heike schellenberger

7/28/2019 Heike Schellenberger

1/10

Hydrocarbons from Coal Coalbed

Methane

Heike Schellenberger

Department of Geology, Bergakademie Freiberg/Saxony, Germany

Abstract. In the Western United States natural gas from coalbedmethane plays an important role and is mentioned to be the cleanestburning fossil fuel. The coalbed methane production has expandedenormously over the past decade. This paper gives an overview of whatcoalbed methane is and where the resources are located both in theUnited States and International, its historical development as well as howthey are extracted. Although CBM by now is mentioned to be a relativelyimportant natural energy source there are some problems, especiallydisposing produced water, which are also named in this paper. The last

part deals with a comparison of coalbed methane and conventionalnatural gas.


2/10

Heike Schellenberger - 2 -

Introduction

Hydrocarbons (CxHy) only consist of the elements carbon (C) and

hydrogen (H). They contain a carbon backbone where hydrogen atomsare bounded. Methane is the simplest hydrocarbon and is also calledswamp or marsh gas. It has one carbon atom and four hydrogen atoms(CH4).Coalbed Methane (CBM) is simply methane found in coal seams. It isgenerated either from a biological process as a result of microbial actionor from a thermal process as a result of increasing heat with depth of thecoal.With increasing coalification, coals become enriched in carbon as largeamounts of volatile matter rich in hydrogen and oxygen are released.Methane, carbon dioxide and water are the most important by-products ofthis devolatilization process. The generation of methane and associatedhydrocarbons is thermal in origin and occurs at ranks of high-volatile

bituminous and higher (shown in Figure 1).

Figure 1: Calculated amounts of gases generated from coal during coalification.Modified from Hunt (1979)

Most of the coalbed methane is stored within the molecular structure ofthe coal and some is stored in the fractures or cleats of the coal ordissolved in the water trapped in the fractures. Coals can generallygenerate more gas than they can absorb and store.

Methane gas is used to heat homes, generate electricity and as a fuel forcars, trucks and public transit.


3/10

- 3 - Hydrocarbons from Coal Coalbed Methane

History of Coalbed Methane

In history of mining, coalbed methane has considered to be one of thegreatest dangers to deep coal mining. The methane gas was intentionallyvented to prevent accidental explosions or asphyxiation, so commercial

extraction of coalbed methane was economically unfeasible. Coalbedmethane production now became practical because of modern extractionmethods.The first attempts to isolate and pipe gas from a coal mine were occurredin Great Britain as early as 1733. The first recorded successful use of avertical borehole to drain gas from virgin coal was occurred in theMansfield Colliery (Ruhr/Germany) in 1943.The first serious research concerning coalbed methane production in theUnited States occurred in the 1970s with a test project in the BlackWarrior basin in Alabama. In the 1980s the Gas Research Institute beganits coalbed methane explores. Their activities dealed with cooperative

well studies, reservoir engineering analysis, fracturing and completionwork as well as operational improvements and recompletion of wells. Anindicator for coalbed methane has emerged as a valuable energy resourceis the increased production of CBM in the Appalachian, Black Warrior,San Juan, Piceance, Powder River and Greater Green River Basins.Coalbed methane production increased to 858 billion cubic feet in 1994.The number of coalbed methane wells in the nation had grown from ahandful in 1982 to more than 6,600 in 1992. From 1992 to 1994 coalbedmethane production of the United States increased by 50 percent.

Coalbed Methane Production

In the United States coalbed methane is produced in commercialquantities since 1981. Coal as gas reservoir can be distinguished in threeways from other gas reservoirs. First: the gas is stored in the adsorbedstate on the surface of the coal; second: the average reservoir pressuremust be reduced before a production in significant quantities can bedone; third: water is usually presented in the reservoir and is normallyco-produced with the gas. These requires a low wellhead pressure,separation of gas and water at the surface, compression of gas to delivery

pressure and procedures to handle and dispose of produced water.

Production statistics in the U.S. show that the largest amount of gas isproduced in the San Juan basin (245 billion cubic feet in 1991). Inaddition coalbed gas is produced in the Black Warrior basin (66 billioncubic feet in 1991), Central Appalachian, Raton, Piceance, Greater GreenRiver, Powder River, Uinta (Utah) and Cherokee (Kansas) basins. Thetotal gas productions of these basins come to 310 billion cubic feet in1991.


4/10


Figure 2: Major Coalbed Methane Basins in the United States.Source: de Albuquerque S. (2002).

International interests in coalbed gas production was originally spurredbecause of mining related gas releases, but is now being viewed as arelatively inexpensive source of energy. Despite of this fact there areongoing activities of natural gas production from coal seams in thefollowing countries: Canada, Australia, China, Poland, United Kingdom,Zimbabwe, Spain, France, Belgium, Hungary and New Zealand (from

Boyer, 1992).In Table 1 the worldwide coalbed methane resources are estimated (fromBoyer 1992).

Table 1: World resources of coalbed gas (1991)

Country Gas Resources (trillion cubic feet)

China 700 - 2800

Soviet Union 1500 - 2800

Australia 350United States 400

Canada 500 - 2600

Poland 50

Europe 250

South Africa 140

Africa 100

TOTALS 3990 - 9490


5/10


Figure 3 illustrates a typical production curve for coalbed methane. Largeamounts of water are initially produced, but water production declines asreservoir pressure is reduced. The onset of gas production is dependenton the relationship between coal gas content, reservoir pressure and thesorption isotherm (McElhiney et al., 1989).

Figure 3: A typical production profile for a coalbed methane well.Source: Brown, W. T. (2002).

Production Equipment

The design of the surface production facilities begins with an area wheresuitable reservoir properties for gas production were determined, but

before drilling a large number of wells. The relationship of surfacefacilities and the location of wells needs to account the expected geologicand reservoir properties, environmental concerns and the topography ofthat area.A coalbed methane well field consists of different parts, which are

production wells, gathering lines, separators, compressors and

dehydration facilities (shown in Figure 4). Also roads, utilities and fieldoffices need to be there.In each development, water and gas from each well site are transported toa single treating site serving water disposal, gas treating, centralcompression and sales.


6/10


Figure 4: Schematic of a typical coalbed methane production facility.Adapted from McCormick Resources brochure, 1991

Typical Production Well Configuration

Coalbed gas production starts at the bottom of the well. CBM wells areoften drilled through several gas-containing formations that can be

perforated in a number of zones. The reservoir pressure is reduced bydrilling the borehole below the lowest producing zone to provide a sump.Thus water drains into this sump before being pumped to the surface. Gasand water are won with a configuration that provides an initiallyseparation in the well bore. This procedure has been adapted from oilfield applications and is known to be very effective. Water is normallylifted by pumping through a tubing string with a diameter of10 or 22 mmand gas is produced up the annulus between the water-production tubing

and the casing (e.g.: diameter of 180 or 200 mm in the San Juan basin).Besides producing water and gas there also solid materials, like coal finesor stimulation sand must be regarded, so different precautions are used toreduce the occurrence of fines migration. The well sump, for example isused for collecting solids to minimize debris that enter pumps or surfaceequipment. Additionally, at the pump intake a screen is used for reducingsolids that enter the production system. However, the screens can get

plugged with fines and so they require cleaning. Slow changes towellhead pressure during the operation as well as the sufficient clearance

between the tubing and casing also reduce the rate of fines migration.


7/10


Problems with Coalbed Methane Development

The problems with coalbed methane developments are composed ofseveral environmental and health hazards.Methods to dispose of produced water and treatment costs depend on the

amount of produced water and its quality. In some states the water-quality from a CBM well is like water from oil or gas well and only candisposed by deep well injection. Often water is discharged into surfacestream or rivers which minimize costs and affect a rapid grow rate ofindustry. This requires a water quality like any other industrial ormunicipal waste stream. For disposing water on this way a NationalPollution Discharge Elimination System permit (NPDES) must beobtained and is given by each state by themself. Dangers of streamdischarge are flooding the property of landowners, causing erosion,damaging soils and plants, as well as posing dangers to fish and aquaticlife. Additionally ways for water disposing are land application,

evaporation, off-site commercial disposal, reuse of water in hydraulicstimulation and reverse osmosis, but only on an experimental basis. Incold regions, it is possible to freeze water in the winter, collect the saltswhich have been separated from the water and dispose or utilize themseparately.Other problems are contamination of aquifers because of gas-migrationthrough vertical fissures; venting and seeping of methane and otherchemicals like carbon dioxide and hydrogen sulphide; underground firesexacerbated by coalbed methane development, when water is removed torelease the gas and oxygen gets in; air pollution for the reason that carbondioxide occur naturally with coalbed methane and many producers vent

this gas directly into the atmosphere; the surface production facilitiesleave scars that will last for decades, wildlife habitats get fragmented andmigration corridors are disrupted.

Coalbed Methane at San Juan basin

The San Juan Basin is located in south-western Colorado and north-western New Mexico. Its expanse amounts about 19,000 km2 andcontains 200 billion metric tons of coal resource and 1.4 trillion m3 ofcoalbed methane of the Upper Fruitland Formation coals. 70 to 850

billion m3 of this gas may be technically recoverable. Additionally coalresources and coalbed methane exists in the Upper Cretaceous MenefeeFormation.The coal thickness in the Fruitland Formation is about 6 to 24 m and thegas content ranges from 8 to 19 m3 per tonnes.The first commercial Fruitland coalbed methane well was drilled in 1953.The well is completed with a conventional technique, producing 1.4

billion cubic feet of gas over the last 30 years. In 1977 AMOCO beginsdevelopment of the Cedar Hill field using conventional open-holecompletions with gravel packs. Problems with coal fines plugging gravel

packs lead to adoption of cased-hole completions for subsequent wells.From 1983 to 1985 cased-hole completions remain predominanttechnique. Also in 1985 the MERIDIAN Company begins experimentingwith the open-hole cavity techniques. Two years later, due to


8/10


MERIDIANs success with the cavity technique, it gains acceptance asthe technique of choice in the high potential fairway region. Since 1988efforts to apply the open-hole cavity technique outside of the fairway

prove unsuccessful, and cased-hole completions remain the prevalenttechnique.

In Figure 5 the evolution of coalbed methane production in the San Juanbasin from 1988 to 2001 is represented. It is shown that the produced gasquantity grown up rapidly from some hundred to several thousandmillion cubic feet per day.

Figure 5: Coalbed methane production of the San Juan basin.Source: C. Cullicott et al. figured in Bryner, G. (2002).

Comparison of Conventional Natural Gas and Coalbed

Methane

Methane is a major component of natural gas, and coalbed methane canbe used in the same way as conventional gas. Conventional gas isgenerated in shale and limestone formations as a consequence of

temperature and pressure transforming organic matter into hydrocarbons.It migrates upward until it is trapped by a geologic fault or fold.Coalbed methane is stored in an adsorbed state on the surface of the coaland before it can be produced in significant quantities, the averagereservoir pressure must be reduced as well as water that is usually presentin the reservoir and normally co-produced with the coalbed methane.The competitiveness of coalbed methane with conventional natural gas isgiven by the rate of gas production, the production costs, markets andeconomies of scale. In conventional gas wells, production peaks earlyfollowed by declining over time whereas water production eventuallyincreases. At coalbed methane extraction, large quantities of water are

produced during the initial phase afterwards the water volume declines asthe pressure of the reservoir falls. The shape of the production curve is afunction of the production techniques that consists of well spacing,


9/10


reservoir permeability and pressure as well as water saturation (Figure 6).The length of time, which is necessary to achieve peaks while gas

production increases in low permeability reservoirs and increased welldensity. CBM wells usually produce gas at lower rates than conventionalgas wells. Also the cost of water disposal is significant less than to

conventional development. Coalbed methane development has a veryshort period of time wells produce gas. In general wells produce gasduring a length of 7 to 10 years.

Figure 6:Comparing coalbed methane and conventional natural gas development.Source: Brown, W. T. (2002).

The economically comparison of CBM and conventional natural gas isusing the following three criteria: gas reserves per well, gas reserves per$ 1 million of investment and finding costs per million cubic feet (Mcf).At Table 2 those criteria are confronted by comparing conventionalnatural gas with the main CBM producing areas in the United States ofthe Warrior basin and San Juan basin.

Table 2: Productivity and investment costs for conventional natural gas compared withthe equivalent costs for coalbed methane.Source: Law, B. E. and D. D. Rice. (1993).

Investment

Reserves / Costs

Reserves/Well $ 1 MM Investment ($/Mcf)

1. Conventional Natural Gas 2 Bcf 1.4 Bcf $0.68

2. Coalbed Methane

- Warrior Basin 0.5 Bcf 1.5 - 1.6 Bcf $0.60 - $0.80

- San Juan Basin 2 Bcf 3 - 4 Bcf $0.25 - $0.35


10/10

Heike Schellenberger - 10

Conclusion

The potential value of coalbed methane as a recoverable resource wasrecognized by Lawall & Morris in 1934 and Burke & Parry in 1936. Inthe U.S. it is produced in commercial quantities since 1981. In Europe it

became widely used throughout the coal fields in the 1940s.Coalbed methane is stored in the coal upon or within the molecularstructure or within the micropores and cleats. Normally, water is co-

produced and usually presented in the reservoir. After drilling the coalsseam, the water, and the gas are separately pumped up to the surface.Coalbeds have a large surface so they can store 6 to 7 times more gasthan the equivalent rock volume in a conventional gas reservoir. In spiteof some problems with CBM development, especially to dispose the

produced water, coalbed methane represents a major new internationalsource of natural gas.So future markets for natural gas are an increased use as industrial boiler

fuel, for electrical power generation, for mass transit in large cities, asfleet fuelling for governments and large companies as well as theconversion of private vehicles as compressed natural gas refuellingstations are built.

References

B. E. Law and D. D. Rice. Hydrocarbons from Coal. AAPG Studies in

Geology #38. The American Association of Petroleum Geologists. Tulsa,

Oklahoma, 1993.

M. Mastalerz, M.Glikson and S. D. Golding. Coalbed Methane:

Scientific, Environmental and Economic Evaluation. Kluwer Academic

Publishers, Dordrecht/Boston/London, 1999.

G. Bryner. Coalbed Methane Development in the intermountain West.

Natural Resources Law Center, University of Colorado School of Law,

July 2002.

C. Cullicott, C. Dunmire, J. Brown and C. Calwell. Coalbed Methane in

the San Juan Basin of Colorado and New Mexico. Ecos Consulting.

W. T. Brown and S. de Albuquerque. Coalbed Methane Conference.

NRLC. April 45, 2002

heike schellenberger

Documents