Source: Alaska's coal database: Alaska Division of Geological and Geophysical Surveys Public-data File – 1984, 76 p.
The coal resources of Alaska (about 5,526 billion short tons; see table 1) contain significant potential economic coalbed methane resources. Methane derived from coal, which has migrated and is stored in interbedded sandstone reservoirs in the Cook Inlet Basin, is presently being developed. Coalbed gas or methane-rich gas is stored (adsorbed) in the coal along fractures, cleats, and pores and (or) within (absorbed) the molecular structure of the coal. Gas is stored in the coal by molecular attraction on the surfaces of the structures of the coal. Methane is a by-product of fermentation during deposition and coalification during burial of peat. The ability of the coal to store gas is a function of rank or grade of coalification (for example, lignite, subbituminous, bituminous) and temperature and pressure. Generally, more methane is stored in higher rank coal and at high pressure whereas higher temperature decreases storage capacity. Methane generated in higher rank coal (for example, bituminous) is thermogenic in origin, and methane produced in lower rank coal (lignite and subbituminous) is biogenic in origin. Biogenic gas is generated during bacterial activity by methanogens or anaerobes that produced methane as a by-product of their metabolism. In most cases methanogens do this by reducing carbon dioxide with hydrogen to produce methane. Biogenic gas generated from lignites in Alaska was determined from a 1994 U.S. Geological Survey test well in the Yukon Basin (Flats), where the coal beds are more than 21 ft (6.4 m) thick.
A major by-product of development of coalbed methane, especially for subbituminous coal, is coproduced water. Volumes of water produced in major methane-producing basins in the conterminous United States vary significantly between bituminous and subbituminous coal. The volume of coproduced water from bituminous coal ranges in average from 48 to 240 barrels (7,632 to 38,160 liters) of water per day per well and from the subbituminous coal the average is about 440 barrels (91,600 liters) (Flores, 2000). Hence, the water:gas ratio for the bituminous coal ranges from 0.029 to 0.51 barrel per thousand cubic feet (16.3 to 286 liters per 100 m3) and from the subbituminous coal is 2.88 barrels per thousand cubic feet (21,360 liters per 100 m3) (Flores, 2000). In order to produce the methane from the coal, the reservoir needs to be dewatered, which results in the depressurization of the reservoir. This water can be disposed of either on the surface, into ponds or existing drainages, or reinjected below the surface. Regulations, quality, and amount of the coproduced water influence the choice of a disposal system. Coproduced water from subbituminous coal of the Tertiary Fort Union Formation being developed in the Powder River Basin of Wyoming is freshwater. It contains concentrations of dissolved solids mainly of bicarbonate, and trace elements and pH values that are generally below and within recommended drinking-water standards (Flores, 2000). Thus gas operators in that basin are permitted to dispose of the coproduced water on the surface; however, the large volume of water being disposed of is affecting the environment (for example, biota, ephemeral drainages, ground-water supply). Water-disposal problems may influence potential development in Alaska where the permafrost (for example, Northern Alaska-Slope coal province) is thick (Ferrians, 1965), and freezing temperatures at the surface for much of the year may curtail surficial disposal by ponding or along preexisting drainages. The quality of water such as concentration of total dissolved solids and location of coalbed gas production where recharge areas are juxtaposed to brackish-marine bodies of water (sea, ocean, bay) may prevent surface disposal or reinjection, which may contaminate ground-water supply.
Smith (1995) reported that Alaska’s in-place coalbed methane resources might be as much as 1,000 trillion cubic feet (tcf) (28 trillion cubic meters [tcm]) based on estimates of the gas content of as much as 245 standard cubic feet per ton (scf/t) for the coals. The high coalbed-methane resource estimate of Smith (1995) utilized 200 scf/t for both the subbituminous and bituminous coals in the Northern Alaska-North Slope coal province and 152 scf/t for the subbituminous coal in the offshore area in the Southern Alaska-Cook Inlet coal province. Our investigations of the subbituminous coals in the Powder River coals indicate gas content ranging from 0 to 99 scf/t, averaging 25 scf/t (Stricker and others, 2001). If the Powder River Basin coalbed-methane content is applied for the Alaska subbituminous coal, Smith’s estimate will be reduced to about one-half the volume.