Consult the TEXTS FOR SUPPLEMENTARY READING and complete the information about the history of oil extraction (Text 42). Be ready to discuss the information you have read.

21. Read the following text and give a brief summary of it:

Classification of Crude Oil

Crude oil usually consists of a mixture of hydrocarbons having varying molecular weights and differing from one another in structure and properties. These various species are separated into groups, or fractions, by a process of distillation called refining. The oil is first heated to a vapor, and then passed upward through a tower containing trays at various levels. The vapors are very hot at the bottom, but become cooler as the rise, so that different fractions condense in the trays at different heights. The lighter the fraction the higher up it condenses.

In an average crude oil the fractions, beginning with the lightest, are: (1) dissolved gases, (2) petroleum ether, (3) gasoline, (4) kerosene, (5) gas oil, (6) lubricating oils, (7) fuel oils, and (8) asphalt.

Because so much gasoline is needed, ways were developed to break down the larger molecules of the heavier fractions to produce it. This process is called «cracking» and may be accomplished by subjecting these fractions to high temperature and pressure, or by employing a chemical catalyst.

The oil industry classifies «crude» by the location of its origin (e.g., «West Texas Intermediate, WTI» or «Brent») and often by its relative weight or viscosity («light», «intermediate» or «heavy»); refiners may also refer to it as «sweet», which means it contains relatively little sulphur, or as «sour», which means it contains substantial amounts of sulphur and requires more refining in order to meet current product specifications. Each crude oil has unique molecular characteristics which are understood by the use of crude oil assay analysis in petroleum laboratories.

Barrels from an area in which the crude oil's molecular characteristics have been determined and the oil has been classified are used as pricing references throughout the world. These references are known as Crude oil benchmarks:

Brent Crude, comprising 15 oils from fields in the Brent and Ninian systems in the East Shetland Basin of the Notrh Sea. The oil is landed at Sullom Voe terminal in the Shetland. Oil production from Europe, Africa and Middle Eastern oil flowing West tends to be priced off the price of this oil, which forms a benchmark.

West Texas Intermediate (WTI) for North American oil.

Dubai, used as benchmark for Middle East oil flowing to the Asia-Pacific region.

Tapis (from Malaysia, used as a reference for light Far East oil)

Minas (from Indonesia, used as a reference for heavy Far East oil)

The OPEC Reference Basket, a weighted average of oil blends from various OPEC (The Organization of the Petroleum Exporting Countries) countries.

22. Read the text below to find answers to the given questions:

Text 11 B

Natural Gas

1. What does a gaseous fossil fuel primarily consist of?

2. Why do you think that methane is an extremely efficient greenhouse gas?

3. What is known about ethane, propane and butane?

4. What are the properties of natural gas?

5. How is liquefied natural gas transported?

6. Why is flaring illegal in many countries now?

7. How is natural gas stored?

Natural gas, commonly referred to as gas, is a gaseous fossil fuel consisting primarily of methane. It is found in oil fields and natural gas fields, and in smaller quantities, coal beds. When methane-rich gases are produced by the anaerobic decay of non-fossil organic material, these are referred to as biogas. Sources of biogas include swamps, which produce swamp gas; marshes, which produce marsh gas; landfills, which produce landfill gas, as well as sewage sludge and manure, by way of anaerobic digesters, in addition to Enteric fermentation particularly in cattle.

Methane is an extremely efficient greenhouse gas which may contribute to enhanced global warming when free in the atmosphere, and such free methane, would then be considered a pollutant rather than a useful energy resource. However, methane in the atmosphere is oxidised, producing carbon dioxide and water, so that the greenhouse effect of released methane is relatively short-lived. Also, natural gas, when burned, produces much less carbon dioxide than more carbonaceous fuel sources, such as coal. Significant biological sources of methane are termites, ruminants and cultivation. Estimated emissions are 15, 75 and 100 million tons per year respectively. The primary component of natural gas is methane, the shortest and lightest hydrocarbon molecule. It may also contain heavier gaseous hydrocarbons such as ethane, propane and butane, as well as other sulphur containing gases, in varying amounts.

Organosulfur compounds and hydrogen sulfide are common contaminants, which must be removed prior to most uses. Gas with a significant amount of sulfur impurities is termed «sour». Natural gas is tasteless and odorless. However, before gas is distributed to end-users, it is odorized by adding thiols, to assist in leak detection. Natural gas is, in itself, harmless to the human body — unlike carbon monoxide, for instance, it is not a poison.

However, natural gas is a simple asphyxiant and can kill if it displaces air to the point where the oxygen content will not support life. Natural gas can also be hazardous to life and property through an explosion. Natural gas is lighter than air, and so tends to dissipate into the atmosphere. But when natural gas is confined, such as within a house, gas concentrations can reach explosive mixtures and, if ignited, result in blasts that could destroy buldings. Methane has a lower explosive limit of 5% in air, and an upper explosive limit of 15%. Explosive concerns with compressed natural gas used in vehicles are almost nonexistent, due to the escaping nature of the gas, and the need to maintain concentrations between 5% and 15% to trigger explosions.

LNG carriers can be used to transport liquefied natural gas (LNG) across oceans, while tank trucks can carry liquefied or compressed natural gas (CNG) over shorter distances. They may transport natural gas directly to end-users or to distribution points, such as pipelines for further transport. These may have a higher cost requiring additional facilities for liquefaction or compression at the production point, and then gasification or decompression at end-use facitilies or into a pipeline.

In the past, the natural gas which was recovered in the course of recovering petroleum could not be profitably sold, and was simply burned at the oil field (known as flaring). This wasteful practice is now illegal in many countries, especially since it adds greenhouse gas pollution to the earth’s atmosphere. Additionally, companies now recognize that value for the gas may be achieved with LNG, CNG, or other transportation methods to end-users in the future. The gas is now re-injected back into the formation for later recovery. This also assists oil pumping by keeping underground pressures higher. In Saudi Arabia, in the late 1970s, a «Master Gas System» was created, ending the need for flaring.

The natural gas is used to generate electricity and heat for desalinization. Similarly, some land-fills that also discharge methane gases have been set-up to capture the methane and generate electricity. Natural gas is often stored in underground caverns formed inside depleted gas reservoirs from previous gas wells, salt domes, or in tanks as liquefied natural gas. The gas is injected during periods of low demand and extracted during periods of higher demand. Storage near the ultimate end-users helps to best meet volatile demands, but this may not always be practical.

TEXT AND VOCABULARY EXERCISES

23. Find in the text the words or phrases which mean the same as: