3/23/13 Coalbed methane - Wikipedia, the free encyclopedia

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Coalbed methaneFrom Wikipedia, the free encyclopedia

Coalbed methane (CBM or Coal Bed Methane),[1] coalbed gas, or coal mine methane (CMM) is a

form of natural gas extracted from coal beds.[2] In recent decades it has become an important source of energyin United States, Canada, and other countries. Australia has rich deposits where it is known as coal seam gas

(abbreviated "CSG"[1]).

The term refers to methane adsorbed into the solid matrix of the coal. It is called 'sweet gas' because of its lackof hydrogen sulfide. 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 or otherconventional gas reservoir, as the methane is stored within the coal by a process called adsorption. The methaneis in a near-liquid state, lining the inside of pores within the coal (called the matrix). The open fractures in thecoal (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 hydrocarbonssuch as propane or butane, and no natural gas condensate. It often contains up to a few percent carbon dioxide.Some coal seams, such as those in certain areas of the Illawarra Coal Measures in NSW, Australia, contain little

methane, with the predominant coal seam gas being carbon dioxide.[3]

Contents

1 Intrinsic properties affecting gas production

1.1 Porosity

1.2 Adsorption capacity

1.3 Fracture permeability

1.4 Thickness of formation and initial reservoir pressure

1.5 Other properties

2 Extraction

3 Environmental impacts

4 Reserves5 References

6 External links

Intrinsic properties affecting gas production

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3/23/13 Coalbed methane - Wikipedia, the free encyclopedia

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Gas contained in coal bed methane is mainly methane and trace quantities of ethane, nitrogen, carbon dioxideand few other gases. Intrinsic properties of coal as found in nature determine the amount of gas that can berecovered.

Porosity

The porosity of coal bed reservoirs is usually very small, ranging from 0.1 to 10%.

Adsorption capacity

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

Fracture permeability

As discussed before, the fracture permeability acts as the major channel for the gas to flow. The higher thepermeability, higher is the gas production. For most coal seams found in the US, the permeability lies in therange of 0.1 to 50 milliDarcies. 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 andproduction operations.

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 of1–2 ft of pay can produce excellent gas rates, whereas an alternative formation with twice the thickness canproduce next to nothing. Some coal (and shale) formations may have high gas concentrations regardless of theformation'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 casewith any producing reservoir in general.

Other properties

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

Extraction

Main article: Coal bed methane extraction

To extract the gas, a steel-encased hole is drilled into the coal seam (100–1500 meters below ground). As thepressure within the coal seam declines due to natural production or the pumping of water from the coalbed, bothgas and 'produced water' come to the surface through tubing. Then the gas is sent to a compressor station and

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into natural gas pipelines. The 'produced water' is either reinjected into isolated formations, released intostreams, used for irrigation, or sent to evaporation ponds. The water typically contains dissolved solids such assodium bicarbonate and chloride.

Coalbed methane wells often produce at lower gas rates than conventional reservoirs, typically peaking at near

300,000 cubic feet (8,500 m3) per day (about 0.100 m³/s), and can have large initial costs. The productionprofiles of CBM wells are typically characterized by a "negative decline" in which the gas production rate initiallyincreases as the water is pumped off and gas begins to desorb and flow. A dry CBM well is similar to astandard gas well.

The methane desorption process follows a curve (of gas content vs. reservoir pressure) called a Langmuirisotherm. The isotherm can be analytically described by a maximum gas content (at infinite pressure), and thepressure at which half that gas exists within the coal. These parameters (called the Langmuir volume andLangmuir pressure, respectively) are properties of the coal, and vary widely. A coal in Alabama and a coal inColorado 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 theporosity and permeability of the coal. This is commonly known as matrix shrinkage/swelling. As the gas isdesorbed, the pressure exerted by the gas inside the pores decreases, causing them to shrink in size andrestricting gas flow through the coal. As the pores shrink, the overall matrix shrinks as well, which mayeventually 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 isrequired for profitable exploitation of CBM. Also important is the maceral composition: maceral is amicroscopic, homogeneous, petrographic entity of a corresponding sedimentary rock. A high vitrinitecomposition 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 toimply higher productivity of the coalbed.

The gas composition must be considered, because natural gas appliances are designed for gas with a heatingvalue of about 1000 BTU (British thermal units) per cubic foot, or nearly pure methane. If the gas contains morethan a few percent non-flammable gases such as nitrogen or carbon dioxide, either these will have to beremoved or it will have to be blended with higher-BTU gas to achieve pipeline quality. If the methanecomposition of the coalbed gas is less than 92%, it may not be commercially marketable.

Environmental impacts

CBM wells are connected by a network of roads, pipelines, and compressor stations. Over time, wells may bespaced more closely in order to extract the remaining methane. Additionally, the produced water may containundesirable concentrations of dissolved substances. Water withdrawal may depress aquifers over a large area

and affect groundwater flows.[4]

The environmental impacts of CBM development are considered by various governmental bodies during the

permitting process and operation, which provide opportunities for public comment and intervention.[5]

Operators are required to obtain building permits for roads, pipelines and structures, obtain wastewater

(produced water) discharge permits, and prepare Environmental Impact Statements.[6] As with other naturalresource utilization activities, the application and effectiveness of environmental laws, regulation, andenforcement vary with location. Violations of applicable laws and regulations are addressed through regulatorybodies and criminal and civil judicial proceedings.

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Reserves

Coalbed methane reserve estimates vary; however a 1997 estimate from the U.S. Geological Survey predictsmore than 700 trillion cubic feet (20 trillion cubic metres) of methane within the US. At a natural gas price ofUS$6.05 per million Btu (US$5.73/GJ), that volume is worth US$4.37trillion. At least 100 trillion cubic feet

(2.8 trillion cubic metres) of it is economically viable to produce.[7]

In Canada, British Columbia is estimated to have approximately 90 trillion cubic feet (2.5 trillion cubic metres)of coalbed gas. Alberta, to date the only province with commercial coalbed methane wells, is estimated to have

approximately 170 trillion cubic feet (4.8 trillion cubic metres) of economically recoverable coalbed methane.[8]

Recent depressed natural gas prices have made CBM less economically viable compared to the past few years.

Currently it is considered a non-renewable resource, there is evidence by the Alberta Research Council, AlbertaGeological Survey and others showing coalbed methane is a renewable resource, because the bacterial actionthat formed the methane is ongoing. The assertion of being renewable, however, has itself become one of debatesince it has also been shown that the dewatering that accompanies CBM production destroys the conditions

needed for the bacteria to produce methane.[9] In addition, the rate of formation of additional methane isundetermined. This debate is currently causing a right of ownership issue in the Canadian province of Alberta, as

only non-renewable resources can legally be owned by the province.[10]

Production forecasting of CBM wells and fields can be performed using specialised Material Balance tools orNumerical simulators, as it requires an explicit coupling of the matrix gas diffusion process with the flow withinfractures (Darcy law applies).

References

1. ̂a b "Jargon Buster" (http://www.bg-group.com/Careers/graduates/howtoapply/Pages/JargonBuster.aspx) . BGGroup. http://www.bg-group.com/Careers/graduates/howtoapply/Pages/JargonBuster.aspx. Retrieved 18 July2010.

2. ^ Coal Gas (http://www.clarke-energy.com/gas-type/coal-gas/) , www.clarke-energy.com, retrieved25.11.2011

3. ^ "Briefing Note - Lord Mayor - Response to SRACSG - Coal Seam Methane"(http://www.nogasmininginsydney.com/Briefing%20Note%20-%20Lord%20Mayor-%20Response%20to%20SRACSG%20-%20Coal%20Seam%20Methane.pdf) .http://www.nogasmininginsydney.com/Briefing%20Note%20-%20Lord%20Mayor-%20Response%20to%20SRACSG%20-%20Coal%20Seam%20Methane.pdf. Retrieved 9 November 2011.

4. ^ Montana State University; Frequently Asked Questions; Coal Bed Methane (CBM)(http://waterquality.montana.edu/docs/methane/cbmfaq.shtml)

5. ^ State of Montana Department of Environment Quality; Coal Bed Methane; Federal, State, and Local Laws,Regulations, and Permits - That May Be Required(http://web.archive.org/web/20080521091129/http://www.deq.state.mt.us/coalbedmethane/Laws_regulations_permits.asp)

6. ^ State of Montana Department of Environment Quality; Final Statewide Oil and Gas EIS and ProposedAmendment of the Powder River and Billings RMPs(http://web.archive.org/web/20080521091200/http://www.deq.state.mt.us/coalbedmethane/finaleis.asp)

7. ^ "NETL: Future Supply - Coalbed Natural Gas" (http://www.netl.doe.gov/technologies/oil-gas/futuresupply/coalbedng/coalbed_ng.html) . Netl.doe.gov. http://www.netl.doe.gov/technologies/oil-gas/futuresupply/coalbedng/coalbed_ng.html. Retrieved 2011-12-21.

8. ^ John Squarek and Mike Dawson, Coalbed methane expands in Canada, Oil & Gas Journal, 24 July 2006,p.37-40.

9. ^ "Renewable Natural Gas? The Discovery of Active Methane Biogenesis in Coalbeds"

3/23/13 Coalbed methane - Wikipedia, the free encyclopedia

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(http://www.greencarcongress.com/2004/11/renewable_natur.html) . Green Car Congress. 2004-11-16.http://www.greencarcongress.com/2004/11/renewable_natur.html. Retrieved 2011-12-21.

10. ^ "TELUS, news, headlines, stories, breaking, canada, canadian, national"(http://www.mytelus.com/ncp_news/article.en.do?pn=tech&articleID=2970245) . Mytelus.com.http://www.mytelus.com/ncp_news/article.en.do?pn=tech&articleID=2970245. Retrieved 2011-12-21.

External links

U.S. Geological Survey page on CBM (http://energy.usgs.gov/factsheets/Coalbed/coalmeth.html)U.S. Environmental Protection Agency page on CBM (http://www.epa.gov/coalbed/)

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

(http://www.em.gov.bc.ca/subwebs/coalbedgas/)Kansas Geological Survey guide to Coalbed Methane

(http://www.kgs.ku.edu/PRS/publication/2004/AAPG/Coalbed/)

Coalbed Methane Gas (http://www.ingenieriadepetroleo.com/2012/10/coalbed-methane-gas.html/)

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