of consumption is increasing by an average of 2.8% annually, faster than other sources of fossil fuels (Figure 2). If consumption continues to grow at this rate, natural gas reserves remain available for another 75 years. However, this number is believed to be optimistic, as natural gas substitutes coal and petroleum in an effort to reduce the effect of greenhouse gases (2).
Besides wells, however, there are other sources of natural gas such as hydrates that if they could be extracted, would provide energy for many hundreds of years. Hydrates are icy deposits of crystallized natural gas and water, buried under the extreme pressures and cold temperatures of the deep oceans and Arctic permafrost that have been formed by the disintegration of certain bacteria. It is estimated that gigantic hydrate fields around the world contain twice the energy of all other forms of fossil fuels combined. These reserves are virtually untapped, as there are major obstacles that have to be overcome before the reserves can be accessed. Any major changes in temperature or pressure may be capable of breaking down the material, releasing huge amounts of methane – a very potent greenhouse gas (See box “Th e Deadly Lake”) (a). The cost of extraction is prohibitively high and the natural gas, even if it is extracted, has to be transported thousands of miles from Arctic regions before it can be used. One solution is to convert it to liquid synthetic fuel, such as methanol, before it is sent through pipelines.
Converting natural gas to liquid first requires breaking its chemical bonds using steam, heat and a nickel-based catalyst to produce a mixture of carbon monoxide and hydrogen known as syngas. Th is process is called steam reforming. Syngas is then blown over various catalysts to transform it into gasoline, diesel, and other liquid hydrocarbons.
References
(1) Energy Information Agency Fact Sheet, http://www.eia.doe.gov.
(2) Meadows, D., Limits to Growth: 30-Year Update, Chelsea Green Publishing Company, 2002, p. 94.
(3) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) A few petroleum geologists have gone so far as to blame the loss of ships and airplanes in the Bermuda Triangle on sudden pulses of methane gas released from a hydrate layer below the Atlantic Ocean. As the ocean boils with a sudden squirt of methane bubbles, ships can be swallowed before having time to make distress calls, and plane engines might be choked by the rising plume of methane gas.
Further Reading
Berkowitz, N., Fossil Hydrocarbons: Chemistry and Technology, Elsevier Academic Press, 1997.
Deff eyes, K. S., Hubbert’s Peak: Th e Impending World Oil Shortage, Princeton University Press, Princeton, N. J., 2001.
Campbell, C. J., Th e Coming Oil Crisis, Multi-Science Publishing Company, 2004.
Tariq Ali, Th e Clash of Fundamentalisms: Crusades, Jihads and Modernity, Verso, 2002.
Pelletiere, S., Iraq and the International Oil System: Why America Went to War in the Gulf, Praeger Publishing, 2001.
Oil and Gas Journal, Technology, news, statistics, special reports, and analysis (http://ogj.pennnet.com).
Journal of Petroleum Technology, The official journal of Society of Petroleum Engineers, Dallas.
The Petroleum Engineer, Petroleum Engineer Pub. Co.
Journal of Petroleum Science and Engineering, Elsevier, covers the fields of petroleum (and natural gas) exploration, production and flow.
External Links
National Energy Technology Laboratory: Th e Strategic Center for Coal (http://www.netl.doe.gov/coal).
National Petroleum Technology Office (http://www.npto.doe.gov).
US Geological Survey (http://www.usgs.gov).
Organization of Petroleum Exporting Countries (OPEC) (http://www.opec.org).
Society of Petroleum Engineers (http://sae.org).