COALBED GAS RESOURCES OF THE U.S., AND ENVIRONMENTAL ISSUES RELATED TO PRODUCTION

Coalbed gas is natural gas associated with coal. It is mainly composed of methane with variable amounts of ethane, nitrogen, and carbon dioxide. Methane gas is adsorbed onto coal surfaces, occurs as free gas in fractures (cleats), and is dissolved in water within the coal beds. Coalbed gas is produced in several basins throughout the U.S., accounting for about 7 to 8 percent of the total U.S. natural gas production. The major coalbed gas areas include the San Juan Basin, Colorado and New Mexico; the Black Warrior Basin, Alabama; the Powder River Basin, Wyoming; the Uinta-Piceance Basins, Colorado and Utah; the Raton Basin, Colorado and New Mexico; and the Appalachian Basin that lies in parts of several states in the eastern U.S. As of November, 2005, the U.S. Geological Survey (USGS) assessed a mean of 67 trillion cubic feet (TCF) of undiscovered, technically recoverable coalbed gas in 14 basins throughout the U.S. To produce coalbed gas, the water in the coal must be removed by pumping, which allows the hydrostatic pressure in the coal to drop sufficiently for the gas to form a separate phase and flow. The amount of water produced from most coalbed gas wells is relatively high compared to conventional natural gas wells. This produced water can range from saline (100,000+ ppm total dissolved solids) to potable (a few hundred ppm total dissolved solids), but in all cases must be disposed of by treatment, and either reinjection and (or) surface discharge in an environmentally acceptable manner. Ongoing studies by the USGS on the composition and volumes of water co-produced with coalbed gas in some of the most active areas of production in the U.S. will be discussed.