COALBED METHANE RESOURCES IN PENNSYLVANIA: FROM OLD HAZARD TO NEW ENERGY

Coalbed methane (CBM), once considered a dangerous, undesirable waste product to underground miners in Pennsylvania and worldwide, has been overlooked and misunderstood as a resource. Interest in commercial development of Pennsylvania CBM grew out of federal research during the 1970's and 1980's.

A preliminary reconnaissance of gas content data from exploratory coal cores and pre-existing data implied that with greater depth and rank, the greater the gas content. The coal seams studied, ranging in age from the Middle Pennsylvanian Allegheny Formation to the Pennsylvanian-Permian Dunkard Group, were from the Main Bituminous field in the Northern Appalachian coal basin, and the Northern and Southern Anthracite fields. Additionally, a geologic mapping investigation of the coal-bearing intervals of southwestern Pennsylvania and northwestern West Virginia in relation to CBM potential showed that the Allegheny Formation and the basal part of the Monongahela Group (Pittsburgh coal) contain the most prospective methane-producing coals. Total in-place CBM resource estimates for this region are 51 Tcf (trillion cubic feet), according to the 1988 Gas Research Institute geologic assessment of natural gas from coal seams. The amount of recoverable CBM resources is projected at 11.48 Tcf by the U.S. Geological Survey oil and gas resource assessment of 1995.

A variety of factors affecting CBM production include the following: 1) tectonic/thermal history (coal rank and quality); 2) depth; 3) total coal thickness; 4) structural setting; 5) depositional and stratigraphic controls; 6) natural fractures, permeability, and porosity of coal and adjacent strata; 7) total gas content per coalbed; 8) hydrogeologic regime; 9) reservoir pressure; and 10) distance from outcrop/mining (although wells intended to extract lower Btu gob gas in active/abandoned mines are becoming more popular).

Pennsylvania has an approximate total of 75 CBM wells in commercial production (in addition to 45 new well permits). Overall target depths range from about 450 to 1450 ft. Production statistics for all CBM wells submitted from 1988 to 1994 yielded a total of 644,918 Mcf.