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Natural Gas: Background

What is Natural Gas?

coal
Molecules of methane react with oxygen during combustion and produce carbon
dioxide and water. Source: Wikipedia

Natural gas consists mainly of a single organic compound called methane. Methane is composed of one carbon and four hydrogen atoms (CH4). Natural gas is both colorless and odorless. Natural gas is a fossil fuel, like petroleum, and often occurs in the same reservoir. It is typically found underground in certain sedimentary rocks. As a source of energy, it has several important properties. It is readily combustible, gives off few emissions, and is fairly abundant in the United States. It is important to recognize that natural gas is different from the gasoline used in cars. It is also different from the gas (propane) used in barbecues and for heating. This gas contains an additive which gives it a "rotten eggs" smell so that leakages can be detected.

coal
Volatile gases are burned off from oil and
gas wells. Source: Wikipedia

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Uses of Natural Gas

Natural gas has a variety of residential, industrial, and transportation uses. It is also used to generate electricity. Common household uses include cooking, space heating, cooling, and drying. Using natural gas for heating and cooking allows for easy temperature control. For cooking, it costs about half as much to run a gas stove as an electric one. According to government statistics, more than half of the homes (62 million) in the United States are heated by natural gas. Today, gas furnaces can achieve efficiencies of 90 percent. This means that only 10 percent of the energy in the methane goes to waste. Another increasing popular use is residential, gas-powered air conditioners. According to the U.S. Department of Energy, natural gas costs approximately 68 percent less, per Btu (British thermal unit), than the cost of electricity. Some industries which rely on a constant flow of electricity without disruption, such as commercial refrigeration, use natural gas to fuel on-site electrical generators.

coal
Cooking fuel is a major use of natural gas.
Source: Wikipedia

Industry is the largest consumer of natural gas, and accounts for 43 percent of demand in the United States. Major industrial uses include heating, cooling, and cooking. Natural gas is also used for metal preheating, glass melting, waste treatment, and incineration. In addition, it is used as a base product for producing a number of chemical products, such as pharmaceuticals and fertilizers. Natural gas is used to make methanol which is used to synthesize commonly used industrial chemicals including formaldehyde, acetic acid and a compound called MTBE which enables gasoline to burn more cleanly.

Natural gas has been used in the transportation sector since the 1930's. According to the Natural Gas Vehicle Coalition, there are 5 million natural gas vehicles (NGV's) worldwide and 150,000 in the United States today. These vehicles use compressed natural gas or CNG. CNG is made by compressing natural gas to less than 1% of the volume it occupies at standard atmospheric pressure. The gas is stored in a tank in the vehicle and the engine operates in a manner similar to a normal combustion engine, but with some modifications. Because CNG is lighter than air, if the fuel tank ruptures, the gas rapidly dissipates. In this respect, CNG is safer than gasoline. Public transport, including buses and taxis, is a major user of natural gas. Some vehicles use liquid natural gas (LNG). While private vehicles that use CNG or LNG can be purchased, they lack popularity due to limitations in both range and refueling infrastructures. However, with increasingly stringent controls on emissions, the advantages of clean-burning natural gas are becoming more apparent.

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How is Natural Gas Formed?

Natural gas is a fossil fuel, like coal and oil. It is formed in a way similar to oil formation, where organic particles are covered in ocean sediments and mud on the ocean floor. Before the organic remains decompose, they are buried and cut off from oxygen which causes decomposition. Over hundreds of thousands of years, the organic matter becomes buried many kilometers below the surface. Here, it is compressed by the mass of overlying sediments, which exert a large amount of pressure. In addition, Earth's inner heat produces an average geothermal gradient of approximately 20° C per kilometer. At depths between 4 and 6 kilometers, the organic material begins to transform due to the heat and pressure. During this process, chemical bonds within the source material are broken. At the top of the hydrocarbon producing window, petroleum is formed. With further heating and transformation of petroleum, natural gas begins to form. For this reason, deeper deposits that have been "cooked" at higher temperatures (due to the geothermal gradient) normally contain more natural gas than petroleum. In shallower deposits, petroleum is the major component. The formation of natural gas by heating and pressurization of organic remains is called thermogenic natural gas.

Biogenic natural gas is different and forms when specialized bacteria called methagens break down organic matter and give off methane. These bacteria mostly live in oxygen-free environments near to the surface of the Earth. They also live in the digestive systems of many kinds of animals e.g. cows and humans. This kind of natural gas is lost to the atmosphere. However, where the bacteria exist below the surface, methane can build up. An example of this is the landfill gas (LFG), methane, formed by the decomposition of buried refuse. The Environmental Protection Agency is presently trying to promote the use of LFG as a fuel source and to minimize its leakage into the environment because it is a greenhouse gas.

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Which Geologic Materials Contain Natural Gas?

Natural Gas Reservoir
Source: USGS

Natural gas is stored or trapped under the ground in geologic structures called traps. Because natural gas forms from organic-rich marine sediments, it is often covered by great thicknesses of sedimentary rocks. Shales are common deep-marine sedimentary rocks associated with hydrocarbon reserves. Tiny spaces in between the grains in sedimentary rocks are called pores. Interconnected pores allow fluids and gases to pass through the rock. This important characteristic is called permeability. Natural gas has a relatively low density, even lower than petroleum, and tends to rise. Under special conditions, the source rocks are overlain by permeable sedimentary rocks that are capped or sealed beneath an impermeable layer through which fluids and gases cannot migrate. These conditions create a trap into which the hydrocarbons fill from below. The best traps also constrain the lateral movement of gas and are shaped like domes, or umbrellas. Domes of various shapes and sizes form naturally in sedimentary layers because pressures cause rocks to bend and buckle. Dome shaped folds in sedimentary layers are called anticlines. In places where stresses in the crust cause large fractures along which rock layers are displaced (called faults), impermeable rocks can be placed alongside and above permeable rocks to form traps.

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Discovering Natural Gas

Since the 1800's, geologists have recognized that certain geologic structures are more likely to contain natural gas than others. Anticlines, with their broad domes, are one example of known reservoir rocks. On land, geologists survey the surface and look for places where rock layers are exposed, such as in gorges. They try to build a picture of what is going on below the surface that allows natural gas to collect. Geologists also gather clues from rock chippings, taken when water wells or irrigation ditches are dug. These allow the geologist to learn about the rock types, and their porosities, beneath the surface. Only by looking at lots of locations and gathering many kinds of data about the rocks and structures in a region can a geologist develop a good understanding of the subsurface structures. Drilling exploratory wells is expensive. Only if there is a very good chance of finding natural gas is drilling used. Sometimes a well does not reach a deposit and is called a "dry well." Logging involves conducting tests on the rock in drill holes. Standard logging involves examining the physical properties of rock chips, such as porosity and fluid content, using powerful microscopes. Electrical logging is done by passing a current into the rocks and measuring the electrical resistance as an indicator of fluid content.

Another technique involves the measurement of minute differences in Earth's gravitational field that result from variations in mass from rocks beneath the surface. Natural gas and hydrocarbons are less dense than rocks and result in minute but measurable anomalies in the gravitational field.

diagram on how coal was formed
Seismic exploration involves the use of charges to
explore the ground with vibrations.
Source: Department of Energy

Geophysicists also conduct seismic tests to collect more subsurface data. During these tests, small seismic waves are artificially emitted into the ground, usually with a truck that creates strong vibrations. When waves strike geologic structures, such as bedding planes or faults, some parts of the waves are reflected back to the surface. Their timing and distribution are recorded by an array of recorders, called geophones, at the surface. The data is applied to a model to produce a picture of the structures as a cross section. Offshore exploration uses similar techniques. Seismic waves are released from a ship and an array of geophones is towed behind the ship to collect the reflected signals. Modern computers can compile the data into 2-, 3-, and even 4-Dimensional models. Visualization labs allow scientists to wear 3-D glasses to explore and move inside geological structures that are located kilometers below the surface. Working in 4-D allows "movies" or time-lapse images to be made of the actual structure and flow of hydrocarbons. These can increase the production from a well by a factor of 2-3 times.

Coal deposits are always interbedded with other sedimentary rocks, mainly sandstones and shales. This occurs because the environments in which sediments are deposited change with time. For example, an area that was a coal swamp may become buried by sand or mud from a nearby river system. Eventually the coal swamp reestablishes itself. Upon burial, the plant material is converted to coal, and the sand and mud form sandstone and shale beds. In this way, there is an alternation of other sedimentary rock types with the coal deposit.

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Drilling and Extraction of Natural Gas

Two main methods are used for onshore drilling for natural gas. Cable tool drilling involves dropping a heavy metal bit on a cable into the ground. This punches a hole into the Earth by fracturing the rocks in the drill hole. It is used for shallow wells, less than 200 meters. Rotary drilling uses a rotating sharpened metal drill bit to cut through the crust. This method is used for deeper wells where rocks are under greater pressure. Modern methods also include horizontal drilling, where lateral wells can extend for more than a kilometer beneath the surface. Such wells minimize the impact at the surface and allow for a large area of a reservoir to be extracted. Directional drilling methods employ a method called "hydraulic fracturing" where the permeability of rocks surrounding the drill hole is enhanced by the introduction of fluids.

diagram on coal production by regions
The growth in natural gas production in 2011
was the largest on record.
Source: DOE

Offshore drilling often requires reaching deposits in deep water. The drilling methods are essentially the same as those employed onshore. However, a platform is needed to house the drilling and extraction machinery and the people who run them. The drilling template is one of the most important components. It is shaped like a cookie cutter and connects the drilling platform with the wells on the sea floor. The template must be lowered to exactly the right location. To do this, GPS co-ordinates are used. It is then fixed to the sea floor and connected to the drilling platform with cables. The platform is allowed to shift slightly with the currents and winds. Another important component is the blowout preventer, which prevents gas and oil from escaping into the ocean. Above this is the marine riser, which extends downwards from the platform to the seafloor and houses the drill bit.

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Transporting Natural Gas to Refineries

After extraction, natural gas is transported to processing plants through a network of gathering pipelines. A gathering system can consist of thousands of kilometers of small diameter pipes in which gas flows at low pressure. The system can connect hundreds of wells in an area. According to the American Gas Association, there was an estimated 50,000 kilometers of gathering system pipelines in the U.S. in 1999.

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Processing Natural Gas

Raw natural gas that comes from underground is less pure than the methane used in homes. It occurs with other hydrocarbons including ethane, propane, butane, and pentanes. It also contains water vapor, hydrogen sulfide (H2S), carbon dioxide, helium, nitrogen, and other compounds. These substances must be separated to produce methane. The byproducts are called natural gas liquids or NGL's and can be separated in refineries by fractionation. They have valuable uses, such as enhancing oil recovery in oil wells, providing raw materials for oil refineries or petrochemical plants, and as sources of energy.

diagram on mining methods
The gas industry: from the well to the user.
Source: Department of Energy

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Transporting Natural Gas from Offshore

Transporting natural gas from offshore reserves is done by pipeline and by ship. There are more than 45,000 kilometers of pipeline on the seafloor of the Gulf of Mexico. Most of these pipes are in shallower settings and a few are in deep water. In areas without pipelines, the gas is liquefied at -162° C to reduce its volume to 1/600 the gaseous volume. Supertankers are fitted with special pressurized tanks that are refrigerated and insulated to keep the liquid gas very cold. Once delivered, the liquid is transformed back into a gas and moved into a gas pipeline system.

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Transporting Natural Gas to Users

Natural Gas transmission grid
Source: DOE

The interstate system is a 350,000 kilometer long superhighway made of pipes for the transport of natural gas to every corner of the U.S. The strong carbon and steel pipes range in size from 15 to 125 cm (though most are 40 to 85 cm). They are coated to protect them against corrosion. Natural gas is moved through this system to areas of high demand, such as cities and major industrial complexes. The gas is transported at high pressures from 200 to 1500 pounds per square inch (psi). Because the gas is compressible, it reduces the volume of the gas by up to 600 times. It also increases the speed at which the gas travels. Compressor stations, located every 40 to 100 miles along the pipeline, ensure that gas remains pressurized. The flow of gas is measured at metering stations. Valves located every 5 to 30 kilometers along the pipeline allow sections to be isolated for such uses as repairs. At the pipeline control systems, the pipeline operators monitor and maintain the flow of natural gas. Control systems also identify problems such as leakages or unexpected changes in pressure or flow. They have customers on both ends of the pipeline: producers and consumers. Pipe inspections are made internally using robots called "smart pigs." They inspect pipes and measure thickness and roundness and check for corrosion and defects that might cause leakages. The cost of laying one mile of interstate pipeline is approximately 3 million dollars.


Gas meters
Gas meters on the outside of a building.
Source: California Energy Commission.

Much of the local gas delivery system is hidden from view in covered trenches, a mandatory 50 centimeters below the surface. Planning future pipe installation is a complicated process that requires many stages of engineering and planning compliance. Once new pipelines are installed, a major effort is made to restore the surface to its original condition. As new commercial and residential areas are built, the distribution network expands. In every state, it consists of thousands of kilometers of smaller diameter pipes. Like interstate networks, the gas is often compressed in stations but at much lower pressures, as little as 3 psi. The gas is depressurized at city stations where the chemical mercaptan is added to give the gas its rotten egg smell. Utility companies own and manage the distribution network. Approximately half of the cost of natural gas comes from transportation and the development of the distribution network.

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History of Natural Gas in North America

Natural gas and petroleum commonly occur in the same reservoir. In 1859, Colonel Edwin Drake struck oil and natural gas at a depth of 69 feet near Titusville, Pennsylvania. This event is considered by many as the beginning of the natural gas industry in North America. To transport the gas, a 5 cm pipeline that extended 8 km was built from the well to the nearest village. This proved that natural gas could be extracted easily from underground and transported safely to its market. However, the "father of natural gas," William Hart, who dug specifically for natural gas in Fredonia, New York, struck gas in a 8 meter well in 1821. The Fredonia Gas Light Company was the first natural gas company in the United States. At this time, natural gas was mostly used for lighting and replaced the use of coal gas, started by the British in the 1780's.

Without pipelines or infrastructure the future of natural gas faced peril as electrical light bulbs replaced gas lamps. Natural gas producers had to find new uses for their product in the face of competition. In 1885, Robert Bunsen invented the Bunsen burner in which gas and air were mixed and ignited. This opened new possibilities for heating and cooking but natural gas was still held back by its lack of transportability. As a result, during mining operations for petroleum, natural gas was mostly burned off or allowed to escape into the atmosphere. In 1891, a long pipeline was constructed from wells in Indiana to Chicago, a distance of 180 kilometers. The pipeline was not very efficient and costly to install. It was in the 1920's that advances in pipe manufacturing and welding led to a renewed interest in pipeline construction. However, it was not until the 1960's that the United States finally constructed thousands of kilometers of efficient pipelines to transport natural gas to residential and industrial buildings.

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Reserves of Natural Gas

Natural Gas Proved Reserves
Source: EIA

Natural gas is a non-renewable resource that takes hundreds of thousands and possibly millions of years to form. Therefore, knowing how much is left and where it is located is important. Measuring reserves in the ground is complicated and requires precise modeling and a good deal of extrapolation. Furthermore, engineers, geologists, and even politicians rarely agree on the same methods. For this reason, estimations vary considerably. The Energy Information Administration calculates that in the United States, there are 2,543 trillion cubic feet (Tcf) of technically recoverable reserves. This includes undiscovered, unproved, and unconventional natural gas. In contrast, the National Petroleum Council has suggested that by 2017 reserves could reach 1887 Tcf. The following graph shows the changes in proved natural gas reserves for the past 30 years. Increases in reserves result from periods where more gas is recovered than is actually used.

map of wet natural gas proved reserves 2010
Source: DOE

Natural gas reserves are unevenly distributed in the United States. Natural gas, like petroleum, is typically found in distinctive geologic regions called basins. The largest basins are located in Texas and the Gulf of Mexico. Recently, natural gas has begun to be extracted from shale. This has enabled production in New York, Pennsylvania, Arkansas and Oklahoma, as well as Texas and Louisiana.

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World Natural Gas Reserves

World Natural gas reserves by geographic region as of 2012
Source: DOE

The Energy Information Agency estimates world proved natural gas reserves to be approximately 6,609 Tcf. Like oil, most of these reserves are located in the Middle East at 40 percent of the world total (around 2,658 Tcf). Europe and the former U.S.S.R. have 35 percent of total world reserves (around 2,331 Tcf). In comparison, the United States accounts for about 4 percent of the world's reserves. The United States currently produces and consumes about 25% of the global natural gas supply each year. In 2009, smaller independent producers without refineries contributed 85% of the natural gas supply.

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Advantages and Disadvantages of Using Natural Gas

The use of natural gas has certain advantages over other fossil fuels. Natural gas is one of the cleanest burning fuels and emits 45% less carbon dioxide than coal and 30% less than oil. Combustion does not produce soot or ash. Natural gas has a high heating value, approximately 24,000 Btu per pound. It can be transported fairly easily via pipelines and compressed during transport and decompressed upon arrival. Transport by pipeline is not affected by bad weather. Natural gas can be piped directly to houses for cooking, lighting, and various appliances. Locations that are not served by pipeline can utilize gas in small tanks. Natural gas can be compressed or liquefied and used as fuel for vehicles, with cleaner properties than gasoline or diesel. Some of the byproducts from refineries can be used to in the production of pharmaceuticals, industrial chemicals, ammonia for fertilizers, as well as paints and plastics. Upon leakage in open spaces, natural gas dissipates quickly and does not pool like heavier hydrocarbons. It is not toxic to humans in small volumes.

The use of natural gas is not without disadvantages. Like all fossil fuels, natural gas is non-renewable. Leakages of methane are costly to the environment because natural gas is a powerful component of Earth's greenhouse, 21 times more heat absorptive than carbon dioxide. The infrastructure required for building interstate, intrastate, and distribution networks is costly. Because of the high transport costs, utility companies need to be closely regulated to protect consumers from monopolies. Natural gas is directly flammable. Therefore where gas leaks in confined spaces there is a strong potential for explosion.

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