(From Grolier Multimedia Encyclopedia)

Natural Gas

Since early in the 19th century, gases manufactured from coal and other fossil fuels have supplied humanity with sources ofenergy. In recent years, however, people have increasingly obtained fuel directly from a previously neglected resource—the vast stores of natural gas trapped within the rock layers of Earth.

Scientists are still not sure how natural gas came to be stored in the crust of the planet. According to one widely accepted theory, countless tiny marine plants and animals, called plankton, were deposited on the ocean floor many millions of years ago. Eventually their remains were covered by layers of mud that had been washed into the sea from shore. As thousands of years passed and more layers of mud were added, the sediments were subjected to extreme pressures and intense heat. Often they were folded and squeezed by movements of Earth's crust. Different layers of sediment turned into various kinds of rocks, some of which were porous. The remains of the once-living animals and plants were converted into gas and oil. Not surprisingly, these two forms of petroleum occur together.

Many people erroneously believe that gas and oil are found in huge subterranean caverns. In reality, they occur in minute pores of sandstone, limestone, and other rock, and are held captive under great pressure by surrounding rock formations that are impervious to seepage. These fuels are released only when the shifting of Earth's surface cracks the caprock—the layer of solid rock above the deposits—or when the caprock is penetrated by drilling.

Humans have known about natural gas for thousands of years, perhaps since before recorded history. Ancient peoples were undoubtedly mystified and perhaps terror-stricken when they accidentally discovered natural gas seeping from the ground or rising through clefts in rocks. They most likely noticed that close to such places, they often became light-headed or weak-kneed and spoke in disjointed fashion. Assuming they must be in the presence of supernatural powers, they erected temples of worship on or near some of these sites. The famous oracle of Delphi in ancient Greece was one such temple.

Eventually people learned that the gas that seeped from the ground was flammable. The Chinese were known to have made use of this property as early as 940 B.C. They piped natural gas through hollow bamboo rods to the ocean shore, where they burned it to evaporate seawater and produce salt.

Such commercial use of natural gas was extraordinarily rare. Indeed, for thousands of years, the gas was considered a natural marvel rather than a marketable commodity. George Washington, for instance, commented with great wonderment in 1775 on a "burning spring" on the banks of the Kanawha River near Charleston, West Virginia. Gas seeping from the ground there had been ignited and provided a perpetual natural torch.

First Use as a Fuel

Natural gas did not become a common fuel until relatively recently. Fuel experts were far more interested in manufactured gas, or "town gas," made from coal and, later, from coke and oil. Manufactured gas was the chief gaseous fuel throughout the 19th century, and virtually every sizable community in the United States had its own gasworks. Manufactured gas is still used in certain places in the United States where natural gas is scarce.

Although natural gas was not a serious competitor of manufactured gas until the 20th century, it was first used on a modest scale in the 1820s. The first natural-gas well in the United States was opened at Fredonia, New York, in 1821. The well was about 26 feet (8 meters) deep, and was capped with a large barrel to maintain pressure. When the Marquis de Lafayette visited Fredonia four years later, he found it lighted with natural gas. A dinner in his honor was even cooked over the gas!

In 1826, another natural-gas well was drilled on the shore of Lake Erie at Westfield, New York. A wooden pipeline less than 0.6 mile (1 kilometer) long was built to carry gas to a lighthouse at Barcelona Harbor, New York. In 1840, in Butler County, Pennsylvania, in an application that recalls the ingenuity of the ancient Chinese, natural gas was used to evaporate brine and produce salt.

The first corporation organized to supply natural gas in the United States was the Fredonia Gas Light and Water Works, formed in 1858 to sell natural gas to business concerns and private homes. By that time, however, some 300 companies had already been established in the United States to manufacture gas from coal. These companies served nearly 5 million customers, about one-sixth of the nation's population at that time.

There was still little demand for natural gas. In 1859, when the first U.S. oil well was discovered in Titusville, Pennsylvania, producers were dismayed to find that natural gas occurred together with the oil. To eliminate the unwanted gas, it was flared—that is, ignited at the wellhead and left to burn. This became a common practice in other parts of the country as well, particularly in the Southwest. A tremendous amount of natural gas was wasted in this way.

Natural-Gas Pipelines

The fact that natural gas is frequently found together with oil contributed to the growth of the natural-gas industry. As more and more oil was discovered in various parts of the United States and the demand for it increased, producers sought to find effective ways of transmitting natural gas to nearby markets. Progress was rather slow. The first "long-distance" pipeline, built in 1870, was made of white-pine logs in which 8-inch (20-centimeter) holes had been bored. The logs, laid end to end, carried gas about 16 miles (25 kilometers) from West Bloomfield to Rochester, New York. After a few years of operation, the project was finally abandoned.

Iron pipe carried natural gas for the first time in 1872, when a line 5.6 miles (9 kilometers) long and 2 inches (5 centimeters) in diameter was built from Newton Wells to Titusville, Pennsylvania. As late as 1890, natural-gas lines were still small in diameter, usually under 8 inches (20 centimeters), and they extended relatively short distances.

By 1900, natural gas had been found in 17 states. Despite such discoveries, the total production of natural gas for that entire year was sold for less than $25 million. Pennsylvania was then the largest gas-producing state. In the decade that followed, immense natural-gas deposits were discovered in Texas, California, and Oklahoma.

Long-distance natural-gas transmission lines got their real start in 1925, when seamless, electrically welded pipe became available to the industry. As a result, collecting and transporting natural gas finally became profitable. By 1930, expanding pipeline systems began to bring natural gas to cities previously served only with manufactured gas.

In the 1940s, the U.S. gas industry acquired two vast pipeline systems—the Big Inch and the Little Big Inch—that had been built by the government during World War II to carry oil from Texas to the East Coast. After the war, these pipelines were converted into natural-gas transmission lines. More than 60 years later, the Little Big Inch is still transporting natural gas to several parts of the country. The U.S. natural-gas distribution grid includes approximately 2.2 million miles (3.5 million kilometers) of underground pipelines and 300,000 miles (482,700 kilometers) of transmission lines.

Natural gas supplies 25 percent of the energy in North America. Nearly 60 percent of U.S. households heat with natural gas. The U.S. Department of Energy (DOE) predicted a 20 percent increase in natural-gas consumption between 2005 and 2030.

From Wells to Homes

In recent years, discoveries of new sources of natural gas have not kept up with consumption. Energy experts are still embroiled in considerable controversy over just how much natural-gas reserve lies undiscovered. In the United States, the majority of proven reserves of natural gas is concentrated in Texas, Wyoming, Oklahoma, the Gulf of Mexico, and Louisiana. The largest proven reserves are in Russia, followed in descending order by Iran, Qatar, Saudi Arabia, the United Arab Emirates, and the United States.

Prospecting

Prospectors searching for natural gas rely on the same techniques used to search for oil. Both substances are difficult to find. Geologists, physicists, chemists, and engineers know that natural gas is found in porous rock, and that it is trapped under a layer of dense rock that is not porous. They look for such rock layers in deserts, on mountains, in swamps, under snow and ice, along the seacoast, or offshore. When they find a place that seems promising, a surveying party makes an accurate map of the area. Geologists and other scientists study the map closely. From clues on the surface, they try to determine how the rock layers below the surface lie, and whether natural gas would be trapped there.

Physicists and geologists then use sophisticated instruments to find evidence of petroleum deposits. They often explode dynamite in the ground to create a shock wave that travels down beneath the surface and bounces back off layers of rock below; a seismograph detects and records the echoes. Scientists can use the seismographic records, along with their knowledge about the shape and nature of the underground rock formations, to predict whether a given region is likely to contain deposits of gas and oil.

Scientists use other methods to discover new deposits. Variations in magnetism can yield clues. An instrument called a magnetometer measures the magnetism in a given region and helps geologists better understand the nature of the rock formations below. The dense, hard rock that traps natural gas is heavier than the porous rock that stores it. Heavy rock has a stronger magnetic pull than light rock.

Drilling

When prospectors are convinced of the likelihood that natural gas or gas combined with oil is trapped in a certain location, the next step is to drill a test hole. The hole is no more than 8 inches (20 centimeters) wide, but may be many thousands of feet deep.

The process of drilling for natural gas is identical to drilling for oil. Drilling engineers and crew are sent to the scene. There they erect special steel towers called derricks, assemble drilling machinery, and start drilling. A drill bit on the end of a turning drill pipe bores into the rock. The device is powered by an engine that often uses natural gas as fuel. At intervals, another length of drill pipe is screwed onto the end of the pipe that disappears into the ground.

As the drill bites its way through rock, it knocks off chips and pieces. To bring these out of the drill hole, water containing clay and chemicals is pumped down the drill pipe. This watery mixture, called drill mud, keeps the drill hole flushed clean of rock chips, and cools and lubricates the drill bit. As the rock chips come to the surface, geologists carefully examine them to see what types of rock layers are being penetrated and their composition.

When a drill bit becomes worn, it must be replaced by a new one. The drill pipe is pulled out of the ground, unscrewed length by length, and stacked on the ground. The worn bit is removed from the hole and is replaced with a new bit. It goes back into the hole, followed, one length at a time, by the drill pipe. Drilling continues night and day. Three crews of five workers each are needed, each crew working for eight hours. All that effort, of course, may be wasted: only 1 of 10 test holes actually yields natural gas.

When gas is found, it rises in the pipe with a roar. The drill pipe and bit are pulled up, and the hole is capped to prevent the gas from escaping and to control its flow. The device used for this purpose is called a Christmas tree, because the collection of valves sticking out from its main stem create a vaguely treelike shape. Some of the valves on the Christmas tree lead the natural gas to instruments that indicate the pressure and supply other information. Other valves control the flow of gas into pipelines.

Many of the drilling methods just described are also employed in offshore operations. Costs are much higher, due in large part to the deeper wells required. On land, natural-gas wells average about 4,000 feet (1,200 meters); offshore wells in the Gulf of Mexico, for example, average more than twice that depth.

Most natural gas can be used just as it comes from the ground without further refining. A small percentage, however, contains water, sulfur, or other impurities. To remove these unwanted substances, the gas is passed through so-called scrubbing towers. Since pure natural gas has no aroma, a chemical compound called an odorant is added as a safety precaution; otherwise it would not be possible for people to detect escaping gas.

The hydrocarbon methane (CH4) forms the bulk of the natural gas burned in homes. Natural gas also contains other valuable hydrocarbons, many of which are separated, or "stripped," from the gas before it is sent on its long pipeline journey to consumers. The stripped hydrocarbons are used for various industrial purposes.

Establishing Pipelines

Laying a long-distance transmission line is a task that calls for highly trained people and many powerful and complicated machines. First, engineers survey and map the pipeline route, which may climb mountains, cross, and run under fields and meadows. Tractors and bulldozers then clear a path overland, while excavating machines dig a trench in which the pipeline will be laid.

The pipes, typically transported to the scene by truck, are made of heavy-duty steel and are usually at least 3 feet (1 meter) in diameter. Tractors equipped with movable beams known as side booms hoist and lower the pipes. First, the pipe lengths are laid end to end. Welders connect the lengths of pipe, making certain that the joints between them are perfectly tight. Inspectors check each pipe joint to be certain that it will not leak.

Before it goes into operation, a pipeline is cleaned inside and out. On the outside, it is scrubbed and then machine-wrapped with layers of specially coated paper that protects the pipe from moisture and from rust formation. The inside of the pipeline is cleaned by a "pig," a machine that has brushes attached to it. The pig is blown through the pipeline by compressed gas, twisting and scrubbing its way through the pipe as it goes. When the pipeline has been cleaned, wrapped, and tested, it is carefully laid in the open trench by the side-boom tractors, and buried. The system is then ready to transmit natural gas from the drill site to the consumer. During a typical workday, a single construction crew can lay down perhaps 1 mile (1.6 kilometers) of pipeline.

Submarine pipelines are increasingly used as more offshore drill sites are established. For example, a 56-mile (90-kilometer) gathering and transmitting pipeline system extends from offshore wells to the mainland in southwestern Louisiana. This extensive pipeline system taps some of the largest known submerged natural-gas reserves in the Gulf of Mexico.

Compressor Stations

Natural gas rushes out of a well because of the pressure that was generated as the gas formed underground centuries ago. This pressure pushes the gas through a pipeline at an initial speed estimated at 60 to 70 miles (95 to 115 kilometers) per hour. As the gas moves along, it "rubs" against the walls of the pipe, creating friction that causes the gas to gradually slow and the pressure in the pipe to drop.

To get the gas moving rapidly again, a compressor station is needed. Within the station, a series of pumps boost the falling pressure by compressing the gas inside the pipeline. As the gas is compressed, it heats up. It must therefore be cooled with water in a cooling tower before being sent back to the pipeline, where the gas continues on to the next compressor station or regulator station. Since natural gas ignites quite easily, precautions must be taken at every step along the way to prevent the possibility of explosion.

Most compressor stations and other pipeline units operate automatically. Computerized, remote-controlled pipelines covering extensive regions can be operated from centralized control consoles. In Nashville, Tennessee, for instance, a single person can regulate the flow of gas through a far-flung pipeline system—one that includes nearly 870 miles (1,400 kilometers) of main line and five remote-controlled compressor stations situated about 185 miles (300 kilometers) apart.

Storage

A community's gas needs change virtually hour to hour. During mealtimes, for example, a great deal of gas is used in a short amount of time. Since gas is used for heating, consumption also varies a great deal from week to week and season to season. Communities in northern regions typically use six times more natural gas in winter than they use in summer.

Gas companies must have enough gas in reserve to meet peak demands. To ensure a reliable supply, they usually store the fuel while consumption is low, holding it in reserve until consumer demands are greater.

Natural gas can be stored in transmission pipelines, a system called pipeline storage or line pack. A large amount of gas can be kept in reserve under high pressure, thanks to the great strength of the pipelines.

If a community is located near a depleted gas or oil field, reserve supplies of natural gas can sometimes be forced back into the ground. A compressor station takes gas from pipelines and "stuffs" it into the layer of porous rocks below. When the gas is needed, it is released again into the pipeline. Such underground storage is often less expensive than stockpiling gas in aboveground tanks.

If no old gas field is available for storage, geologists can sometimes locate layers of porous rock that have the appropriately dense rock formation needed to trap and hold gas. Abandoned coal mines can also be used in some cases.

Often, however, gas must be stored in aboveground tanks. One way to store a great deal of gas in a relatively small space is through liquefaction. Natural gas can be changed into liquid form by cooling it to −256° F (−160° C). Once liquefied, natural gas fills less than 1/600th of the storage space required by gaseous natural gas. Liquid natural gas is often shipped long distances in tankers.

Distribution

When the gas in a pipeline reaches a community, the flow needs to be controlled in order to meet changing needs. For example, pipeline pressure must be reduced before gas can be used in ranges, furnaces, water heaters, and other appliances. The flow of the gas must also be measured so a utility company can monitor consumption and charge accordingly.

At a regulator station, some valves control the flow of gas, while others regulate the pressure. Meanwhile, instruments indicate the temperature and pressure of the gas, and display how much of it is passing through to the community. Many of these valves and instruments are automatic. For example, in the morning, when showers are using hot water and ovens are being lighted, valves in the regulator station let more gas through without human intervention. Technicians need only oversee the operation to ensure that it is working properly.

Many communities use only pure natural gas, transmitted through pipelines from wells and controlled by means of compressor and regulator stations. In some towns and cities, natural gas is combined with manufactured gas in carefully regulated amounts in a place called a mixing house.

Manufactured Gas

Early in the 19th century, manufactured gas was made only from coal or its residue, coke. In the United States today, most manufactured gas is derived from oil; it accounts for only a small percentage of all the gas consumed. Manufactured gas is used principally as a standby fuel for use during peak-load periods in areas served by natural gas. Comparatively few companies now market straight manufactured gas or mix it with natural gas. This may change, however, because of expected future shortages of natural gas.

Applications

Gas—natural, manufactured, or a mixture of the two—is used in homes for a variety of applications, including cooking, heating water, central heating, and laundering. It is also used in certain types of refrigerators and air conditioners.

For many years, the gas industry has conscientiously researched innovative ways to make gas appliances more effective and efficient. As a result, many new developments have emerged into the general consumer marketplace. Gas ranges can come equipped with burners the size of nickels. Tiny pilot lights, called "minipilots," give off less heat than the lightbulb in the back of an oven. Top-burner heat control makes every pot, pan, and skillet a completely independent appliance. An all-gas kitchen-appliance center combines cooking, laundering, refrigeration, house heating, and water heating in one efficient unit.

Stores, restaurants, laundries, and hospitals all constitute important gas customers. The restaurant industry, in particular, seems to favor gas for cooking, judging from the fact that more than 95 percent of the meals served daily in public dining rooms in the United States are cooked by this fuel. Food in canneries is often cooked with gas as well. Laundry and dry-cleaning establishments also use great quantities of gas, largely because it is the cleanest-burning of all fuels.

Natural gas plays a vital role in industry. It is employed in the processing of bricks, glass, cement, and steel and many other metals; gas is used to bake enamel on prefabricated houses, to prepare chemicals, and to manufacture paint and varnish. The textile industry also uses vast quantities of natural gas.

Natural gas is also widely used in agriculture. Freshly mowed hay and alfalfa are processed in gas-fired devices to provide food for livestock during the winter. Natural gas heats the sterilizers that disinfect hatchery equipment, and the brooders and incubators in which eggs hatch and chicks are kept warm.

Natural gas is not only a fuel, but an ingredient of many industrial products, including carbon black, used in the processing of natural rubber, and butadiene, from which certain kinds of synthetic rubber and plastics are produced by the chemical industry. Natural gas also yields potash, used in fertilizers and explosives and in the manufacture of such synthetic fabrics as nylon, Dacron, Acrilan, and Orlon.

Of all fossil fuels, natural gas has the cleanest environmental record. When it is burned, it emits only carbon dioxide and water. Although there is concern about the production of carbon dioxide—the "greenhouse gas" primarily responsible for global warming—other environmental consequences are minimal. Unlike coal and oil, natural gas emits no sulfur dioxide, and thus does not add to the formation of acid rain; nor does it contribute to smog or solid-waste problems.

As a motor-vehicle fuel, compressed natural gas (CNG) produces only a fraction of the pollutants generated by gasoline. Honda, General Motors, Toyota, and Ford are among the automakers developing CNG-powered models. There are now more than 130,000 CNG vehicles on U.S. roadways, and one in every five new public buses runs on natural gas.