Coal-Bed Methane Production Potential in Complex Geologic Settings

Terry E. Mitchell, Steven P. Pappajohn

Establishing the economic production of coal-bed methane is a viable resource development opportunity as proven by the steady increase in the number of successful wells drilled during the past decade, primarily in the pioneering projects in the Warrior basin of Alabama and the San Juan basin of Colorado and New Mexico. If coal-bed methane is to contribute more significantly to the world's energy supply, however, production potential also will have to be demonstrated in basins characterized by a more complex tectonic history than those in which the initial successes have occurred. Steep bedding dips, heterogeneous hydrologic conditions, and related structural complexities previously have hindered the exploration for and the production of coal-bed methane resources in the m re disturbed basins.

The first attempt to produce coal-bed methane in a complex geologic setting was undertaken recently by Geo Trends, Inc., in western Washington where a closely spaced five-well pattern was drilled to an average depth of 3,900 ft and completed in an aggregate of 40 ft of medium to high-volatile bituminous coal comprised of three to five seams with 60° dips in a 1,200-ft stratigraphic interval of the Eocene Puget Group. Production modeling of the initial test results demonstrated an average per-well production potential of over 500 mcf/day of pipeline-quality natural gas (99% methane, up to 1,003 btu) from an in-place reserve base of greater than 14 bcf/mi². Full-scale development (on the order of 500 wells) of the seven Tertiary coal basins in Washington could result in he long-term production (average 20 years) of approximately 140 mmcf/day, a volume equivalent to nearly 50% of the current demand, which is supplied entirely by gas imported into this nonproducing state.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.