INTRODUCTION
Production of water and natural gas from coal beds (coalbed methane, CBM) has increased dramatically over the past ten years and the gas currently accounts for about 6% of the total produced in the United States. The Powder River Basin (PRB) in Wyoming and Montana (Fig. 1) has emerged as one of the most active new areas of CBM production since 1997. Gas and water are being produced from thick coals in the Paleocene age Fort Union Formation primarily in the eastern part of the basin, although development is expanding to the northwest in the basin at the time of this report. The number of producing wells has increased from 270 in March, 1997 to 2,469 as of March, 2000 (Wyoming Oil and Gas Conservation Commission (WOGCC)). CBM production in the same period has increased from 34,529 thousand cubic feet per day (mcf/day) to over 333,000 mcf/day (WOGCC, 2000). Estimates from State and federal officials and industry representatives of the total number of wells expected in the basin over the next 20-30 years vary from 15,000-70,000. 

Water is also brought to the surface during production of coalbed methane. The water in coal beds contributes to pressure in the coal beds that keeps methane gas adsorbed to the coal. During production, this water is pumped to the ground surface to lower the pressure in the reservoir and stimulate desorption of methane from the coal. As with gas production, water production in the PRB has also increased in the three-year period between 1997 and 2000 from about 130,000 barrels per day to over 1.28 million barrels per day (WOGCC, 2000), a ten-fold increase. As the number of CBM wells increases, the amount of water produced will also increase. Water production from a CBM well typically declines over the life of the well, and declining water production is anticipated and has been observed in CBM wells that have produced for several years. Decline in water production in developing areas of the basin and the basin as a whole is not expected to occur until most of the CBM wells have been developed and produced for a number of years.

Reliable data on the composition of the water produced from the CBM wells are needed so that State and federal land use managers can make informed decisions on handling, disposal, and possible beneficial use of water produced with CBM. Previous studies of water associated with coal beds in the PRB have focused on small areas near surface coal mines (Drever and others, 1977; Larson, 1988). Composition data on groundwater in the Fort Union Formation presented previously (Larson and Daddow, 1984) and other data acquired by the State of Wyoming Department of Environmental Quality through the discharge permit process are not coal bed specific. The data may represent co-mingled water from multiple coal beds and/or surface water or water from strata in the Fort Union Formation distinctly different from water produced from coal bed methane wells. Compositional data for CBM water can provide information on the heterogeneity of the CBM reservoirs, the potential flow paths in the Fort Union Formation, and the source and compositional evolution of the water. 

In an effort to provide a better understanding of CBM resources and associated water, the U.S. Geological Survey, in cooperation with the U. S. Bureau of Land Management and coalbed methane production companies in the PRB is conducting multidisciplinary studies in the Powder River Basin. These studies are investigating regional geology and hydrology, coal composition, gas composition, methane desorption, and water composition. This report provides preliminary compositional data on water from 47 CBM wells sampled between June, 1999 and May, 2000 in the Powder River Basin, Wyoming. Data on major, minor, and trace elements are included. Other analyses on these samples, including deuterium, oxygen, and carbon stable isotopes, and dissolved organic carbon are not yet available. Additional sampling in the basin is planned over the next year to include other areas brought into development.