Evaluating Coal-Bed
Methane Commercial Viability in
Northern and Central Appalachian Basins
David Decker
According to studies conducted by the Gas Research Institute and the U.S. Department of Energy, approximately 109 tcf of pipeline quality gas occluded onto coals in the Northern and Central Appalachian basins. Commercial viability of this large resource will depend on coal permeability, pressure, saturation, and gas content.
Coal-bed
methane, as with any gas exploration program, requires prediction of
the optimum reservoir properties essential in achieving commercial production.
However, geologic methods for predicting ideal coal reservoir properties have
been modified from conventional exploration practices. Gas-in-place estimates
are determined from total coal thickness and gas content of the coal. Coal gas
content may be measured directly from coal core using modified techniques from
the coal mining industry. Where direct measurements are not available, source
rock evaluation techniques may be used to estimate gas contents. Geologic
techniques, such as curvature analysis and remote-imagery fracture analysis, can
be applied effectively to determine areas of structurally enhanced coal
permeability. Local knowledge of in-situ stresses are necessary to evaluate
whether these fracture systems are open. Determining whether coal cleats are
currently saturated with water or gas can only be evaluated by close examination
of drilling records from wells penetrating coal objectives. Coal reservoir
pressure may be established using the available drill-stem and production tests
or from hydrologic data.
Aggressive coal-bed
methane exploitation programs are under way in the
Warrior basin of Alabama and the San Juan basin of southwest Colorado and
northwest New Mexico. Ownership of coal-
bed
methane between coal lessor and oil
or gas lessor is being clarified in the Appalachian coalfields. Geologists
working these regions will shortly be challenged to select areas for commercial
coal degasification.
AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.