"Large amounts of methane-rich gas are generated and stored in coalbeds. Recently, commercial production of coalbed methane has been undertaken in the United States, and this development will be expanded to other countries. The U.S. Geological Survey is engaged in detailed studies of the controls, distribution, and recoverable resources of coalbed methane, particularly as they affect the release of gas and water to the environment."
Dudley Rice, U.S. Geological Survey
Coal is the most abundant energy source in the world, and it is a major source of hydrocarbons, particularly gas.
The coalification process, whereby plant material is progressively converted to coal, generates large quantities of methane-rich gas which are stored within the coal. The presence of this gas has been long-recognized due to explosions and outbursts associated with underground coal mining. Only recently has coal been recognized as a reservoir rock as well as a source rock, thus representing an enormous undeveloped "unconventional" energy resource. But production of coalbed methane is accompanied by significant environmental challenges, including prevention of unintended loss of methane to the atmosphere during underground mining, and disposal of large quantities of water, sometimes saline, that are unavoidably produced with the gas.
Methane from coalbed reservoirs can be recovered economically, but disposal of water is an environmental concern.
Most gas in coal is stored on the internal surfaces of organic matter. Because of its large internal surface area, coal stores 6 to 7 times more gas than the equivalent rock volume of a conventional gas reservoir. Gas content generally increases with coal rank, with depth of burial of the coalbed, and with reservoir pressure. Fractures, or cleats, that permeate coalbeds are usually filled with water; the deeper the coalbed, the less water is present, but the more saline it becomes. In order for gas to be released from the coal, its partial pressure must be reduced, and this is accomplished by removing water from the coalbed. Large amounts of water, sometimes saline, are produced from coalbed methane wells, especially in the early stages of production. While economic quantities of methane can be produced, water disposal options that are environmentally acceptable and yet economically feasible, are a concern. Water may be discharged on the surface if it is relatively fresh, but often it is injected into rock at a depth where the quality of the injected water is less than that of the host rock. Another alternative, not yet attempted, is to evaporate the water and collect the potentially saleable solid residues; this scheme might be feasible in regions having high evaporation rates.
Human activity accounts for about 60 percent of atmospheric methane emissions.
Analysis of gases trapped in ice indicates that the amount of atmospheric methane has more than doubled over the past three centuries and that it has increased at the rate of 1 percent per year during the past 15 years. This increase is correlative with the growth of human population and increase in human activity. Natural systems such as wetlands and decomposing forested areas account for about 40 percent of the methane released to the atmosphere. Significantly, the balance is largely the result of human activities such as rice cultivation (19 percent), livestock (11.5 percent), landfills (8 percent), biomass burning (11.5 percent), venting from oil and gas wells (4 percent), and coal mining (6 percent). Most experts agree that little reduction can be gained from altering livestock and rice cultivation methods; however, some important reductions can be achieved by improving techniques for recovery and utilization of methane from coal mines.
U.S. Geological Survey (USGS) researchers have undertaken a worldwide study to determine the contribution of coal beds to the atmospheric methane budget.
Pressure reduction during coal mining results in the release of coalbed methane. Most of this gas is presently emitted to the atmosphere through mine ventilation systems and by mine degasification systems as a safety measure. Nearly 10 percent of atmospheric methane resulting from human activity is derived from coal mining. Fully one-third of this amount is derived from underground coal mines in China. However, quantitative data are insufficient to satisfactorily assess methane emissions from coal resulting from natural causes. Estimates of total methane flux from coal mines around the world are based on figures obtained from mines in the United States, and therefore, are considered to be less reliable. USGS scientists are working in China under the auspices of the Global Environmental Facility of the United Nations, and in Poland and the Czech Republic, to make these countries aware of the environmental concerns and the enormous resource potential of coalbed methane.
As a part of the President's 1993 Climate Change Action Plan, coal mine operators are being encouraged to recover and use coalbed methane.
A goal of the Climate Change Action Plan is the reduction of atmospheric methane to the 1990 level by the year 2000. Reducing emissions from underground coal mines will be an important part of this effort, particularly in the Appalachian basin which accounts for about two-thirds of the emissions in the United States. These steps would mitigate environmental concerns and could provide significant economic benefits. There are, however, related political and legal issues that require resolution. The USGS, in cooperation with the Department of Energy and the Environmental Protection Agency, has the opportunity to play a vital role in addressing this important environmental issue.
USGS scientists are conducting studies of the contamination of shallow aquifers resulting from coalbed methane development.
Contamination of aquifers due to coalbed methane development represents another environmental concern. Such contamination has been a problem in the San Juan basin where major development of natural gas from sandstone reservoirs began in the 1950's, followed more recently by development of coalbed methane in the mid-1980's. Studies by USGS scientists, in cooperation with State and Federal regulatory agencies, indicate that there are multiple sources of contamination--some from natural causes along fractures and from shallow biogenic gas; some from older, deteriorating gas wells completed in sandstone reservoirs; and some from recently completed coalbed methane wells. In other areas, USGS scientists work with the same agencies to determine baseline conditions before large-scale development of coalbed methane takes place.
Major resources of coalbed methane are associated with immense amounts of coal, but are accompanied by significant environmental challenges.
Coalbed methane can be used as an energy source that is environmentally more acceptable than mining and combustion of coal. It can partly replace coal as a fossil energy source, and it sometimes occurs where other conventional resources of oil and gas are not present. Coalbed methane accumulations are widespread, commonly basinwide, and are characterized by large in-place resources. Although most wells will encounter gas in these widespread accumulations, production rates will be highly variable, even within a small area, because of the heterogeneous nature of coalbeds. The in-place coalbed methane resources of the United States are estimated to be more than 700 trillion cubic feet (Tcf), but less than 100 Tcf may be economically recoverable. Worldwide estimates of in-place resources are as much as 7,500 Tcf, but this number is uncertain because of the scarcity of basic data on coal resources and gas content. Basin-wide studies are needed to determine controls of the occurrence, availability, and recoverability of coalbed methane in the United States and other countries that need clean energy resources. Underground coal-mining areas, such as the Appalachian basin, should be emphasized because of the need to reduce atmospheric methane emissions. Most previous exploration and research efforts have been in the San Juan basin and the Black Warrior basin. However, since each coal-bearing basin has unique attributes, coalbed methane issues need to be studied separately in each basin.
Email: vnuccio@usgs.gov
This USGS Fact Sheet can be found at <URL:http://energy.usgs.gov/factsheets/Coalbed/coalmeth.html>
Maintained by L. Friedrich Last updated 17-Jan-1997