Coalbed methane (CBM) is produced commercially from Pennsylvanian-age coal beds in four states: Pennsylvania, West Virginia, Virginia, and Alabama. The Appalachian Basin is divided into four smaller basins (subbasins): the Dunkard Basin in Pennsylvania, Ohio, and northern West Virginia, the Pocahontas Basin in Virginia and southern West Virginia, and the Black Warrior and Cahaba Basins in Alabama. Kentucky and Ohio have potential for commercial CBM production from Pennsylvanian coals, and there is some potential for the production of CBM from Mississippian coal beds in Virginia. CBM reservoirs are multistoried coal beds that are widespread throughout the Appalachian Basin.
CBM tests are commercial where the cumulative coal thickness completed in wells is greater than three meters, the depth of burial of the coal beds is greater than 100 meters, and thermal maturation exceeds 0.8 percent vitrinite reflectance (%Ro). Commercial production is enhanced by secondary fracture porosity, which is commonly related to ubiquitous cleating, and by local supplemental fractures within coal beds. For example, in Alabama, fracture porosity is enhanced by interstratal slip along major folds and by extensional faults; in Virginia, increased fracture porosity is locally related to decollement within coal beds. Most wells are dewatered prior to commercial production of gas. The first large-scale commercial CBM production in the Appalachian Basin began in the early 1980’s, in the Black Warrior (BW) and Pocahontas Basins (P).
Commercial CBM production began in the Dunkard Basin (D) in northern West Virginia and Pennsylvania later in the decade. To date, almost 10,000 CBM wells have been drilled in the Appalachian Basin and have produced almost 736 billion cubic meters (Bcm) of methane ((BW-510 Bcm; P - 216 Bcm; D - 3.2 Bcm)). In areas with significant CBM production, maximum gas-in-place values of methane, based on canister desorption tests, range from about 47 to 220 cubic meters of methane per tonne of coal. As of 2002, the U.S. Geological Survey assessed the technically recoverable undiscovered CBM resources in this area as about 439 Bcm with almost half in the BW.
Two Total Petroleum Systems (TPS) were defined by the USGS during the assessment, the Pottsville Coal-bed gas TPS in Alabama, and the Carboniferous Coal-bed Gas TPS in Pennsylvania, West Virginia, and Virginia. These were divided into seven assessment units, of which three had sufficient data to be assessed. Most of the production is from relatively small areas (counties) within the Appalachian Basin, where geologic conditions are favorable for generation of methane in commercial amounts. Recent increases in prices and demand have stimulated drilling for CBM so that the Basin currently produces about 57 Bcm annually. In horizontal wells drilled in relatively thick coal beds, production rates are higher than in vertical wells, recovery per unit area is greater, and environmental impact is decreased.