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natural gas,natural mixture of gaseous hydrocarbonshydrocarbon
, any organic compound composed solely of the elements hydrogen and carbon. The hydrocarbons differ both in the total number of carbon and hydrogen atoms in their molecules and in the proportion of hydrogen to carbon.
..... Click the link for more information. found issuing from the ground or obtained from specially driven wells. The composition of natural gas varies in different localities. Its chief component, methanemethane
, CH4, colorless, odorless, gaseous saturated hydrocarbon; the simplest alkane. It is less dense than air, melts at −184°C;, and boils at −161.4°C;. It is combustible and can form explosive mixtures with air.
..... Click the link for more information. , usually makes up from 80% to 95%, and the balance is composed of varying amounts of ethaneethane
, CH3CH3, gaseous hydrocarbon. It is a continuous-chain alkane. As a constituent of natural gas, it is used for fuel. It can be prepared by cracking and fractional distillation of petroleum.
..... Click the link for more information. , propanepropane,
CH3CH2CH3, colorless, gaseous alkane. It is readily liquefied by compression and cooling. It melts at −189.9°C; and boils at −42.2°C;.
..... Click the link for more information. , butanebutane
, C4H10, gaseous alkane, a hydrocarbon that is obtained from natural gas or by refining petroleum. It can be liquefied at room temperature by compression. There are two structural isomers of butane.
..... Click the link for more information. , and other hydrocarbon compounds. Some of the hydrocarbons found in gasolinegasoline
light, volatile mixture of hydrocarbons for use in the internal-combustion engine and as an organic solvent, obtained primarily by fractional distillation and "cracking" of petroleum, but also obtained from natural gas, by destructive distillation
..... Click the link for more information. also occur as vapors in natural gas; by liquefying these hydrocarbons, gasoline can be obtained.
Although commonly associated with petroleumpetroleum,
oily, flammable liquid that occurs naturally in deposits, usually beneath the surface of the earth; it is also called crude oil. It consists principally of a mixture of hydrocarbons, with traces of various nitrogenous and sulfurous compounds.
..... Click the link for more information. deposits it also occurs separately in sand, sandstone, limestone, and shale deposits. Some geologists theorize that natural gas is a byproduct of decaying vegetable matter in underground strata, while others think it may be primordial gases that rise up from the mantle. Because of its flammability and high calorific value, natural gas is used extensively as an illuminant and a fuel.
Natural gas was known to the ancients but was considered by them to be a supernatural phenomenon because, noticed only when ignited, it appeared as a mysterious fire bursting from the ground. One of the earliest attempts to harness it for economic use occurred in the early 19th cent. in Fredonia, N.Y. Toward the latter part of the 19th cent., large industrial cities began to make use of natural gas, and extensive pipeline systems have been constructed to transport gas. Since the late 20th cent., improvements in hydraulic fracturing, or "fracking"—in which pressurized fluids are injected into a well to induce rock fractures that allow the release of natural gas—and its use in combination with horizontal drilling has permitted natural gas to extracted from previously untappable deposits of shale.
Liquefied natural gas, or LNG, is natural gas that has been pressurized and cooled so as to liquefy it for convenience in shipping and storage. The boiling point of natural gas is extremely low, and only in the 1970s did cryogenic technology (see low-temperature physicslow-temperature physics,
science concerned with the production and maintenance of temperatures much below normal, down to almost absolute zero, and with various phenomena that occur only at such temperatures.
..... Click the link for more information. ) advance enough to make the production and transport of LNG commerically feasible. Some of the natural gas moved to and from the United States is carried as LNG in special tankers.
gaseous hydrocarbons that form in the earth’s crust.
General information and geology. Commercial deposits of natural gas occur in the form of individual accumulations that are not associated with any other mineral, as gas and oil deposits in which the gaseous hydrocarbons are completely or partially dissolved in petroleum or occur in a free state and fill the raised part of a bed (gas caps) or the upper parts of the layers of an oil and gas formation that are in contact with each other, and as gas-condensate deposits, in which the gas is enriched with liquid, primarily low-boiling, hydrocarbons.
Natural gas consists of methane, ethane, propane, and butane and sometimes contains admixtures of low-boiling liquid hydrocarbons, such as pentane and hexane; carbon dioxide, nitrogen, hydrogen sulfide, and inert gases are also present in it. Many natural-gas deposits that lie at depths of no more than 1.5 km consist almost wholly of methane, with small admixtures of its homologues (ethane, propane, and butane), nitrogen, argon, and occasionally carbon dioxide and hydrogen sulfide; the content of methane homologues usually increases with depth. In gas-condensate deposits the content of methane homologues is considerably higher than the methane content. This is typical of petroleum by-product gases. A high content of carbon dioxide, hydrogen sulfide, and nitrogen is observed in individual deposits.
Natural gas is found in deposits of all geological systems, beginning with the end of the Proterozoic (see Table 1), and at various depths, but most frequently down to 3 km. It is formed mainly as a result of the catagenetic transformation of an organic substance of sedimentary rocks. Natural-gas deposits form in natural traps in the paths of gas migration. Migration occurs as the result of the static or dynamic load of rock, which forces the gas out, and also during the free diffusion of gas from high-pressure areas to zones of lower pressure. A distinction is made between extrareservoir regional migration, through thick layers of rock mass of varying permeability by way of capillaries, pores, fractures, and fissures; and intrareservoir local migration, within highly permeable gas-collecting beds.
|Table 1. Relationship of natural gas to various geological systems|
|Quantity of gas (trillion cu m)|
Gas deposits are divided into two groups according to their structural features: bedded deposits and massive deposits (see Figure 1). In bedded deposits, gas accumulations are confined to certain collector beds. Massive deposits do not conform to specific beds in their localization. The most common bedded deposits are the vaulted types, which are preserved by a thick argillaceous or halogenous cap rock. Sandstones, sandy siltstones, and siltstones, often interbedded with clays, serve as the natural underground reservoirs for 85 percent of the total number of gas and gas-condensate deposits; carbonaceous rocks serve as gas collectors in the remaining 15 percent of cases. A series of beds subordinated to a single geological structure constitutes individual deposits. The structures of deposits are different for folded and platform conditions.
Two groups of structures, which are associated with anticlines and monoclines, are prominent in folded regions. In platform regions, four groups of structures are distinguished: dome-shaped and brachyanticlinal uplifts, erosion and reef masses, monoclines, and synclinal down warps. All gas and petroleum-gas deposits are confined to one or another oil-and-gas-bearing sedimentary basin, which is a large and elongate autonomous depression area in the present structure of the earth’s crust. Four groups of these are distinguished: those confined to intraplatform down warps (for example, the Michigan and Illinois basins in North America and the Volga-Ural region of the USSR), those confined to the collapsed marginal parts of platforms (the Western Siberian basin in the USSR), those controlled by the depressions of rejuvenated mountains (the basins of the Rocky Mountains in the USA and of the Fergana and Tadzhik depressions in the USSR), and those associated with the piedmont and interior basins of young alpine mountain structures (the California basin in the USA and the Sakhalin basin in the USSR). Gas deposits are continually being discovered in the shelf zone and in shallow basins—for example, the large gas deposits at West Soil, Hewitt, and Lehman-Bank in the North Sea.
The world geological reserves of natural gas on the continents, in shelf zones, and in shallow seas are estimated to be several trillion cubic meters, which is equivalent to 1012 tons of petroleum.
The USSR possesses enormous natural-gas resources. The largest deposits are Iambursk (4.4 trillion cu m), Urengoi (4.2 trillion cu m), Medvezh’e (3.6 trillion cu m), and Zapolinaroe (2.7 trillion cu m), which are confined to the Cretaceous deposits of the West Siberian basin; Vuktyl (460 billion cu m) and Orenburg (above 2 trillion cu m) in the Volga-Ural region; Gazli (1 trillion cu m) and Shatlyk (2.4 trillion cu m) in Central Asia; Shebelinka (390 billion cu m) in the Ukraine; and Stavropol’ (220 billion cu m) in the Northern Caucasus. Among other countries, the largest natural-gas reserves (estimated total reserves in trillions of cubic meters) are located in the USA (8.3), Algeria (4.0), Iran (3.1), and the Netherlands (2.3); the largest deposits in foreign countries (in trillions of cubic meters) are Pan-handle-Hugoton in the USA (1.96), Slochteren (Groningen) in the Netherlands (1.65), and Hassi-R’Mel in Algeria (approximately 1).
N. B. VASSOEVICH
Use Natural gas is a highly economical energy-producing fuel; its heat of combustion is 32.7 megajoules per cu m (7,800 kilocalories per cu m) and higher. It is widely used as a fuel in electric power plants, in ferrous and nonferrous metallurgy, in the cement and glass industries, in the production of building materials, and for municipal and domestic needs.
The hydrocarbons that are part of natural gas constitute the raw material for the production of methyl alcohol, formaldehyde, acetaldehyde, acetic acid, acetone, and other organic compounds. Synthesis gas (CO + H2), which is commonly used for producing ammonia, alcohols, and other organic products, is derived from methane—the chief constituent of natural fuel gases—by oxygen or steam conversion. Acetylene, carbon black, and hydrogen, which is used mainly for the synthesis of ammonia, are produced from methane by pyrolysis and dehydrogenation. Natural gas is also used in the production of olefin hydrocarbons, primarily ethylene and propylene, which in turn are the raw materials for further organic synthesis. Plastics, synthetic rubbers, and artificial fibers are produced from these materials.
S. F. GUDKOV
Extraction The extraction of natural gas includes the recovery of the gas from the earth and the collection, metering, and preparation of the gas for transportation to the consumer (so-called development of gas deposits), as well as the operation of wells and surface equipment. A distinctive feature of natural-gas production as opposed to the mining of solid minerals is the fact that the entire complex journey of the gas from the deposit to the consumer is hermetically sealed.
Discharges of natural gas from natural sources (the “eternal fires” in Dagestan, Azerbaijan, and Iran) have been utilized by mankind from time immemorial. The natural gas produced from pits and wells began to be used later (for example, during the first millennium A.D. the Tzuliuching deposit was discovered during the drilling of wells for salt in the province of Szechwan, China; gas from this deposit was used for the evaporation of salt from solutions). The occasional use of natural gas extracted from accidentally discovered deposits continued for many centuries. The use of natural gas as an industrial fuel dates to the mid-19th century (glass production was established on the basis of the Dage-stanskie Ogni deposit). The exploration and development of gas deposits did not take place until the 1920’s, when pure gas deposits were first developed commercially: deposits lying at shallow depths (hundreds of meters) were worked at first, followed by those at greater depths. The working of deposits during this period was conducted in a primitive manner: boreholes were spaced at an average of 1 mile (1.6 km) from each other in a uniform grid over the deposit. The production from the wells was 10-20 percent of the potential well output (of absolute free yield); in some cases, because of favorable geological conditions and the characteristics of the bed, the production yields were greater.
A new type of deposit—gas-condensate deposits—was discovered in the 1930’s, owing to the development of drilling technology and the shift to great depths (1,500-3,000 m and more); the working of these deposits demanded the creation of a whole new technology.
The late 1940’s were characterized by intensive development of Soviet gas industry and the adoption of scientific methods in the development of gas and gas-condensate deposits. The first scientifically grounded plan for the working of a gas deposit (Sultangulovo, Kuibyshev Oblast) was created in 1948 under the direction of the Soviet scientist B. B. Lapuk. In subsequent years commercial deposits of natural gas were worked according to plans formulated on the basis of the latest achievements in industrial geology, hydrodynamics, and other fields. An important stage in the exploitation of a deposit is prospecting. The detailed surveying of a gas deposit requires the drilling of a large number of deep wells; the number of test wells frequently exceeds the required number of producing wells.
New methods for the working of gas deposits were created by Soviet scientists and introduced during the postwar period. The experimental commercial exploitation of a gas bed takes place during the first phase of harnessing a gas deposit, in the course of which (two to five years) more precise determinations are made of the characteristics of the deposit—the properties of the bed, the gas reserves, the well output, the degree of mobility of stratal water, and so on. The deposit is tied in to the nearest gas pipeline or serves as a supply for local consumers. The second phase is commercial operation, which is based on sufficiently complete information on the deposit that was gathered during the experimental commercial development phase. Three principal periods are distinguished during the commercial phase: periods of increasing, constant, and decreasing production. The first period lasts three to five years and is associated with the drilling of wells and fitting out of the gas field. During this period 10-20 percent of the total gas reserves are extracted. The second period continues for approximately ten years, during which time 55-60 percent of the gas reserves are removed from the deposit. The number of wells increases during this time, since the productivity of each well drops off separately, although the total recovery of gas from the deposit remains unchanged. When the pressure in the bed falls to 5-6 meganewtons per sq m (MN/m2), or 50-60 kilograms-force per sq cm (kgf/cm2), a pressure-maintenance gas compressor station, which raises the pressure of the gas to be removed from the deposits to the level at which the main gas pipeline normally operates, is put into operation. The third period, that of falling production, has no set time limit. The working of a gas deposit takes place mainly over a period of 15-20 years. During this period 80-90 percent of the gas reserves are recovered.
Expenditures for the construction of operating wells make up 40-60 percent of the prime cost of natural-gas production. A well drilled through to a gas-bearing bed needs only to be opened in order to produce gas, but it is impossible to fully open high-discharge wells, since the free escape of gas could damage the bed and the shaft of the well and cause flooding because of the inflow of stratal water, and the energy of the gas under pressure in the bed would be spent irrationally. Therefore, the gas flow rate is limited by the use of a connecting pipe (a local narrowing of the pipe), which is usually installed at the wellhead. The daily operating output of wells ranges from dozens to several million cu m.
Superhigh-capacity wells with operating columns 8-12 inches (200-300 mm) in diameter were sunk in the USSR in the late 1960’s for the first time in world drilling practice.
The productivity of gas wells depends on the properties of the bed, the method of stripping, and the construction of the face. The thicker and more permeable the bed and the better the communication between the bed and the inside part of the well, the more productive the well. To increase the productivity of a gas well in carbonaceous rocks (limestones and dolomites), the face of the well is treated with hydrochloric acid, which reacts with the rock and expands channels for the inflow of gas; in hard rock the face is torpedoed, creating a network of fissures in the area around the face that facilitate the movement of gas. The inflow of gas is also intensified by perforation of the casing string by a water-sand jet, which improves the communication between the bed and the borehole, and by hydraulic bed fracturing, in which one or more large fissures filled with coarse sand that has low filtration resistance are created in the bed. In choosing a system for the arrangement of wells at a gas deposit, consideration is given not only to the properties of the formation but also to land forms, the system of collecting the gas, the nature of depletion of the deposit, and the period during which the compressor plant is to be put into operation. The wells may be evenly spaced in the deposit area, in a square or triangular pattern, or unevenly, in groups. The grouped arrangement, which facilitates servicing of the wells and permits complete automation of the product collecting, metering, and processing operations, is frequently used. This system usually proves to be highly advantageous and economical. For example, in the Severo-Stavropol’skoe gas deposit, the grouped arrangement of wells in the central portion of the bed (as opposed to uniform spacing) made it possible to cut the number of operating wells in half, yielding a saving of approximately ten million rubles.
Gas-condensate fields are worked by three basic methods. The first method, which is widely used in the USA, consists in the maintenance of sufficiently high pressure in the bed by pumping in gas from which the heavy hydrocarbons have been extracted at the surface (the so-called gas cycling process); as a result, the condensate does not precipitate in the bed and is delivered to the surface in a gaseous state. The recovery of condensate and the back pumping of lean gas (with no more than 10 percent heavy hydrocarbons) into the bed is continued until most of the condensate has been recovered from the deposit. In this method the reserves of gas are conserved over a long period of time. The second method consists in pumping water into the gas-bearing beds to maintain the formational pressure. This makes possible the use of the recovered gas immediately after it has been separated from the condensate. However, the pumping of water can lead to losses of both the gas and the condensate as a result of so-called gas binding (incomplete displacement of the gas by water). This method is rarely used. In the third method, gas-condensate fields are worked in the same way as pure gas deposits. This method is used in cases when the amount of condensate in the gas is not great, or if the total gas reserves in the deposit are small.
A gas deposit is worked at a gas field, which is a complex industrial facility spread out over a large area. A gas field of average size has dozens of wells, which are located in an area of hundreds of square kilometers. The basic industrial task of the gas field is to provide for planned operating conditions of the wells, the collection of gas in wells, the metering of the gas, and its preparation for delivery (the separation of solid and liquid impurities from the gas and of heavy hydrocarbons from the condensate, drying the gas, and the removal of hydrogen sulfide, the content of which must not exceed 2 g per 100 cu m).
The method of separating the condensate depends on the temperature, pressure, and composition of the gas and on whether the gas to be processed comes from a pure gas deposit or a gas-condensate deposit. Natural gas coming from a bed always contains a certain amount of water; combined with hydrocarbons, it forms a snowlike substance—hydrocarbon hydrates. Hydrates complicate the production and transportation of gas.
Before natural gas is transported to points of consumption, it undergoes processing for the removal of mechanical impurities, noxious components (H2S), heavy hydrocarbon gases (propane, butane, and so on), and water vapors. Various types of separators are used for the removal of mechanical impurities. Moisture is removed from the gas by low-temperature separation—that is, the condensation of water vapor at low temperatures (to -30° C), which are developed in the separators by throttling of the gas (reduction of the gas pressure by a factor of 2 to 4)—or by sorption of the water vapors by solids (adsorption) or liquids (absorption). The same methods are also used to separate heavy hydrocarbon gases from the gas, with the extraction of raw casinghead (natural) gas, which is later split (fractionation) into stable casinghead (natural) gas and commercial light hydrocarbons (industrial propane, industrial butane, propane-butane mixture, and other fractions). If necessary, noxious substances, chiefly hydrogen sulfide, are also removed from natural gas. Sulfur is removed from gas by the use of a series of solid and liquid substances that bind it. After processing in the field, the gas is sent through a drying collector under a pressure of 4.5-5.5 MN/m2 (45-55 kgf/cm2) to an industrial gas-collection point or to the head structures of a main gas pipeline. Natural gas from pure gas deposits generally undergoes only drying and the purification process for removing solid impurities.
The transition to complete integrated planning of the working of gas deposits, the intensification of gas inflow to wells, and the automation of gas field installations have made it possible to increase considerably the producing outputs of wells, improve the processing of gas for delivery, and reduce the net cost of natural gas.
E. V. LEVYKIN
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