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Natural Gas September 25, 2009

Posted by nzafee in Natural Gas Processing.
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Natural gas is a subcategory of petroleum that is a naturally occurring, complex mixture of hydrocarbons, with a minor amount of inorganic compounds. Geologists and chemists agree that petroleum originates from plants and animal remains that accumulate on the sea/lake floor along with the sediments that form sedimentary rocks. The processes by which the parent organic material is converted into petroleum are not understood. The contributing factors are thought to be bacterial action; shearing pressure during compaction, heat, and natural distillation at depth; possible addition of hydrogen from deep-seated sources; presence of catalysts; and time (Allison and Palmer 1980).

Table 1-1 shows composition of a typical natural gas. It indicates that methane is a major component of the gas mixture. The inorganic compounds nitrogen, carbon dioxide, and hydrogen sulfide are not desirable because they are not combustible and cause corrosion and other problems in gas production and processing systems. Depending upon gas composition, especially the content of inorganic compounds, the heating value of natural gas usually varies from 700 Btu/scf to 1,600 Btu/scf. Natural gas accumulations in geological traps can be classified as reservoir, field, or pool. A reservoir is a porous and permeable underground formation containing an individual bank of hydrocarbons confined by impermeable rock or water barriers and is characterized by a single natural pressure system. A field is an area that consists of one or more reservoirs all related to the same structural feature. A pool contains one or more reservoirs in isolated structures. Wells in the same field can be classified as gas wells, condensate wells, and oil wells. Gas wells are wells with producing gas-oil-ration (GOR) being greater than 100,000 scf/stb;

Table 1-1 Composition of a Typical Natural Gas

Compound

Mole Fraction

Methane

0.8407

Ethane

0.0586

Propane

0.0220

i-Butane

0.0035

n-Butane

0.0058

i-Pentane

0.0027

n-Pentane

0.0025

Hexane

0.0028

Heptanes and Heavier

0.0076

Carbon Dioxide

0.0130

Hydrogen Sulfide

0.0063

Nitrogen

0.0345

Total

1.0000

condensate wells are those with producing GOR being less than 100,000 scf/stb but greater than 5,000 scf/stb; and wells with producing GOR being less than 5,000 scf/stb are classified as oil wells.Because natural gas is petroleum in a gaseous state, it is always accompanied by oil that is liquid petroleum. There are three types of natural gases: nonassociated gas, associated gas, and gas condensate. Nonassociated gas is from reservoirs with minimal oil. Associated gas is the gas dissolved in oil under natural conditions in the oil reservoir. Gas condensate refers to gas with high content of liquid hydrocarbon at reduced pressures and temperatures.

Dr. Boyun Guo & Dr. AIi Ghalambor

University of Louisiana at Lafayette

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