Extraction

Date: 2015-10-07; view: 409.

Ex. 8

a. Look through the text. Find the equivalents for the terms from exercise 7. Make corrections in your summary.

b. Translate the text orally. Make sure you know the following words and word combinations:

well site geologist; depleted; economically viable; primary oil recovery; natural drives; higher production rates; beam pumps; submersible pumps; thermally enhanced oil; detergents.

Locating an oil field is the first obstacle to be overcome. Today, geologists use seismic surveys to search for geological structures that may form oil reservoirs. The "classic" method includes making underground explosion nearby and observing the seismic response that provides information about the geological structures under the ground. However the "passive" methods that extract information from naturally-occurring seismic waves are also known .

Other instruments such as gravimeters and magnetometers are also sometimes used in the search for petroleum. Generally, the first stage in the extraction of crude oil is to drill a well into the underground reservoir. When an oil bearing structure has been tapped, the wellsite geologist will note its presence. Historically, in the USA, some oil fields existed where the oil rose naturally to the surface, but most of these fields have long since been depleted, except for certain remote locations in Alaska. Often many wells are drilled into the same reservoir, to ensure that the extraction rate will be economically viable. Also, some wells may be used to pump water, steam, acids or various gas mixtures into the reservoir to raise or maintain the reservoir pressure, and so maintain an economic extraction rate.

If the underground pressure in the oil reservoir is sufficient, then the oil will be forced to the surface under this pressure. Gaseous fuels, natural gas or water are usually present, which also supply needed underground pressure. In this situation it is sufficient to place a complex arrangement of valves (the Christmas tree) on the well head to connect the well to a pipeline network for storage and processing. This is called primary oil recovery. Usually, only about 20% of the oil in a reservoir can be extracted this way.

The amount of oil that is recoverable is determined by a number of factors including the permeability of the rocks, the strength of natural drives (the gas present, pressure from adjacent water or gravity), and the viscosity of the oil. When the reservoir rocks are "tight" such as shale, oil generally cannot flow through but when they are permeable such as in sandstone, oil flows freely. The flow of oil is often helped by natural pressures surrounding the reservoir rocks including natural gas that may be dissolved in the oil, natural gas present above the oil, water below the oil and the strength of gravity. Oils tend to span a large range of viscosity from liquids as light as gasoline to heavy as tar. The lightest forms tend to result in higher production rates.

Over the lifetime of the well the pressure will fall, and at some point there will be insufficient underground pressure to force the oil to the surface. If economical, as often is, the remaining oil in the well is extracted using secondary oil recovery methods. Secondary oil recovery uses various techniques to aid in recovering oil from depleted or low-pressure reservoirs. Sometimes pumps, such as beam pumps and electrical submersible pumps (ESPs), are used to bring the oil to the surface. Other secondary recovery techniques increase the reservoir's pressure by water injection, natural gas reinjection and gas lift, which injects air, carbon dioxide or some other gas into the reservoir. Together, primary and secondary recovery allow 25% to 35% of the reservoir's oil to be recovered.

Tertiary oil recovery reduces the oil's viscosity to increase oil production. Tertiary recovery is started when secondary oil recovery techniques are no longer enough to sustain production, but only when the oil can still be extracted profitably. This depends on the cost of the extraction method and the current price of crude oil. When prices are high, previously unprofitable wells are brought back into production and when they are low, production is curtailed. Thermally enhanced oil recovery methods (TEOR) are tertiary recovery techniques that heat the oil and make it easier to extract. Steam injection is the most common form of TEOR, and is often done with a cogeneration plant. In this type of cogeneration plant, a gas turbine is used to generate electricity and the waste heat is used to produce steam, which is then injected into the reservoir. In-situ burning is another form of TEOR, but instead of steam, some of the oil is burned to heat the surrounding oil. Occasionally, detergents are also used to decrease oil viscosity. Tertiary recovery allows another 5% to 15% of the reservoir's oil to be recovered.