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Composition

Natural gas is composed of hydrocarbon molecules that range from one to four carbon atoms in length. The gas with one carbon atom in the molecule is methane (CH4), two is ethane (С2Н4), three is propane (C3H8), and four is butane (C4H10). All are paraffin-type hydrocarbon molecules. A typical natural gas composition is shown in Table 1—5.

Ethane – этан

Propane – пропан

Butane – бутан

Pure – чистый, беспримесный

Distill – очищать

These percentages vary from field to field, but methane gas is by far the most common hydrocarbon. Many natural gas fields contain almost pure methane. The gas from pipelines that is burned in homes and industry is methane gas. Propane and butane burn giving off more heat than methane. They are often distilled from natural gas and sold separately. Liquefied petroleum gas (LPG) is made from propane gas.

The nonhydrocarbon, gaseous impurities that don’t burn in natural gas are called inerts. A common inert is water vapor (steam). Another inert is carbon dioxide (CO2), a colorless, odorless gas. Because it doesn’t burn, the more carbon dioxide natural gas contains, the less valuable the gas is. In some gas reservoirs, carbon dioxide is greater than 99% of the gas. Large fields of almost pure carbon dioxide probably formed by the chemical reaction of volcanic heat on limestone rock. Carbon dioxide can be used for inert gas injection, an enhanced oil recovery process, in depleted oil fields. Nitrogen (N), another inert, is also a colorless, odorless gas that can be used for inert gas injection. Helium is a light gas used in electronic manufacturing and filling dirigibles. Gas from the Hugoton gas field in western Texas, Oklahoma, and Kansas contains 0.5 to 2% helium. It is thought to have been formed by the radioactive decay of K40 in granite. Amarillo, Texas, near the giant gas field, is called the “helium capital of the world.”

Hydrogen sulfide (H2S) is a gas that can occur mixed with natural gas or by itself. It is not an inert and is a very poisonous gas that is lethal in very low concentrations. The gas has the foul odor of rotten eggs and can be smelled in extremely small amounts. It is associated with the salt domes of the Gulf of Mexico and ancient limestone reefs of Mexico, West Texas, and Lou-

45

Volcanic – вулка-

нический

Helium – гелий

Decay – разлагаться

Giant – огромный

Hydrogen sulfide –

сероводород

Poisonous – ядови-

тый

Lethal – смертельный; летальный

Foul – вонючий, омерзительный, отталкивающий

Ancient – древний

Overthrust belt –

надвиговая зона

Occurrence

Because of high pressure in the subsurface reservoir, a considerable volume of natural gas occurs dissolved in crude oil. The formation, dissolved or solution gas/oil ratio is the cubic feet of natural gas dissolved in one barrel of oil in that reservoir under subsurface conditions. The volume measurements are reported under surface conditions. In general, as the pressure of the reservoir increases with depth, the amount of natural gas that can be dissolved in crude oil increases.

When crude oil is lifted up a well to the surface (Fig. 1—6), the pressure is relieved, and the natural gas, called solution gas, bubbles out of the oil. The producing gas-oil ratio of a well is the number of cubic feet of

Measurement –

снятие мерок, измерение

Relieve – ослаб-

лять, уменьшать

Solution gas –

природный газ, растворённый в нефти коллектора

Bubble – пузы-

риться; подниматься пузырьками

Condensate

In some subsurface gas reservoirs, at high temperatures, shorter-chain liquid hydrocarbons, primarily those with five to seven carbon atoms in length, occur as a gas. When this gas is produced, the temperature decreases, and the liquid hydrocarbons condense out of the gas. This liquid, called condensate, is almost pure gasoline, is clear to yellowish to bluish in color and has 45 °API to 62 °API. Condensate is commonly called casinghead gasoline, drip gasoline, white gas, or natural gasoline. Condensate can be added to crude oil in the field to decrease the °API and increase the volume of the oil. Condensate removed from natural gas in the field is classified as crude oil by regulatory agencies.

Refiners pay almost as much for condensate as crude oil. It doesn’t have a high octane and must be mixed with high-octane gasoline made by cracking in the refinery. Because of the low octane, the posted price for condensate is usually slightly less than that for crude oil. Natural gas that contains condensate is called wet gas, whereas natural gas lacking the condensate is called dry gas. The condensate along with butane, propane, and ethane that can be removed from natural gas is called natural gas liquids (NGL).

Condense – кон-

денсировать

Casinghead gasoline – газовый бензин (получаемый в промысловом сепараторе или абсорбционной установке из нефтяного газа)

Regulatory agencies – органы государственного регулирования

Measurement

The English unit of volume measurement for natural gas is a cubic foot (cf). Because gas expands and contracts with pressure and temperature changes, the measurement is made under or is converted to standard conditions defined by law. It is usually 60°F and 14.65 psi (15°C and 101.325 kPa) and is called standard cubic feet (scf). The abbreviation for 1000 cubic feet is Mcf, a million cubic feet is MMcf, a billion cubic feet is Bcf and a trillion cubic feet is Tcf Condensate content is measured in barrels per million cubic feet of gas (BCPMM).

The unit used to measure heat content of fuel in the English system is the British thermal unit (Btu). One Btu is about the amount of heat given off by burning one wooden match. Pipeline natural gas ranges from 900 to 1200 Btus per cubic foot and is commonly 1000 Btu. The heat content varies with the hydrocarbon composition and the amount of inerts in the natural gas. Natural gas is sold to a pipeline by volume in thousands of cubic feet, by the amount of heat when burned in Btus or by a combination of both. If the pipeline contract has a Btu adjustment clause, the gas is bought at a certain price per Mcf, and the price is then adjusted for the Btu content of the gas.

In the metric system, the volume of gas is measured in cubic meters (m3). A cubic meter is equal to 35.315 cf. Heat is measured in kilojoules. A kilojoule is equal to about 1 Btu.

The Btus in one average barrel of crude oil are equivalent to the Btus in 6040 cubic feet of average natural gas and is called barrel of oil equivalent (BOE). Different companies often have a slightly different BOE numbers depending on the oil and gas composition of their production.

Answer the questions:

Unit – единица

Contract – уплот-

няться, сжиматься

Convert – преоб-

разовывать; превращать

Content – доля,

процент, содержание (чего-л. в чём-

л.)

Match – спичка

Adjust –

регулировать, корректировать

Hydrocarbon reservoir

Chemists classify reservoir hydrocarbons into 1) black oil, 2) volatile oil, 3) retrograde gas, 4) wet gas, and 5) dry gas. Laboratory analysis of a sample is used to determine type.

Both black and volatile oils are liquid in the subsurface reservoir. Black oil or low-shrinkage oil has a relatively high percentage of long, heavy, nonvolatile molecules. It is usually black but can have a greenish or brownish color. Black oil has an initial producing gasoil ratio of 2000 scf/bbl or less. The °API is below 45.

Volatile oil or high-shrinkage oil has relatively more intermediate size molecules and less longer size molecules than black oil. The color is brown, orange, or green. Volatile oil has an initial producing gas-oil ratio between 2000 and 3300 scf/bbl. The °API is 40 or above.

Retrograde gas is a gas in the reservoir under original pressure but liquid condensate forms in the subsurface reservoir as the pressure decreases with production. The initial gas-oil ratio is 3300 scf/bbl or higher.

Wet gas occurs entirely as a gas in the reservoir, even during production, but produces a liquid condensate on the surface. It often has an initial producing gas-oil ratio of 50,000 scf/bbl or higher.

Dry gas is pure methane. It does not produce condensate either in the reservoir or on the surface.

Classify – класси-

фицировать

Sample – образец, экземпляр

Low-shrinkage oil

– нефть с низкой усадкой

High-shrinkage oil

– нефть с высокой усадкой