Natural gas

Natural gas

May 8, 2013, 5:54 pm
Content Cover Image

Blue flame generated by natural gas. Source: U.S. Department of Energy

Natural gas is a gas consisting primarily of methane (CH4). It is found as a raw naturally occurring gas in underground reservoirs, as gas associated with underground reservoirs of petroleum crude oil, as undersea methane hydrates and as coalbed methane in underground coal mines. It is an important fuel source and a major feedstock for producing ammonia, hydrogen, petrochemicals and fertilizers.

Natural gas is often informally referred to as simply gas or fuel gas, especially when compared to other fossil fuel energy sources such as diesel oil, fuel oil or coal..

There are a great many different natural gas reservoirs worldwide and each of those gas deposits has a different composition. However, in general, most of them contain at least 80 to 90 volume percent of methane.

Before raw natural gas extracted from those reservoirs can be used as a fuel, it must undergo extensive processing to remove almost all materials other than methane.

The by-products of that processing include ethane, propane, butanes, natural gasoline consisting of pentanes plus higher molecular weight hydrocarbons (often referred to as C5+ ), elemental sulfur, and sometimes helium and nitrogen

Formation of natural gas

While the origins of natural gas deposits are not known with certainty, and the different reservoirs vary in their geologic history, it is generally believed that the methane-rich gas was formed over millions of years from organic matter, usually from former marine or coastal ecosystems. Decomposed organic matter from dead plants and animals became covered in layers of mud and other sediments, and as more mud and other sediments piled on top of the organic matter, the pressure and temperature increased. The increased pressure and temperature is thought to have caused the organic matter to slowly transform into methane-rich gas and oil. Once formed, gas deposits tended to rise towards the surface of the earth through fissures and pores, until they become trapped underneath less permeable rock layers and formed gas reservoirs.

Composition of raw natural gas

Raw natural gas typically consists primarily of methane (CH4), the most compact and lightest hydrocarbon molecule. It also contains varying amounts of:

  • Higher molecular weight gaseous hydrocarbons: Raw natural gas may contain a total of as much as 20 volume per cent of ethane (C2H6), propane (C3H8), normal butane (n-C4H10), isobutane (i-C4H10), pentanes and even higher molecular weight hydrocarbons. When processed and purified into finished by-products, all of these are very often collectively referred to as NGL (Natural Gas Liquids).
  • Acid gases: Carbon dioxide (CO2), hydrogen sulfide (H2S) and mercaptans such as methanethiol (CH3SH) and ethanethiol (C2H5SH).
  • Other gases: Nitrogen (N2) and helium (He).
  • Water: Water vapor and liquid water.
  • Liquid hydrocarbons: Perhaps some "natural gas condensate", a low-boiling point mixture of liquid hydrocarbons (sometimes also called natural gasoline, casinghead gasoline or simply condensate).
  • Mercury: Very small amounts of mercury primarily in elemental form, but chlorides and other species are possibly present.

Measurement units and heating values

Quantities of natural gas are usually measured in "normal cubic meters" at 0 °C and 101.325 kPa absolute pressure or in "standard cubic feet" at 60 °F and 14.496 psi absolute pressure.

The "higher heating value" (HHV) of commercial, processed natural gas is about 40 MJ per "normal" cubic meter which, in the USA, is equivalent to about 1,015 Btu per "standard" cubic foot. However, those values can vary by several percent from one natural gas to another depending upon their source reservoir and upon their degree of processing.

The heating value of a fuel gas when the water formed during combustion does not condense is the called the "lower heating value" (LHV) and can be as much as 10% less than the HHV.

In the United States, retail sales of processed natural gas to end users are often measured in units of "therms" with one therm being 100,000 Btu (equivalent to the higher heating value of approximately 100 standard cubic feet).

Finding and extracting natural gas

Exploration

Exploration, in the oil and gas industry, is the search for underground or undersea natural gas reservoirs. Recoverable reserves of natural gas tend to occur where impermeable rocks (called ''caprocks'') constrain the upward movement of gas through the underground soil and rocks. Geologists and geophysicists usually use seismology to find areas which have the right conditions for a gas or oil deposit. High-speed computers that help develop three-dimensional underground maps as well as satellite image technology are also widely used to search for natural gas reservoirs. The various worldwide areas having significantly large reservoirs of natural gas are referred to as "gas fields" and most gas fields have been given a unique name, such as the North Field in Qatar. Many gas fields are in underwater gas reservoirs and are referred to as "offshore fields", such as in the North Sea, the Gulf of Mexico, the Persian Gulf and elsewhere.

Extraction

After an initial exploration, wells are drilled to confirm the existence of a reservoir and its size. Once a reservoir of natural gas has been confirmed, production wells are drilled to extract the gas (see adjacent photograph). Once reached by a well, the gas comes up to the surface under its own pressure. Over time, the gas flow rate decreases, but may be maintained by drilling reinjection wells and using gas compressors to reinject some compressed gas downward into the reservoir.

Also over time, as gas is extracted, the pressure in the underground gas reservoir declines. When it decreases to below its "hydrocarbon dew point", any propane, butanes and pentanes and higher molecular weight hydrocarbons that it contains will partially condense into liquids. That formation of liquid hydrocarbons in a gas reservoir is called "retrograde condensation" because some of the gas condenses into a liquid under isothermal conditions instead of expanding or vaporizing when the pressure is decreased. The liquids thus formed may get trapped by depositing in the pores of the gas reservoir. One method of mitigating this problem is to reinject dried gas, free of condensate, to maintain the underground pressure and to allow re-evaporation and extraction of the condensate.

The upper adjacent photograph depicts the extent of a gas field and the many wells drilled to extract natural gas from the underground gas reservoir. The lower photograph depicts a typical gas drilling rig.

Sources of natural gas

Types of natural gas sources

Raw natural gas is presently extracted primarily from any of the conventional underground sources, namely petroleum crude oil wells, gas wells, and condensate wells.

Raw natural gas that comes from petroleum crude oil wells is typically termed associated gas. This gas can exist separate from the crude oil in the underground formation, or dissolved in the crude oil.

Raw natural gas from gas wells and from condensate wells contains little or no crude oil and is termed non-associated gas. Gas wells typically produce only raw natural gas, while condensate wells produce raw natural gas along with natural gas condensate.

Other non-conventional sources, such as listed below, are not yet being fully exploited:

  • Methane-rich gas can also come from methane deposits in the pores of some coal seams. Such gas is referred to as coalbed gas or coalbed methane (CBM) and it is also called sweet gas because it is relatively free of hydrogen sulfide. It consists mainly of methane with only trace quantities of ethane, nitrogen and carbon dioxide. It does not contain propane, butanes, pentanes or condensate.
  • Natural gas hydrate (also called "clathrate hydrate", "methane clathrate" or "methane hydrate) is a solid in which methane is trapped within the crystal structure of ice. Large deposits of methane clathrate have been found on Earth's ocean floors and in the arctic regions. The amount of the worldwide reserves of methane clathrates is poorly known, and estimates of those reserves have decreased many orders of magnitude since it was first recognized that methane clathrates could exist at the bottom of certain seas of the world. Estimates range from six to 29 times the worldwide proven natural gas reserves of 175.4 × 1012 m3 from conventional sources as shown in Figure 2. As of 2009, there was no available technology for economically extracting and exploiting methane from the hydrate reservoirs.

Biogas, landfill gas, town gas and syngas (produced by coal gasification) are man-made fuels. They are not natural gases.

Natural gas reserves

The world's total proven reserves of natural gas as of 2007 amounted to 175.4 × 1012 cubic metres; proven reserves are those quantities of natural gas, which, by analysis of geological and engineering data, can be estimated with a high degree of confidence to be commercially recoverable from a given date forward, from known reservoirs and under current economic conditions. The country having the largest proven reserves is Russia, with 44.7 × 1012 cubic metres (see Figure 2 below). As shown in Figure 1, Russia was also the world's largest natural gas producer in 2007.

The world's largest gas field is the offshore North Field in Qatar, estimated to have 25 × 109 cubic metres of gas in place. The second largest natural gas field is the South Pars Gas Field in Iranian waters in the Persian Gulf. Connected to Qatar's North Field, it has estimated reserves of 8 to 14 × 109 cubic metres.

Natural gas processing

There are a great many ways in which to configure the various unit processes used in the processing of raw natural gas.

The schematic block flow diagram below is a generalized, typical configuration for the processing of raw natural gas from non-associated gas wells. It shows how raw natural gas is processed into sales gas pipelined to the end user markets.

The block flow diagram below also depicts how processing of the raw natural gas yields these byproducts: 

  • Natural gas condensate
  • Sulfur
  • Ethane
  • Natural gas liquids (NGL): propane, butanes and C5+ (which is the commonly used term for pentanes plus higher molecular weight hydrocarbons)

Transportation and storage

The major means of transporting large volumes of natural gas overland is by pipelines and pipelines have been established as the preferred transportation mode for overland distances of up to about 4000 kilometres (2500 miles).

However, transporting significant amounts of natural gas across oceans in pipelines is too expensive and using ordinary seagoing tankers is not feasible because of the very large volume of a gas compared to its liquid volume. However, natural gas can be and has been cryogenically cooled to – 161 °C (– 258 °F), condensed into liquefied natural gas (LNG) and transported in large LNG sea-going carriers capable of carrying about 150,000 m3 of LNG at that temperature and at essentially atmospheric pressure. Transforming natural gas to a liquid is accompanied by a volume reduction of approximately 600 to 1. Thus, each large tanker can transport LNG which can be reconverted to 90,000,000 m3 of natural gas at atmospheric pressure when delivered at the end of its journey.

For LNG transportation by seagoing tankers, an LNG liquefaction plant is needed at the exporting origin and an LNG regasification plant is needed at the receiving LNG terminal. LNG transportation by seagoing LNG carriers has been established as the preferred technology for long distance transportation across oceans.

LNG carriers can be used to transport LNG across oceans, while tank trucks can carry LNG or compressed natural gas (CNG) over shorter distances. Sea transport using CNG carrier ships that are now under development may be competitive with LNG transport in specific conditions.

Compressed natural gas (CNG) is a substitute fuel for the use of gasoline, diesel fuel, or propane in automotive vehicles. CNG is natural gas that has been compressed to an absolute pressure of about 200 to 220 bar (2,900 to 3,190 psi) which results in reducing the gas volume to about 0.5% of its volume at atmospheric pressure. CNG is transported by rail or in tanker trucks overland to end-users or to distribution points such as pipelines for further transport. As of 2009, plans and designs have been developed for using seagoing CNG tankers for oversea transport in the near future and such transport is expected to be competitive to LNG oversea transport in LNG tankers.

Natural gas is often stored underground inside depleted gas reservoirs from previous gas wells, salt domes, or in storage tanks as LNG. The gas is stored during periods of low demand and extracted during periods of higher demand.

Uses of natural gas

See main article: Natural gas as fuel

Power generation

Natural gas is a major source of electricity generation through the use of gas turbines and steam turbines. Most grid peaking power plants and some off-grid engine-generators use natural gas. Particularly high efficiencies can be achieved through combining gas turbines with a steam turbine in combined cycle mode. Natural gas burns more cleanly than other fossil fuels, such as oil and coal, and produces less carbon dioxide per unit energy released. For an equivalent amount of heat, burning natural gas produces about 30% less carbon dioxide than burning fuel oil and about 45% less than burning coal. Combined cycle power generation using natural gas is thus the cleanest source of power available using fossil fuels, and this technology is widely used wherever gas can be obtained at a reasonable cost. Fuel cell technology may eventually provide cleaner options for converting natural gas into electricity, but as yet it is not price-competitive.

Domestic use

Natural gas is supplied to homes, where it is used for such purposes as cooking in natural gas-powered ranges and/or ovens, natural gas-heated clothes dryers, air conditioners and central heating. Home or other building heating may include boilers, furnaces, and water heaters. Compressed natural gas (CNG) is used in rural homes without connections to piped-in public utility services, or homes with portable food grills. However, due to CNG being less economical than liquified petroleum gas (LPG), the dominant source of rural gas is LPG. (LPG is a designation for liquified propane, liquified butane or a mixture of the two chemicals.)

Transportation fuel

Compressed natural gas (CNG) is a cleaner alternative to other automobile fuels such as gasoline (petrol) and diesel fuel. As of December 2008, the countries with the highest number of CNG vehicles, ranked numerically, were Pakistan, Argentina, Brazil, Iran and India. The energy efficiency is generally equal to that of gasoline engines, but lower compared with modern diesel engines. Gasoline/petrol vehicles converted to run on natural gas suffer because of the low compression ratio of their engines, resulting in a cropping of delivered power (by about 10 to 15 percent) while running on natural gas. CNG-specific engines, however, use a higher compression ratio due to this fuel's higher octane number of 120 to 130

Fertilizer

Natural gas is a major feedstock for the production of ammonia used in fertilizer production.

Hydrogen

Natural gas is also a major feedstock for the production of hydrogen via the steam-methane reforming (SMR) process. Hydrogen has various applications as a primary feedstock for the chemical industry, a hydrogenating agent, an important commodity for oil refineries, and a fuel source in hydrogen vehicles.

Petrochemicals

Natural gas is commonly used as a feedstock in the production of petrochemicals.

Safety

Processed natural gas is colorless and essentially odorless. Since it is also very flammable, it is important that natural gas leaks be detected before a fire or explosion occurs. For that reason, very small amounts of a mercaptan such as t-butyl mercaptan, with a strongly unpleasant smell, is added to the natural gas as an odorant. Such odorants are considered non-toxic in the extremely low concentrations at which they are used. Odorizing of natural gas began in the United States after the 1937 New London School explosion in the town of New London, Texas. The buildup of gas in the school's basement went unnoticed, killing three hundred students and faculty when it ignited.

In coal mines, where any coalbed methane that may be present has no odor, methane sensors are used, and mining equipment has been specifically developed to avoid ignition sources.

Explosions caused by natural gas leaks occur a few times each year. Individual homes, small businesses and boats are most frequently affected when an internal leak builds up gas inside the structure. Frequently, the blast will be enough to significantly damage a building but leave it standing. In such cases, the people inside tend to have minor to moderate injuries. Occasionally, the gas can collect in high enough quantities to cause a deadly explosion, disintegrating one or more buildings in the process. The gas usually dissipates readily outdoors, but can sometimes collect in dangerous quantities if weather conditions are right. However, considering the tens of millions of structures that use the fuel, the individual risk of using natural gas is very low.

Natural gas heating systems are a minor source of carbon monoxide (CO) deaths in the United States. According to the US Consumer Product Safety Commission, 56% of unintentional deaths from non-fire carbon monoxide poisoning were associated with engine-driven tools like gas-powered generators and lawn mowers. Natural gas heating systems accounted for 4% of these deaths. Improvements in natural gas furnace designs have greatly reduced CO poisoning concerns. Carbon monoxide detectors are also available that warn of unsafe levels of carbon monoxide and/or explosive gas (e.g. methane, propane).

References

 Notes

  • Orders of magnitude are powers of 10. One order of magnitude is factor of 10, two orders of magnitude is a factor of 100, three orders of magnitude is a factor of 1000, etc.
  • The "higher heating value" is also referred to as the "gross heating value" (GHV).
  • The "lower heating value"  is also referred to as the net heating value (NHV).
  • In the United Kingdom and some other countries, heating values are referred to as "caloric values".

 

Glossary

Citation

Beychok, M. (2013). Natural gas. Retrieved from http://www.eoearth.org/view/article/51cbf20c7896bb431f6a7a52

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