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Date: 2015-10-07; view: 499.

Unit 1.What is Natural Gas?

Chapter 2.

C. Discussion

Make a presentation on different types of oil traps. Draw a picture of principal types of oil traps and describe them.

LEARNING OBJECTIVES:

Having worked through this chapter the student will be able to:

· define the typical composition of natural gas

· describe chemical and physical properties of natural gas

· describe the discovery and early uses of natural gas

· list and describe the ways in which methane may be formed

· describe conventional gas reservoirs

· list and describe unconventional gas reservoirs

· list and describe the use and ways of measurement of natural gas

Natural gas, in itself, might be considered a very uninteresting gas - it is colorless, shapeless, and odorless in its pure form. Quite uninteresting - except that natural gas is combustible, and when burned it gives off a great deal of energy. Unlike other fossil fuels, however, natural gas is clean burning and emits lower levels of potentially harmful byproducts into the air. We require energy constantly, to heat our homes, cook our food, and generate our electricity. It is this need for energy that has elevated natural gas to such a level of importance in our society, and in our lives.

Natural gas is a hydrocarbon mixture consisting primarily of methane and ethane, both of which are gaseous under atmospheric conditions. The mixture also may contain other hydrocarbons, such as propane, butane, pentane, and hexane. In natural gas reservoirs even the heavier hydrocarbons occur for the most part in gaseous form because of the higher pressures. They liquefy at the surface (at atmospheric pressure) and are referred to as natural gas liquids, gas condensate, natural gasoline, or liquefied petroleum gas. They may separate in some reservoirs through retrograde condensation or may be separated at the surface either in field separators or in gas processing plants by means of condensation, absorption, adsorption, or other modification.

Other gases that commonly occur in association with the hydrocarbon gases are nitrogen, carbon dioxide, hydrogen, hydrogen sulfide, and such noble gases as helium and argon. Because natural gas and formation water occur together in the reservoir, gas recovered from a well contains water vapour, which is partially condensed during transmission to the processing plant.

The composition of natural gas can vary widely, but below is a chart outlining the typical makeup of natural gas before it is refined.

Natural gas is considered 'dry' when it is almost pure methane, having had most of the other commonly associated hydrocarbons removed. When other hydrocarbons are present, the natural gas is 'wet'.

The physical properties of natural gas include colour, odour, and flammability. The principal ingredient of gas is methane, which is colourless, odourless, and highly flammable. However, some of the associated gases in natural gas, especially hydrogen sulfide, have a distinct and penetrating odour, and a few parts per million is sufficient to impart a decided odour to natural gas.

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 buildings. Methane has a lower explosive limit of 5% in air and an upper explosive limit of 15%.

Natural gas can be measured in a number of different ways. As a gas, it can be measured by the volume it takes up at a pressure of 750 millimetres of mercury and a temperature of 15° С, commonly expressed in cubic metres (or in cubic feet at an absolute pressure of 14.73 pounds per square inch and a temperature of 60° F). While measuring by volume is useful, natural gas can also be measured as a source of energy. Like other forms of energy, natural gas is commonly measured and expressed in British thermal units (Btu). One Btu is the amount of natural gas that will produce enough energy to heat one pound of water by one degree at normal pressure. To give an idea, one cubic foot of natural gas contains about 1,027 Btus. When natural gas is delivered to a residence, it is measured by the gas utility in 'therms' for billing purposes. A therm is equivalent to 100,000 Btu's, or just over 97 cubic feet, of natural gas.

Natural gas has many uses, residentially, commercially, and industrially. Found in reservoirs underneath the earth, natural gas is commonly associated with oil deposits. Production companies search for evidence of these reservoirs by using sophisticated technology that helps to find the location of the natural gas, and drill wells in the earth where it is likely to be found. Once brought from underground, the natural gas is refined to remove impurities like water, other gases, sand, and other compounds. Some hydrocarbons are removed and sold separately, including propane and butane. Other impurities are also removed, like hydrogen sulfide (the refining of which can produce sulfur, which is then also sold separately). After refining, the clean natural gas is transmitted through a network of pipeline. From these pipelines, natural gas is delivered to its point of use.