Assessment of Unconventional Coalbed Methane Resource Potential: The U.S. Experience

Schenk, Christopher J., Troy Cook, Richard M. Pollastro, U.S. Geological Survey, Denver, CO

Historically, the quantitative assessment of U.S. coalbed methane resources has been limited by insufficient coalbed methane production data. However, the past decade has seen a dramatic increase in the numbers of wells producing methane from coal beds in several basins in the U.S., including coals in the Powder River Basin, San Juan Basin, Raton Basin, Uinta-Piceance Basin, Appalachian Basin, Black Warrior Basin, and the Cherokee Platform. Today, coalbed methane production accounts for about 7 percent of total annual U.S. natu­ral gas production. The availability of exploration and production data from several thousand wells (many with several years of production data) has allowed us to construct decline curves, and to construct Estimated Ultimate Recovery (EUR) distributions for thousands of wells that produce gas from coals in these basins. The EUR distributions served as a quan­titative guide to the assessment of future coalbed methane resources in these U.S. basins, but these distributions might also be valuable as analog production curves for the assess­ment of coalbed methane in frontier areas, such as the Alaskan North Slope, or in other countries where coalbed methane production is limited. The proper application of these EUR distributions as analogs for assessment purposes depends strongly upon the careful geo­logic analysis of coal-bearing intervals in each frontier area, including coal composition, rank, thickness, and distribution; methane content; cleat density, orientation, and geometry; depth of burial and overburden thickness.