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Coalbed methane (CBM or coal-bed methane), coalbed gas, coal seam gas (CSG), or coal-mine methane (CMM) is a form of
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
extracted from
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...
beds. In recent decades it has become an important source of energy in United States, Canada, Australia, and other countries. The term refers to
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
absorbed into the solid matrix of the coal. It is called 'sweet gas' because of its lack of
hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The under ...
. The presence of this gas is well known from its occurrence in underground coal mining, where it presents a serious safety risk. Coalbed methane is distinct from a typical
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...
or other conventional gas reservoir, as the methane is stored within the coal by a process called
adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
. The methane is in a near-liquid state, lining the inside of pores within the coal (called the matrix). The open fractures in the coal (called the cleats) can also contain free gas or can be saturated with water. Unlike much natural gas from conventional reservoirs, coalbed methane contains very little heavier hydrocarbons such as
propane Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as ...
or
butane Butane () or ''n''-butane is an alkane with the formula C4H10. Butane is a gas at room temperature and atmospheric pressure. Butane is a highly flammable, colorless, easily liquefied gas that quickly vaporizes at room temperature. The name but ...
, and no
natural-gas condensate Natural-gas condensate, also called natural gas liquids, is a low-density mixture of hydrocarbon liquids that are present as gaseous components in the raw natural gas produced from many natural gas fields. Some gas species within the raw natu ...
. It often contains up to a few percent
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
. Coalbed methane is generally formed due to thermal maturation of kerogen and organic matter. However, coal seams with regular
groundwater recharge Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs ...
see methane generated by microbial communities living in situ.


History

Coalbed methane grew out of venting methane from coal seams. Some coal beds have long been known to be "gassy," and as a safety measure, boreholes were drilled into the seams from the surface, and the methane allowed to vent before mining. Coalbed methane as a natural-gas resource received a major push from the US federal government in the late 1970s. Federal price controls were discouraging natural gas drilling by keeping natural gas prices below market levels; at the same time, the government wanted to encourage more gas production. The US Department of Energy funded research into a number of unconventional gas sources, including coalbed methane. Coalbed methane was exempted from federal price controls, and was also given a federal tax credit. In Australia, commercial extraction of coal seam gas began in 1996 in the
Bowen Basin The Bowen Basin contains the largest coal reserves in Australia. This major coal-producing region contains one of the world's largest deposits of bituminous coal. The Basin contains much of the known Permian coal resources in Queensland inclu ...
of
Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , establishe ...
.


Reservoir properties

Gas contained in coal bed methane is mainly methane and trace quantities of ethane,
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
,
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
and few other gases. Intrinsic properties of coal as found in nature determine the amount of gas that can be recovered.


Porosity

Coalbed
methane reservoirs Methane reservoirs on Earth are mainly found in * Oil and gas reservoirs as natural gas * Coalbeds *the seabed and the Arctic and other permafrost regions as methane clathrate *the atmosphere *decaying organic material with the exact distribution so ...
are considered as a dual-porosity reservoirs. Dual porosity reservoirs are reservoirs in which porosity related to cleats (natural fractures) are responsible for flow behavior and reservoir porosity of the matrix is responsible for the storage of gas. The
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...
of a coalbed methane reservoir can vary from 10%-20 %; However, the cleat porosity of the reservoir is estimated to be in the range of 0.1%-1 %


Adsorption capacity

Adsorption capacity of coal is defined as the volume of gas adsorbed per unit mass of coal usually expressed in SCF (''standard cubic feet'', the volume at standard pressure and temperature conditions) gas/ton of coal. The capacity to adsorb depends on the rank and quality of coal. The range is usually between 100 and 800 SCF/ton for most coal seams found in the US. Most of the gas in coal beds is in the adsorbed form. When the reservoir is put into production, water in the fracture spaces is pumped off first. This leads to a reduction of pressure enhancing desorption of gas from the matrix.


Fracture permeability

Fracture permeability acts as the major channel for the gas to flow. The higher the permeability, the higher the gas production. For most coal seams found in the US, the permeability lies in the range of 0.1–50 milliDarcys. The permeability of fractured reservoirs changes with the stress applied to them. Coal displays a stress-sensitive permeability and this process plays an important role during stimulation and production operations.S. Saurabh, S. Harpalani, V.K. Singh, Implications of stress re-distribution and rock failure with continued gas depletion in coalbed methane reservoirs, International Journal of Coal Geology, Volume 162, 2016, Pages 183-192, ISSN 0166-5162, https://dx.doi.org/10.1016/j.coal.2016.06.006. Fracture permeability in Coalbed methane reservoir tends to increase with depletion of gas; in contrast to conventional reservoirs. This unique behavior is because of shrinking of coal, when methane is released from its matrix, which results in opening up of fractures and increased permeability. It is also believed that due to shrinkage of coal matrix at lower reservoir pressures, there is a loss of horizontal stress in the reservoir which induces in-situ failure of coal. Such a failure has been attributed to sudden decrease in the fracture permeability of the reservoir


Thickness of formation and initial reservoir pressure

The thickness of the formation may not be directly proportional to the volume of gas produced in some areas. For example, it has been observed in the Cherokee Basin in Southeast Kansas that a well with a single zone of of pay can produce excellent gas rates, whereas an alternative formation with twice the thickness can produce next to nothing. Some coal (and shale) formations may have high gas concentrations regardless of the formation's thickness, probably due to other factors of the area's geology. The pressure difference between the well block and the sand face should be as high as possible as is the case with any producing reservoir in general.


Other properties

Other affecting parameters include coal density, initial gas-phase concentration, critical gas saturation, irreducible water saturation, relative permeability to water and gas at conditions of Sw = 1.0 and Sg = 1-Sw irreducible respectively.


Extraction

To extract the gas, a steel-encased hole is drilled into the coal seam below ground. As the pressure within the coal seam declines due to natural production or the pumping of water from the coalbed, both gas and ''produced water'' come to the surface through tubing. Then the gas is sent to a compressor station and into natural gas pipelines. The produced water is either reinjected into isolated formations, released into streams, used for irrigation, or sent to evaporation ponds. The water typically contains dissolved solids such as sodium bicarbonate and
chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...
but varies depending on the formation geology. Coalbed methane wells often produce at lower gas rates than conventional reservoirs, typically peaking at near per day (about 0.100 m³/s), and can have large initial costs. The production profiles of CBM wells are typically characterized by a " negative decline" in which the gas production rate initially increases as the water is pumped off and gas begins to desorb and flow. A dry CBM well is similar to a standard gas well. The methane desorption process follows a curve (of gas content vs. reservoir pressure) called a
Langmuir isotherm The Langmuir adsorption model explains adsorption by assuming an adsorbate behaves as an ideal gas at isothermal conditions. According to the model, adsorption and desorption are reversible processes. This model even explains the effect of pressu ...
. The isotherm can be analytically described by a maximum gas content (at infinite pressure), and the pressure at which half that gas exists within the coal. These parameters (called the Langmuir volume and Langmuir pressure, respectively) are properties of the coal, and vary widely. A coal in
Alabama (We dare defend our rights) , anthem = " Alabama" , image_map = Alabama in United States.svg , seat = Montgomery , LargestCity = Huntsville , LargestCounty = Baldwin County , LargestMetro = Greater Birmingham , area_total_km2 = 135,7 ...
and a coal in
Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of the ...
may have radically different Langmuir parameters, despite otherwise similar coal properties. As production occurs from a coal reservoir, the changes in pressure are believed to cause changes in the porosity and permeability of the coal. This is commonly known as matrix shrinkage/swelling. As the gas is desorbed, the pressure exerted by the gas inside the pores decreases, causing them to shrink in size and restricting gas flow through the coal. As the pores shrink, the overall matrix shrinks as well, which may eventually increase the space the gas can travel through (the cleats), increasing gas flow. The potential of a particular coalbed as a CBM source depends on the following criteria. Cleat density/intensity: cleats are joints confined within coal sheets. They impart permeability to the coal seam. A high cleat density is required for profitable exploitation of CBM. Also important is the maceral composition: maceral is a microscopic, homogeneous, petrographic entity of a corresponding sedimentary rock. A high vitrinite composition is ideal for CBM extraction, while inertinite hampers the same. The rank of coal has also been linked to CBM content: a vitrinite reflectance of 0.8–1.5% has been found to imply higher productivity of the coalbed. The gas composition must be considered, because natural gas appliances are designed for gas with a heating value of about 1,000 BTU (
British thermal unit The British thermal unit (BTU or Btu) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. The modern SI ...
s) per cubic foot, or nearly pure methane. If the gas contains more than a few percent non-flammable gases such as
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
or
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
, either these will have to be removed or it will have to be blended with higher-BTU gas to achieve ''pipeline quality''. If the methane composition of the coalbed gas is less than 92%, it may not be commercially marketable.


Environmental impacts


Methane

As with all carbon-based fossil fuels, burning coalbed methane releases carbon dioxide (CO2) into the atmosphere. Its effect as greenhouse gas was first analyzed by chemist and physicist Svante Arrhenius. CBM production also entails leaks of fugitive methane into the atmosphere. Methane is rated as having 72 times the effect on global warming per unit of mass than CO2. over 20 years, reducing to 25 times over 100 years and 7.5 times over 500 years. IPCC Fourth Assessment Report
Table 2.14, Chap. 2, p. 212
/ref> Analysis of
life-cycle greenhouse gas emissions of energy sources Greenhouse gas emissions are one of the environmental impacts of electricity generation. Measurement of life-cycle greenhouse gas emissions involves calculating the global-warming potential of energy sources through life-cycle assessment. These ...
indicates that generating electricity from CBM, as with conventional natural gas, has less than half the greenhouse gas effect of coal. In the United States, methane escaping from coal during mining amounts to seven percent of total methane emissions. Recovery of coal mine methane in advance of mining is seen as a major opportunity to reduce methane emissions. Companies like
CNX Resources CNX Resources Corporation is a natural gas company based in Pittsburgh with operations in the Appalachian Basin, primarily in the Marcellus Shale and Utica Shale in Pennsylvania, Ohio and West Virginia. It also develops coalbed methane properties ...
have methane abatement programs to reduce greenhouse gas emissions from active and closed mines.


Infrastructure

CBM wells are connected by a network of roads, pipelines, and compressor stations. Over time, wells may be spaced more closely in order to extract the remaining methane.


Produced water

The
produced water Produced water is a term used in the oil industry or geothermal industry to describe water that is produced as a byproduct during the extraction of oil and natural gas, or used as a medium for heat extraction. Produced water is the kind of brackis ...
brought to the surface as a byproduct of gas extraction varies greatly in quality from area to area, but may contain undesirable concentrations of dissolved substances such as salts, naturally present chemicals,
heavy metals upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead Heavy metals are generally defined as ...
and
radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transfer ...
s. In many producing regions the water is treated, such as through a Reverse Osmosis plant and used beneficially for irrigation, water for livestock, urban and industrial uses, or dust suppression.


Pilliga Scrub

In 2012 Eastern Star Gas was fined for "discharging polluting water containing high levels of salt into Bohena Creek" in the Pilliga Scrub. There were "16 spills or leaks of contaminated water" including "serious spills of saline water into woodland and a creek." In 2012, a NSW Legislative Council inquiry criticised the use of open holding ponds, recommending that "the NSW Government ban the open storage of produced water."


Powder River Basin

Not all coalbed methane produced water is saline or otherwise undesirable. Water from coalbed methane wells in the Powder River Basin of Wyoming, US, commonly meets federal drinking water standards, and is widely used in the area to water livestock. Its use for irrigation is limited by its relatively high sodium adsorption ratio.


Groundwater

Depending on aquifer connectivity, water withdrawal may depress aquifers over a large area and affect groundwater flows. In Australia, the CBM industry estimates extraction of to of groundwater per year; while the
National Water Commission The National Water Commission (NWC) was an independent statutory authority in Australia established by the ''National Water Commission Act 2004'' to implement the National Water Initiative and reform the broader national water agenda. The agency ...
estimates extraction above a year.


Power generation

In 2012, the
Aspen Skiing Company The Aspen Skiing Company, known locally as Ski Co, is a commercial enterprise based in Aspen, Colorado. The Aspen Skiing Company operates the Aspen/Snowmass resort complex, comprising four ski areas: Aspen Mountain, Aspen Highlands, Buttermilk, a ...
built a 3-megawatt methane-to-electricity plant in Somerset, Colorado at Oxbow Carbon's Elk Creek Mine.


Coalbed methane producing areas


Australia

Coal Seam Gas resources are in the major coal basins in Queensland and New South Wales, with further potential resources in South Australia. Commercial recovery of coal seam gas (CSG) began in Australia in 1996. As of 2014, coal seam gas, from Queensland and New South Wales, made up about ten percent of Australia's gas production. Demonstrated reserves were estimated to be 33 trillion cubic feet (35 905 petajoules) as of January 2014. *
Bowen Basin The Bowen Basin contains the largest coal reserves in Australia. This major coal-producing region contains one of the world's largest deposits of bituminous coal. The Basin contains much of the known Permian coal resources in Queensland inclu ...
* Surat Basin *
Sydney Basin The Sydney Basin is an interim Australian bioregion and is both a structural entity and a depositional area, now preserved on the east coast of New South Wales, Australia and with some of its eastern side now subsided beneath the Tasman Sea ...


Canada

In Canada,
British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, for ...
is estimated to have approximately of coalbed gas.
Alberta Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest T ...
, in 2013, was the only province with commercial coalbed methane wells and is estimated to have approximately of economically recoverable coalbed methane, with overall reserves totaling up to . Coalbed methane is considered a
non-renewable resource A non-renewable resource (also called a finite resource) is a natural resource that cannot be readily replaced by natural means at a pace quick enough to keep up with consumption. An example is carbon-based fossil fuels. The original organic ma ...
, although the
Alberta Research Council Alberta Innovates (AI) is an Alberta government provincial corporation whose appointed Board of Directors is accountable to the Minister of Jobs, Economy and Innovation and is responsible for promoting innovation in the province. AI was created ...
, Alberta Geological Survey and others have argued coalbed methane is a
renewable resource A renewable resource, also known as a flow resource, is a natural resource which will replenish to replace the portion depleted by usage and consumption, either through natural reproduction or other recurring processes in a finite amount of ti ...
because the bacterial action that formed the methane is ongoing. This is subject to debate since it has also been shown that the dewatering that accompanies CBM production destroys the conditions needed for the bacteria to produce methane and the rate of formation of additional methane is undetermined. This debate is currently causing a right of ownership issue in the Canadian province of
Alberta Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest T ...
, as only non-renewable resources can legally be owned by the province.


United Kingdom

Although gas in place in Britain's coal fields has been estimated to be 2,900 billion cubic meters, it may be that as little as one percent might be economically recoverable. Britain's CBM potential is largely untested. Some methane is extracted by coal mine venting operations, and burned to generate electricity. Assessment by private industry of coalbed methane wells independent of mining began in 2008, when 55 onshore exploration licences were issued, covering 7,000 square kilometers of potential coalbed methane areas. IGas Energy became the first in the UK to commercially extract coalbed methane separate from mine venting; as of 2012, the Igas coalbed methane wells at Doe Green, extracting gas for electrical generation, were the only commercial CBM wells in the UK. The use of CBM (in GWh) for electricity generation in the UK is as shown.


United States

United States coalbed methane production in 2017 was 1.76 trillion cubic feet (TCF), 3.6 percent of all US dry gas production that year. The 2017 production was down from the peak of 1.97 TCF in 2008. Most CBM production came from the Rocky Mountain states of Colorado, Wyoming, and New Mexico.


Kazakhstan

Kazakhstan could witness the development of a large coalbed methane (CBM) sector over the coming decades, according to industry professionals. Preliminary research suggests there may be as much as 900 billion m3 of gas in Kazakhstan’s main coalfields – 85% of all reserves in Kazakhstan.


India

Great Eastern Energy (GEECL) was the first company with a field development plan approved. With the completion of the drilling of 23 vertical production wells by GEECL, coalbed methane became commercially available in India on 14 July 2007 with CNG priced at ₹30 per kg. Initially 90% of the CBM would be distributed as CNG gas to fuel vehicles. GEECL is responsible for Southeast Asia's first CBM station and is also locating one in the
West Bengal West Bengal (, Bengali: ''Poshchim Bongo'', , abbr. WB) is a state in the eastern portion of India. It is situated along the Bay of Bengal, along with a population of over 91 million inhabitants within an area of . West Bengal is the fou ...
city of
Asansol Asansol is a (Tier-II) metropolitan city in the Indian state of West Bengal. It is the second largest and most populated city of West Bengal and the 33rd largest urban agglomeration in India. Asansol is the district headquarters of Paschim B ...
. Prashant Modi, President and Chief Operating Officer of GEECL, said, "With the nation requiring higher energy sources to sustain its development pace, we are confident that CBM will play an important role as one of the prime energy sources for the future generations."
Essar Group Essar Group is an Indian multinational conglomerate company, founded by Shashi Ruia and Ravi Ruia in 1969. As Essar Global Fund Limited (EGFL), the company controls a number of assets across the core sectors of energy (oil refining, oil an ...
's Essar Oil and Gas Exploration and Production Ltd.'s CBM portfolio includes 5 blocks. Currently, only one of them, Raniganj East, is currently operational. The others include Rajmahal in Jharkhand, Talcher and Ib Valley in Odisha and Sohagpur in Madhya Pradesh. The 5 blocks possess an estimated 10 trillion cubic feet (CBF) of CBM reserves.


See also

* Firedamp * Underground coal gasification * Western Canadian Sedimentary Basin


References


External links


The latest CBM news



U.S. Environmental Protection Agency page on CBM

British Columbia Ministry of Energy, Mines and Petroleum Resources guide to coalbed gas

Kansas Geological Survey guide to Coalbed Methane



Coal seam gas: by the numbers
Australian Broadcasting Corporation {{DEFAULTSORT:Coalbed Methane Alternative fuels Coal technology Methane Natural gas safety Unconventional gas