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.
Natural Gas is a mixture of gaseous hydrocarbons occurring in reservoirs of porous rock (commonly sand or sandstone) capped by impervious strata. It is often associated with petroleum, with which it has a common origin in the decomposition of organic matter in sedimentary deposits. Natural gas consists largely of methane (CH4) and ethane (C2H6), with also propane (C3H8) and butane (C4H10)(separated for bottled gas), some higher alkanes (C5H12 and above) (used for gasoline), nitrogen (N2) , oxygen (O2), carbon dioxide (CO2), hydrogen sulfide (H2S), and sometimes valuable helium (He). It is used as an industrial and domestic fuel, and also to make carbon-black and chemical synthesis. Natural gas is transported by large pipelines or (as a liquid) in refrigerated tankers. Natural gas is combustible mixture of hydrocarbon gases, and when burned it gives off a great deal of energy. We require energy constantly, to heat our homes, cook our food, and generate our electricity . Unlike other fossil fuels, however, natural gas is clean burning and emits lower levels of potentially harmful byproducts into the air. It is this need for energy that has elevated natural gas to such a level of importance in our society, and in our lives.
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.
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, 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.
The Components of Natural Gas
Although the principal use of natural gas is the production of pipeline quality gas for distribution to residential and industrial consumers for fuel, a number of components in natural gas are often separated from the bulk gas and sold separately.
1. METHANE
The principal use of methane is as a fuel; it is the dominant constituent of pipeline quality natural gas. Considerable quantities of methane are used as feedstock in
the production of industrial chemicals, principally ammonia and methanol.
2. ETHANE
The majority of the ethane used in the United States comes from gas plants, and refineries and imports account for the remainder. In addition to being left in the
gas for use as a fuel, ethane is used for the production of ethylene, the feedstock for polyethylene.
3. PROPANE
Gas plants produce about 45% of the propane used in the United States, refineries contribute about 44%, and imports account for the remainder. The principal uses
are petrochemical (47%), residential (39%), farm (8%), industrial (4%), and transportation (2%) . A special grade of propane, called HD-5, is sold as fuel.
4. ETHANE–PROPANE MIX
When NGL is fractionated into various hydrocarbon streams, the butanes along with part of the propane are sometimes separated for use in local markets because
they are transportable by truck. The remaining light ends, an ethane−propane mix (E-P mix), is then pipelined to a customer as a chemical or refining feedstock.
5. ISOBUTANE
Approximately 42% of the United States supply of isobutene comes from gas plants, refineries supply about 5% (this percentage does not include consumption
of isobutane within the refinery), and imports are responsible for about 12%. The remaining isobutane on the market is furnished by isomerization plants that
convert n-butane to isobutane. The three primary markets for isobutane are as a feedstock for MTBE (methyl tertiary butyl ether) production (which is being
phased out), as a feedstock in the production of reformulated gasoline, and as a feedstock for the production of propylene oxide.
6. n-BUTANE
Gas plant production of n-butane accounts for about 63% of the total supply, refineries contribute approximately 31%, and imports account for the remainder.
Domestic usage of n-butane is predominantly in gasoline, either as a blending component or through isomerization to isobutane. Specially produced mixtures
of butanes and propane have replaced halocarbons as the preferred propellant in aerosols.
7. NATURAL GAS LIQUIDS
Natural gas liquids (NGL) include all hydrocarbons liquefied in the field or in processing plants, including ethane, propane, butanes, and natural gasoline. Such mixtures generated in gas plants are usually referred to as “Y-grade” or “raw product.”
8 NATURAL GASOLINE
Natural gasoline, a mixture of hydrocarbons that consist mostly of pentanes and heavier hydrocarbons and meet GPA product specifications, should not be confused
with natural gas liquids (NGL), a term used to designate all hydrocarbon liquids produced in field facilities or in gas plants.
The major uses of natural gasoline are in refineries, for direct blending into gasoline and as a feedstock for C5/C6 isomerization. It is used in the petrochemical industry for ethylene production.
9. SULFUR
Current sulfur production in the United States is approximately 15,000 metric tons per day (15 MMkg/d); about 85% comes from gas processing plants that
convert H2S to elemental sulfur. Some major uses of sulfur include rubber vulcanization, production of sulfuric acid, and manufacture of black gunpowder
References:
1. Natural Gas Engineering Handbook, Dr. Boyun Guo and Dr. AIi Ghalambor
2. Natural Gas, by Primož Potočnik.
3. Fundamentals of Natural Gas, Arthur J. Kidnay & William R. Parrish