Introduction:

Natural gas is a gas comprising mainly of methane. It is found related with fossil fuels, in coal beds, as methane clathrates, and is formed by methanogenic organisms in marshes, bogs and landfills. This is a significant fuel source, a main feedstock for fertilizers and a potent greenhouse gas. This is often informally termed to as simply gas, particularly whenever compared to other energy sources like electricity.

Origin of Natural Gas Accumulations:

The decomposition of organic material in the oxygen-poor environment, having the assistance of anaerobic bacteria, yields in the formation of methane. As organic matter is present in the younger sediments of the earth, therefore is methane. Whenever all of the existing methane could be collected, it could provide most of the world's energy for hundreds of years. Unluckily, most is too diffuse to be commercially recovered.  Natural gas, a hydrocarbon mixture comprising mainly of methane and ethane, is derived from both land plant and marine organic matter. Over geologic time, nearly all natural gas reaches the surface of earth and is lost to the atmosphere. Whenever its upward migration is interrupted via a geologic trap (that is, an upwardly convex permeable reservoir rock sealed above through impermeable cap rock) commercial quantities of gas can accumulate. This gas is known as non associated gas. Commercial amounts of gas as well can accumulate as a gas cap above the oil pool or, if reservoir pressure is adequately high, dissolved in the oil.  This natural gas is known as associated gas.

Natural gas generation and migration takes place over an extensive vertical zone which comprises shallow biogenic gas, intermediate dissolved gas of the oil window and deep thermal gas. The production of biogenic methane needs anaerobic microbial activity and is limited to poorly drained swamps, a few lake bottoms, and marine atmospheres below the zone of active sulphate reduction. The gas of predominantly biogenic origin comprises more than 20 percent of global gas reserves. The mature phase of petroleum generation takes place at depths between around 6,500 and 16,000 feet, based on the geothermal gradient. At such temperatures and pressures, the complete range of hydrocarbons is produced in the oil window and significant amounts of thermal methane gas frequently are produced all along with the oil. Below around 9,500 feet, mainly wet gas which comprises liquid hydrocarbons is formed. In the post mature phase, beneath around 16,000 feet, oil is no longer stable and the major hydrocarbon product is thermal methane gas which is a product of the cracking of the existing liquid hydrocarbons.

The gas shows an initial low concentration and high dispersibility, making sufficient seals very significant to conventional gas accumulation. Due to the differences in physical properties of gas and oil, likewise sized oil traps have more recoverable energy (on a Btu basis) as compare to gas traps, however  more  than  three-quarters  of  the  in-place  gas  frequently  can  be recovered.  Less than one percent (1%) of the gas fields of the world are of huge size, originally having at least 3 trillion cubic feet of recoverable gas. These fields, though, all along by the related gas in giant  oil  fields,  account  for  around  80%  of  the  world's  proved  and produced gas reserves. Oil is mainly derived from the marine or Lancastrian source rocks; however, as gas can be derived from land plants too, all source rocks encompass the potential for gas generation.  Most of the large gas accumulations appear to be related by the coal deposits.

Unconventional Gas Accumulations:

The border line or boundary between conventional gas and unconventional gas resources is not well stated, as they result from a continuum of geologic conditions. Coal seam, shale and tight gas take place in rocks of low permeability and need special treatment for recovery. The method via which vegetation is transformed to coal over geologic time produces large amounts of natural gas. Much of this gas becomes concentrated as conventional gas deposits in the permeable sediments adjacent to the coal; however some gas remains in the coal as unconventional 'continuous' gas deposits. The coal doesn't make a continuous reservoir over the whole basin, however takes place in individual non-communicating coal seams separated via other strata.  Coal seams are compartmentalized gas reservoirs bounded via facies changes or faults and the coal itself outcomes extremely variable amounts of gas. The coal seams that are deeply buried show considerably reduced permeability and, therefore, reduced gas recoverability.

The productivity of coal seam gas well is mainly based on the reservoir pressure and water saturation. Multi-well patterns are essential to take away water from the coal and to establish a favorable pressure gradient. As the gas is adsorbed on the surface of the coal and trapped via reservoir pressure, initially there is low gas production and high water production. Thus, an additional expense associates to the disposal of coal bed water, which might be saline, acidic or alkaline. As production carries on, water production declines and gas production rises, before eventually starting a long decline. In common, though, coal seam gas recovery rates have been low and unpredictable. Average per-well conventional gas production in a mature gas-rich basin is around five times more than average per-well coal seam gas production. Therefore, several times as many wells have to be drilled in coal seams than in the conventional gas accumulations to accomplish identical gas recovery levels.

Large continuous gas accumulations are at times present in low permeability (tight) sandstones, siltstones, shale, sandy carbonates, limestone, dolomites and chalk. These gas deposits are generally categorized as unconventional due to their reservoir features differ from conventional reservoirs and they need stimulation to be produced economically.  The tight gas is contained in lenticular or blanket reservoirs which are relatively impermeable and can take place downdip from water-saturated rocks and cut across lithologic boundaries. They often have a big amount of in-place gas, however show low recovery rates. Gas can be economically recovered from the better quality continuous tight reservoirs via creating down hole fractures having explosives or hydraulic pumping.  The almost vertical fractures give a pressure sink and channel for the gas, making a larger collecting area in such a way that the gas recovery is at a faster rate. At times massive hydraulic fracturing is needed, by using a half million gallons of gelled fluid and a million pounds of sand to keep the fractures open subsequent to the fluid has been drained away.

Town Gas:

Town gas is the mixture of methane and other gases, mostly the highly toxic carbon monoxide which can be employed in a similar manner to natural gas and can be produced via treating coal chemically. This is a historic technology, still employed as 'best solution' in some local conditions; however coal gasification is not generally economic at current gas prices. Though, based on infrastructure considerations, it remains a future possibility.

Most of the town 'gas houses' positioned in the eastern United States in the late 19th and early 20th centuries were simple by-product of coke ovens which heated  bituminous coal in air-tight  chambers. The gas driven off from the coal was collected and distributed via town-wide networks of pipes to residences and other buildings where it was employed for cooking and lighting purposes. (Gas heating didn't come into widespread utilize until the last half of the 20th century.) The coal tar which collected in the bottoms of the gas house ovens was often employed for roofing and other water-proofing purposes, and as well, whenever mixed by sand and gravel, was employed for creating Bitumen for the surfacing of local streets.

Bio Gas:

Whenever methane-rich gases are made by the anaerobic decay of non-fossil organic matter (that is, biomass), these are termed to as biogas (or natural biogas). Sources of biogas comprise swamps, marshes and landfills, and also sewage sludge and manure by way of anaerobic digesters, in addition to enteric fermentation specifically in cattle.  

Methanogenic Archaea are accountable for all the biological sources of methane, some in   symbiotic relationships with other life forms, comprising termites, ruminants and cultivated crops. Methane discharged directly to the atmosphere would be considered as a pollutant, though; methane in the atmosphere is oxidized, producing carbon-dioxide and water. Methane in the atmosphere consists of a half life of seven years, signifying that every seven years, half of the methane present is transformed to carbon-dioxide and water.

Future sources of methane, the main component of natural gas comprise landfill gas, biogas and methane hydrate. Biogases and particularly landfill gas are already in used in certain areas; however their use could be greatly expanded. Landfill gas is a kind of biogas; however biogas generally refers to gas produced from the organic material which has not been mixed by other waste.

Landfill gas is made from the decomposition of waste in the landfills. If the gas is not eliminated, the pressure might get so high that it works its way to the surface, causing damage to the landfill structure, unpleasant odor, vegetation die-off and an explosion hazard.  The gas can be vented to the environment, flared or burned to generate electricity or heat.

Once water vapor is eliminated, around half of landfill gas is methane. Nearly all of the rest is carbon-dioxide; however there are as well small quantities of nitrogen, oxygen and hydrogen. There are generally trace amounts of hydrogen sulphide and siloxanes, however their concentration differs broadly. Landfill gas can't be distributed via natural gas pipelines unless it is cleaned up to the similar quality. It is generally more economical to combust the gas on site or in a short distance of the landfill by employing a dedicated pipeline. Water vapor is often eliminated, even if the gas is combusted on site.  The other non-methane components might   as well be eliminated in order to meet up emission standards, to prevent fouling of the equipment or for ecological considerations. Co-firing landfill gas having natural gas enhances combustion that lowers the emissions.

Biogas is generally produced by using agricultural waste materials, like unusable portions of plants and manure. Biogas can as well be generated via separating organic materials from waste that or else goes to landfills.

This is more proficient as compare to just capturing the landfill gas it generates. The use of materials that would otherwise produce no income or even cost money to get rid of enhances the profitability and energy balance of the biogas production.

Anaerobic lagoons generate biogas from manure, whereas biogas reactors can be employed for manure or plant parts. Similar to landfill gas, biogas is generally methane and carbon-dioxide, having small amounts of nitrogen, oxygen and hydrogen. Though, by the exception of pesticides, there are generally lower levels of contaminants.

Hydrates:

Big quantities of natural gas (mainly methane) exist in the form of hydrates under sediment on offshore continental shelves and on land in the arctic regions which experience permafrost like those in Siberia (hydrates need a combination of high pressure and low temperature to form).  Though, as at 2009, no technology has been developed to produce natural gas economically from the hydrates.

Natural Gas Production:

The figure shown below is a schematic block flow diagram of a typical natural gas processing plant. It exhibits the different unit processes employed to transform raw natural gas into sales gas pipelined to the end user markets.

Fig: Flow diagram of a typical Natural gas processing plant 

The block flow diagram as well represents how processing of the raw natural gas yields via product sulphur, by product ethane and natural gas liquids (NGL) propane, butane and natural gasoline (represented as pentanes +).

Uses of Natural Gas:

1) Power Generation:

The natural gas is a main source of electricity generation via the use of gas turbines and steam turbines. Most of the grid peaking power plants and some off-grid engine-generators make use of natural gas. This is due to high efficiencies can be accomplished via combining gas turbines having a steam turbine in the combined cycle mode. Natural gas burns more cleanly than other fossil fuels, like oil and coal, and generates less carbon-dioxide per unit energy discharged or liberated.

For an equivalent quantity of heat, burning natural gas produces around 30% less carbon-dioxide than burning petroleum and around 45% less as compare to burning coal. Combined cycle power generation by using natural gas is therefore the cleanest source of power available by using fossil fuels and this technology is broadly employed wherever gas can be acquired at a reasonable cost. Fuel cell technology might ultimately give cleaner options for transforming natural gas into electricity; however it is not price-competitive.

2) Domestic Use:

The natural gas is supplied to homes, where it is employed for such purposes as cooking in natural gas-powered engines and/or ovens, natural gas-heated clothes dryers, heating/cooling and central heating.  Home or other building heating might comprise boilers, furnaces and water heaters. Methane as well termed as compressed natural gas (CNG) is employed in rural homes without connections to piped-in public utility services, or by portable grills. Though, because of CNG being less economical than liquefied natural gas, LPG (Propane) is the dominant source of the rural gas.

3) Transportation Fuel:

Compressed natural gas (CNG) is a cleaner alternative to other automobile fuels like gasoline (petrol) and diesel.  The energy efficiency is usually equivalent to that of gasoline engines, however lower compared with modern diesel engines. Gasoline or petrol vehicles transformed to run on natural gas suffer due to the low compression ratio of their engines, resultant in a cropping of delivered power as running on natural gas (10 to 15%). CNG-specific engines, though, make use of a higher compression ratio because of its fuel's higher octane number of 120-130.

4) Fertilizer:

Natural gas is a main feedstock for the production of ammonia, through the Haber process, for utilization in the fertilizer production.

5) Aviation:

Russian aircraft manufacturer, Tupolev claim that at current market prices, an LNG-powered aircraft would cost around 5,000 rubles (~ $218/ £112) less to operate per ton, approximately equivalent to 60%, having considerable reductions to carbon monoxide, hydrocarbon and nitrogen oxide emissions.

The benefits of liquid methane as a jet engine fuel are that it consists of more particular energy than the standard kerosene mixes and that its low temperature can assist to cool the air that the engine compresses for greater volumetric efficiency, in effect substituting an intercooler. Alternatively, it can be employed to lower the temperature of the exhaust.

6) Hydrogen:

Natural gas can be employed to produce hydrogen, with one common process being the hydrogen reformer. Hydrogen consists of different applications: it is a main feedstock for the chemical industry, a hydrogenating agent, a significant commodity for oil refineries, and a fuel source in the hydrogen vehicles.

7) Others:

Natural gas is as well employed in the manufacture of fabrics, glass, steel, paint, plastics and other products.

Storage and Transport:

The main difficulty in the use of natural gas is transportation and storage due to its low density. Natural gas pipelines are economical, however are impractical across the oceans.

LNG carriers can be employed to transport liquefied natural gas (that is, LNG) across oceans, whereas tank trucks can carry liquefied or compressed natural gas (that is, CNG) over shorter distances. Sea transport by using CNG carrier ships which are now under development might be competitive with the LNG transport in particular conditions.

For LNG transport, a liquefaction plant is required at the exporting end and regasification   equipment at the receiving terminal.  Shipborne regasification equipment is as well practicable.  LNG transportation is established as the favored technology for long distance, high volume transportation of natural gas, while pipeline transport is favored for transport for distances up to 4.000 km overland and around half that distance overseas.  Though,  for  CNG  transport,  high  pressure, usually  above  200  bars,  is  employed.  Compressors and decompression equipment are less capital intensive and might be economical in smaller unit sizes than liquefaction or regasification plants. For CNG mode, the crucial problem is the investment and operating cost of carriers. Natural gas trucks and carriers might transport natural gas directly to end-users, or to distribution points like pipelines for additional transport.

In the  past,  the  natural  gas  that  was  recovered  in  the  course  of recovering  petroleum  could  not  be  profitably  sold,  and  was  only burned at the oil field (termed as flaring). This wasteful practice is now illegal in numerous countries. Furthermore, companies now recognize that value for the gas might be achieved by LNG, CNG or other transportation processes to end-users in the future. The gas is now re-injected back to the formation for later recovery. This as well helps oil pumping by keeping underground pressures higher. The natural gas is employed to produce electricity and heat for the desalination. Likewise, some of the landfills which as well discharge methane gases have been set up to capture the methane and produce electricity.

Natural gas is most of the time stored underground within depleted gas reservoirs from previous gas wells, salt domes or in tanks as liquefied natural gas. The gas is injected throughout periods of low demand and extracted throughout periods of higher demand. Storage close to the ultimate end-users assists to best meet up volatile demands; however this might not for all time be practicable.

With 15 nations accounting for 84% of the universal production, access to natural gas has become an important factor in international economics and politics. In this respect, control over the pipelines is a main strategic factor.

Environmental Effect:

1) Climate Change:

Natural gas is often illustrated as the cleanest fossil fuel, producing less carbon-dioxide per joule delivered as compare to either coal or oil, and far fewer pollutants than other fossil fuels.  Though, in absolute terms it does contribute substantially to global carbon emissions, and this contribution is projected to grow. According to the IPCC Fourth Assessment Report, in the year 2004 natural gas produced around 5,300 Mt/yr of CO₂ emissions, whereas coal and oil produced 10,600 and 10,200 correspondingly however by the year 2030, according to an updated version of the SRES B2 emissions scenario, natural gas would be the source of 11,000 Mt/yr, by coal and oil now 8,400 and 17,200 correspondingly. (Total global emissions for the year 2004 were anticipated at over 27,200 Mt). Moreover, natural gas itself is a greenhouse gas far more potent than carbon-dioxide whenever liberated into the atmosphere however is not of main concern because of the small amounts in which this takes place. Natural gas is usually comprised of methane, which consists of a radioactive forcing twenty times more than carbon-dioxide.  This signifies, though, a ton of methane in the atmosphere traps in as much radiation as 20 tons of carbon-dioxide. Carbon dioxide still gets the lion's share of attention over greenhouse gases as it is in much higher concentration.

2) Pollutants:

Natural gas generates far lower amounts of sulphur-dioxide and nitrous oxides as compare to any other fossil fuel.

Safety:

In any form, minute quantities of odorant like t-butyl mercaptan, having a rotting-cabbage-like smell, is added to or else colourless and nearly odorless gas, in such a way that leaks can be detected before a fire or explosion takes place. At times a related compound, thiophane is employed, by a rotten-egg smell. Adding odorant to natural gas start in the United States after the year 1937 New London School explosion.  The buildup  of  gas  in  the  school  went  unnoticed,  killing  three  hundred students in a faculty if it ignited. Odorants are considered non-toxic in the very low concentrations taking place in natural gas delivered to the end user.

In mines, where methane seeping from rock formations has no odor, sensors are employed, and mining apparatus have been specifically developed to ignore ignition sources, example: the Davy lamp.

Explosions caused due to natural gas leaks take place a few times each year. Individual homes,   small businesses and boats are most often influenced whenever an internal leak builds up gas within the structure. Often, the blast will be adequate to significantly damage a building however leaves it standing. In such cases, the people within tend to encompass minor to moderate injuries. Occasionally, the gas can collect in high adequate quantities to cause a deadly explosion, disintegrating one or more buildings in the procedure. The gas generally dissipates readily outdoors, however can at times collect in dangerous quantities if weather conditions are right. Though, considering the tens of millions of structures which use the fuel, the individual risk of employing natural gas is extremely low.

Several gas fields yield sour gas having hydrogen sulphide (H₂S). This untreated gas is toxic. Amine gas treating which is an industrial scale procedure to remove acidic gaseous components is often employed to get rid of hydrogen sulphide from natural gas.

The extraction of natural gas (or oil) leads to reduction in pressure in the reservoir.  This in turn might lead to subsidence at ground level. Subsidence might influence ecosystems, waterways, sewer and water supply systems, foundations and so on.

The natural gas heating systems are a minor source of carbon monoxide deaths in the United States. According to the US Consumer Product Safety Commission (year 2008), 56% of unintentional deaths from non-fire CO poisoning were related by engine-driven tools such as gas-powered generators and lawn mowers. Natural gas heating systems accounted for 4% of these deaths. Enhancements in natural gas furnace designs have greatly reduced CO poisoning concerns. Detectors are as well available that warn of carbon monoxide and/or explosive gas (that is, methane, propane and so on).

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