Introduction to Coalbed Methane
Coal is one of the most abundant non-renewable energy sources in the world, and Alaska has substantial coal resources. The majority of Alaska’s coal is located in the North Slope, followed by the Cook Inlet region, Interior Alaska (mainly Healy), the Alaska Peninsula, Copper River Basin, and numerous smaller basins and individual coal localities throughout the state. Until 1981, gas in coal seams, or coalbed methane (CBM), was considered a dangerous hazard to underground mining operations and was vented to the surface. Beginning in 1981, this ‘waste’ methane was successfully produced, initially from underground mines, as a viable energy resource. Today the production of coalbed methane from coal seams in the Lower 48 accounts for about 1.6 billion cubic feet of gas, or about 10% of the gas production in the United States.
How Does Coalbed Methane Work?
Coalbed methane is a clean-burning fuel, comparable in heating value (~1,000 Btu/scf) to conventional natural gas. Unlike conventional natural gas, with CBM the coal serves as the source rock and as the gas reservoir. Methane is formed along with water, nitrogen, and carbon dioxide when buried plant material is converted into coal by heat, pressure, and chemical processes over millions of years. This coalification process generates methane-rich gas, which often is held in pores, fractures, and spaces within the coal reservoir. As a reservoir, coal is a microporous hydrocarbon mineral capable of holding a large quantity of gas that is generated internally. This gas cannot be extracted from the coal reservoir unless these small micropores are connected through a well-developed fracture system called coal ‘cleats.’
Permeability is the measurement of how well a fluid or gas moves through a rock when the pores are connected through a cleat or fracture system. Even if there is sufficient coalbed gas, it cannot be produced if there are few fractures resulting in low permeability.
Coal must also reach critical threshold of thermal maturity or ‘coal rank’ before large volumes of thermogenic methane gas are generated. Lower rank lignite to sub-bituminous coals contain mostly biogenic gas. The gas results from bacterial action on organic material, in the same manner that methane is generated by bacteria in shallow garbage landfills.
It is important to note that there is no current production of biogenic gas from lignite coals because they lack a well-developed natural fracture system. Production of biogenic gas from very thick (50-200ft-thick) sub-bituminous coals is occurring in the Powder River Basin. There, gas contents average less than 35 cubic feet per ton. Most commercially viable coalbed methane production is from coals within the range of high volatile A bituminous to low volatile bituminous. These coals provide both optimum gas content (as high as 800 cubic feet of gas per ton) and well-developed, natural fracture cleat systems to provide a pathway to the well bore.
Finally, coal seams are usually saturated with water, with the hydrostatic pressure keeping the methane within the coal. Sufficient hydrostatic pressure must be present throughout the geologic history of the coal seam for gas to be retained. If pressure is reduced enough by erosion, uplift, or other means, the gas can escape from the coal leaving little or no gas behind.
Challenges in Coalbed Methane
Coalbed Methane in Alaska
In 2004, USGS and BLM, in association with DOE, DGGS, UAF, Doyon Native Corporation, and the village of Fort Yukon, organized and funded the drilling of a coalbed site to test for coalbed methane (CBM) developmental potential.
To read the full report, go here.
The results showed that Fort Yukon coal deposit does not have sufficient gas content or permeability to meet the energy needs of Fort Yukon. However, the project did show that slim hole drilling with portable rigs is a technically feasible method of CBM production in rural Alaska.1
|TECHNOLOGY SNAPSHOT: COALBED METHANE|
|Current production (Worldwide)||Worldwide resources estimated between 3,000 to 9000 tcf. USA production is about 1.6 bcf from about 20,000 wells in 2007.|
|Current production (Alaska)||None|
|Resource Distribution||Coalbed gas is generally limited to sub-bituminous to bituminous coal rank. Coalbed gas content is unknown in most Alaska coal basins.|
|Number of communities impacted||Unknown|
|Technology Readiness||Successful and established commercial CBM production in Lower 48, however no production or infrastructure in Alaska yet. Production through permafrost is unproven|
|Environmental Impact||Water disposal must not adversely affect surface waters or subsurface aquifers.|
|Economic Status||Uncertain in Alaska|
Alaska Specific Technology Challenges
Links, Resources and Documents
- Clark, Art (USGS) and Bob Fisk (BLM). "Results from the Coalbed Methane Expolration and Testing Project, Wainwright, Alaska 2007-2008." Presentation from 2008 Alaska Rural Energy Conference.
- Clough, Jim, Jim Albright, Charles Barker, Bob Fisk, Rick Miller, and David Ogbe. "Coalbed Methane for Remote Villages." September 2002. Presentation.
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