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| Production of natural gas associated with coal beds accounts for about 7 percent of the total natural gas produced annually in the United States. Several important U.S. coal-bed gas-producing areas are in the Rocky Mountain region, where extensive shallow coal beds have produced or have the potential to produce coal-bed gas (fig.
1). We summarize the technically recoverable coal-bed gas resources of six geologic provinces in the Rocky Mountain region herein. |
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| Coal-Bed Gas Provinces of the Rocky Mountain
Region |
| The Rocky Mountain region contains several sedimentary prov-inces with extensive coal deposits and significant accumulations of coal-bed gas. This summary includes coal-bed gas resources in the Powder River Basin (Wyoming and Montana), Wind River Basin (Wyoming), Southwest Wyoming (Greater Green River Basin of Wyoming, Colorado, and Utah), Uinta-Piceance Basin (Colorado and Utah), Raton Basin (Colorado and New Mexico), and San Juan Basin (Colorado and New Mexico). Other provinces in the Rocky Moun-tain region may contain significant coal-bed gas resources, but these resource estimates are not available at this time. |
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| What is Coal-Bed Gas? |
| Coal-bed gas is natural gas associated with, and sourced by, coal. Coal-bed gas is mainly composed of methane with variable amounts of ethane, nitrogen, and carbon dioxide. Methane can be generated by biogenic (microbial) or thermogenic (thermal) processes. Bio-genic methane is formed when microbes act upon coals very near the surface, whereas thermogenic methane is formed as the temperature of a coal bed increases with increasing depth of burial. Generally, the amount of thermogenic gas in a coal increases with higher tempera-tures. Methane gas occurs as gas adsorbed onto coal surfaces, as free gas in fractures, cleats, or other porosity, and as gas dissolved in ground water within the coal beds. The amount of methane gas generated in a particular coal bed depends upon the depth of burial and related pressure, the temperature to which a coal has been raised, coal thickness, and coal composition. To produce coal-bed gas, the water in the coal must be removed by pumping, allowing the pressure in the coal to drop sufficiently for the gas to flow. Water produced during the coal-bed methane operation varies in quantity and quality among, and within, coal-bed methane plays. In some plays, the produced water may be sufficiently fresh to discharge onto the surface, whereas in other plays the water may require special
Printed treatment and (or) reinjection. The necessity of proper disposal of produced water increases the technological complexity of coal-bed gas production and results in water production being a governing factor in the production process. |
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Figure 1. Selected coal-bed gas provinces of the Rocky Mountain region assessed for this study.
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| How is Coal-Bed Gas Recovered? |
| Once a coal bed is known to contain recoverable coal-bed gas through a program of testing and analysis of exploratory wells, other wells are drilled into the coal to produce the gas (fig.
2). Each well is fitted with a pump to first remove water from the coal, a process that may take months or even years to complete. Some wells may require artificial fracturing to induce sufficient permeability for the water and gas to move out of the coal. As the coal is de-watered, methane gas begins to be produced with the water, and the gas is separated from the water at the surface. A coal bed may extend beneath a large area of a province, and because a single well only recovers the gas from a small part of the coal bed, many hundreds to thousands of wells may be required to recover the gas. |
| Assessment of Coal-Bed Gas |
| The U.S. Geological Survey National Oil and Gas Resource Assessment
Team (1995) and Gautier and others (1996) defined and assessed the
major coal-bed gas accumulations of the United States. The USGS has
an ongoing effort to reassess the coal-bed gas resources of the United
States. The Powder River Basin, Uinta-Piceance Basin, San Juan Basin,
and SW Wyoming provinces were |
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