Coal-Bed Methane: Potential and Concerns

Introduction Stable production Decline Methane (natural gas), while perhaps Dewatering stage stage stage most closely related in our minds with Powder Methane pr petroleum, also occurs in association with Wasatch River oduction rates Plateau Basin VOLUME coal, the Nation’s most abundant fossil fuel resource. Conservative estimates Water production rates (Rice, 1997) suggest that in the contermi- TIME nous United States more than 700 trillion Coal fields Figure 2. Typical production curves for a coal-bed cubic feet (TCF) of coal-bed methane methane well showing relative volumes of methane and water through time. Modified from Kuuskraa and exists in place, with perhaps 100 TCF Figure 1. Major coal fields of the conterminous Brandenberg (1989). economically recoverable with existing United States. The Wasatch Plateau and the Powder technology—the equivalent of about a 5- River Basin are producing coal-bed methane and are being studied by the USGS. to trap heat and thus contributes to global year supply at present rates of use. Coal- warming. bed methane now accounts for about 7.5 can hold. In addition, much of the coal, percent of total natural gas production in and thus much of the methane, lies at USGS Activity the United States. shallow depths, making wells easy to drill The USGS research will provide Scientific understanding of, and pro- and inexpensive to complete. With greater information that is vital to understanding duction experience with, coal-bed depth, increased pressure closes fractures the potential for, and the problems of, methane are both in the early learning (cleats) in the coal, which reduces perme- developing coal-bed methane as a part of stages. Much is yet to be learned (1) ability and the ability of the gas to move the national energy mix. USGS studies about the controls on the occurrence and through and out of the coal. Exploration will— recoverability of coal-bed methane—the costs for coal-bed methane are low, and • Establish detailed models of the devel- geologic, geochemical, engineering, tech- the wells are cost effective to drill. opment and evolution of coal-bed nological, and economic factors, for Methane occurs in most coals, and the methane plays in the Wasatch Plateau example—and (2) about the environmen- location of the Nation’s coal resources is and Powder River Basin (fig. 1) and tal implications of developing the already well known (fig. 1). Methane • Improve assessment techniques for resource. The coal-bed methane industry from coal is thus an attractive resource. coal-bed methane resources. is still relatively young, and few studies Increased production of coal-bed exist of the development and evolution of methane, however, carries with it some Wasatch Plateau and Powder River Basin an individual coal-bed methane play (a technological and environmental difficul- The Wasatch Plateau, Utah, and the group of strata characterized by similar ties and costs. In a conventional oil or gas Powder River Basin, Wyoming and aspects of methane occurrence); thus, few reservoir, for example, gas lies on top of Montana, are two of the newest, most pro- models are available for planning the oil which, in turn, lies on top of water. An ductive areas of coal-bed methane activity development of coal-bed methane oil or gas well draws only from the petro- in the United States (fig. 1). The USGS is resources on a broader scale. Studies now leum that is extracted without producing a studying these regions to characterize the underway by the U.S. Geological Survey large volume of water. But water perme- geologic and hydrologic controls on coal- (USGS) are designed to develop such ates coal beds, and its pressure traps bed methane generation and production. models and to further our ability to assess methane within the coal. To produce This study will— accurately the potential U.S. coal-bed methane from coal beds, water must be • Determine the distribution, thickness, methane resource. drawn off first, lowering the pressure so rank, gas content, and burial history of methane can flow out of the coal and to coal beds. The Resource: Methane from Coal the well bore (fig. 2). This water, which is • Characterize the cleats that allow During coalification, large quantities commonly saline but in some areas can be methane to move through the bed. This of methane-rich gas are generated and potable, must be disposed of in an envi- work is instrumental to developing pre- stored within the coal on internal surfaces. ronmentally acceptable manner. Surface dictive models for recovery of coal-bed Because coal has such a large internal sur- disposal of large volumes of potable water methane. face area, it can store surprisingly large can affect streams and other habitats, and • Analyze the organic and geochemical volumes of methane-rich gas; six or seven subsurface reinjection makes production composition of the coal and how these times as much gas as a conventional nat- more costly. In addition, methane is a factors relate to gas content, composi- ural gas reservoir of equal rock volume greenhouse gas; in the atmosphere it acts tion, distribution, and recoverability.

U.S. Department of the Interior USGS Fact Sheet FS–123–00 U.S. Geological Survey October 2000 • Develop regional models of fluid flow water is allowed to flow into surficial to understand the variability of drainages or is put into evaporation ponds methane production and to help plan (fig. 4). In cold regions, it is possible to for the disposal of produced water. freeze the water in the winter, collect the • Analyze the composition of produced salts that separate out, and dispose of or waters to construct models that predict utilize them independently of the water, water quality and assess the effects of which can be discharged. injected waters on the strata into which Methane in the Atmosphere they are placed. Figure 4. Production-water evaporation pond. Methane in the atmosphere has been Photograph provided by Cyndi Rice, USGS. Coal-Bed Methane Assessment increasing at a rate of 1 percent per year Methodology during the past 15 years (Rice, 1997). and production of methane from coals The 1995 National Assessment of Natural systems—wetlands and swamps, prior to mining reduces the amount of United States Oil and Gas Resources for example, and decomposing forest methane released to the atmosphere dur- (Gautier and others, 1996) provided materials—account for about 40 percent ing the mining process. Producing assessments of only selected coal-bed of the methane released to the atmos- methane then can serve a double purpose methane plays. Now the USGS is devel- phere. The balance is the result of human in the campaign to reduce the release of oping new assessment tools and modify- activity, though only about 10 percent of gases that contribute to global warming. ing established methods to provide a more this is attributed to methane production; Increased production and use of coal- comprehensive assessment of coal-bed the remainder comes from such activities bed methane, however, require a new methane resources in the United States. as rice cultivation, livestock, landfills, and understanding of its origin and distribu- This study will— biomass burning. Production of methane tion within coals, new approaches that • Review coal-bed methane plays used in from coal beds may actually reduce will provide a variety of appropriate alter- the 1995 assessment and reanalyze methane emissions to the atmosphere by natives for the disposal of produced those plays as new data are incorporated, removing the gas that is otherwise waters, and engineering studies that will • Define possible new coal-bed methane released during coal mining. As part of augment the current understanding and plays, the President’s Climate Change Action recovery of this unique resource. Studies • Modify old and develop new assess- Plan, established in 1993, coal-mine oper- now underway as part of the USGS ment methodologies, and ators are being encouraged to recover and Energy Resources Program speak to all • Assess coal-bed methane resources of use coal-bed methane for energy. these issues. the United States. Methane Migration References Cited The Concerns In some areas, methane migration Gautier, D.L., Dolton, G.L., Takahashi, may have contaminated ground-water Produced Water K.I., and Varnes, K.L., eds., 1996, sources, and methane may have migrated In a coal-bed methane well, water is 1995 National Assessment of United into residential neighborhoods. The con- produced in large volumes, especially in States Oil and Gas Resources— trols on methane migration, however, are the early stages of production; as the Results, methodology, and supporting unclear. Some contamination may come amount of water in the coal decreases, gas data: U.S. Geological Survey Digital from migration of methane along natural production increases (figs. 2 and 3). The Data Series DDS–30, release 2, one fractures; some may come from older gas water must be disposed of safely. Most CD-ROM. wells that tapped reservoirs in sandstones frequently, water is reinjected into subsur- Kuuskraa, V.A., and Brandenberg, C.F., associated with the coals. Some may face rock formations. In some cases, the 1989, Coalbed methane sparks a new come from new coal-bed methane wells. energy industry: Oil and Gas Journal, Reports from the 1800’s document gas v. 87, no. 41, p. 49–56. Pump Water to bubbles in water wells, in streams, and in Rice, D.D., 1997, Coalbed methane—An Gas to separator fields after heavy rains; this evidence sug- untapped energy resource and an for reinjection pipeline or discharge gests that migration has always existed. It environmental concern: U.S. has now become a problem because of Geological Survey Fact Sheet new residential development near the FS–019–97. (Available only on the methane migration pathways. Studies by web at http://energy.usgs.gov/ the USGS will help clarify the nature of factsheets/Coalbed/coalmeth.html) methane migration. For more information, please contact: The Bottom Line The conterminous United States has Vito Nuccio estimated coal-bed methane resources of U.S. Geological Survey, MS 939 at least 700 TCF; about 100 TCF appears Box 25046 to be economically recoverable with exist- Denver Federal Center ing technology. Burning methane adds Denver, CO 80225–0046 Telephone: 303–236–1654 Figure 3. Diagram showing the production scheme considerably less carbon dioxide to the of gas and water for a typical coal-bed methane well. atmosphere than does the burning of coal, E-mail: [email protected]

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