Active vocabulary. 1. gas cap газовая шапка (в коллекторе нефти) 2. percussion удар; толчок; ударное действие 3. bit

Natural gas is often found dissolved in oil at the high pressures, existing in a reservoir, and it also can be present as a gas cap above the oil. Such natural gas is known as associated gas. There are also reservoirs that contain gas and no oil. This gas is termed nonassociated gas.

Discovery and early uses.

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, 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 ВС. It was in China in 211 BC that the first known well was drilled for natural gas to reported depths of 140 metres. The Chinese drilled their wells with bam­boo poles and primitive percussion bits for the express purpose of searching for gas in Upper Triassic limestones (about 190,000,000 years old) in an anticline west of mod­ern Chungking. The gas was burned to dry the rock salt found bedded in the limestone. Eventually wells were drilled to depths exceeding 915 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 distilled from coal (known as coal gas or manufactured gas) became the primary fuel for il­luminating streets and houses throughout much of Europe from 1790 on. In North America, the first commercial application of a petroleum product was the utilization of natural gas from a shallow well in Fredonia, N.Y., in 1821. The gas was distributed through a small-bore lead pipe to consumers for lighting and cooking.

Throughout the 19th century, the use of natural gas remained localized because there was no way to trans­port large quantities of gas over long distances. Natural gas remained on the sidelines of industrial development, which was based primarily on coal and oil. An important breakthrough in gas-transportation technology occurred in 1890 with the invention of leakproof pipeline coupling. Nonetheless, materials and construction techniques re­mained so cumbersome that gas could not be used more than 160 kilometres (100 miles) from a source of supply. Thus, associated gas was mostly flared (i.e., burned at the wellhead) and nonassociated gas was left in the ground, while coal gas was manufactured for use in the cities.

Long-distance gas transmission became practical during the late 1920s because of further advances in pipeline tech­nology. From 1927 to 1931 more than 10 major transmis­sion systems were constructed in the United States. Each of these systems was equipped with pipes having diameters of approximately 51 centimetres (20 inches) and extended more than 320 kilometres. Following World War II, a large number of even longer pipelines of increasing diam­eter were constructed. The fabrication of pipes having a diameter of up to 142 centimetres became possible. Since the early 1970s, the longest gas pipelines have been laid in the Soviet Union. For example, the 5,470-kilometre-long Northern Lights pipeline crosses the Ural Mountains and some 700 rivers and streams, linking Eastern Europe with the West Siberian gas fields on the Arctic Circle. As a result, gas from the Urengoy field, the world's largest, is transported to Eastern Europe and then on to Western Europe for consumption. Another gas pipeline, shorter but also of great engineering difficulty, is the 51-centimetre line that extends from Algeria across the Mediterranean Sea to Sicily. The sea is more than 610 metres deep along some parts of the route.

Natural gas as a premium fuel.

As recently as 1960, associated gas was a nuisance by-product of oil production in many areas of the world. The gas was separated from the crude oil stream and eliminated as cheaply as possible, often by flaring. Only since the crude oil shortages of the late 1960s and early 1970s natural gas has become an important world energy source.

Even in the United States, the home-heating market for natural gas was limited until the 1930s when manufac­tured gas began to be replaced by abundant and cheaper supplies of natural gas, which contained twice the heating value of its synthetic predecessor. Also, when natural gas burns completely, carbon dioxide and water are normally formed. The combustion of gas is relatively free of soot, carbon monoxide, and the nitrogen oxides associated with the burning of other fossil fuels. In addition, sulfur dioxide emissions, another major air pollutant, are almost nonex­istent. As a consequence, natural gas is often a preferred fuel for environmental reasons.