Gas Desorption Tests: Application for Powder River Basin Gas Production Prediction

Margaret S. Ellis¹, Gary D. Stricker², and Romeo M. Flores². (1) Central Energy Resources Team, U.S. Geological Survey, Box 25046, MS 939 Denver Federal Center, Denver, CO 80225-0046, phone: 303-236-7775, fax: 303-236-0459, [email protected], (2) Central Energy Resources Team, U.S. Geological Survey, Box 25046, MS 939 Denver Federal Center, Denver, CO 80225-0046

Production of coalbed methane (CBM) from about 20,000 CBM wells from Paleocene-age Fort Union Formation coal in the Powder River Basin (PRB) was about 1.5 trillion cubic feet through 2004. The Bureau of Land Management predicted average lifetime recoveries of 0.20 to 0.50 billion cubic feet per well.

Since 1999, the U.S. Geological Survey and the Bureau of Land Management have collected coal cores with the cooperation of gas operators for use in a CBM desorption study. We applied a variation of Bodden and Ehrlich's (1998) method for projecting gas production from bituminous coal in the Black Warrior Basin in Alabama. They suggested a relationship between a well's first year of CBM production and cumulative measured gas that had been desorbed for 120 days divided by the depth of the coal. Using PRB subbituminous coal, we plotted data for the first 12 months of production against desorbed gas for 5, 40, and 120-day desorption time periods divided by the core depth. The plots showed correlation coefficients of 0.72 for the 5-day interval, and 0.74 for the 40-day and 120-day intervals. Therefore, for PRB subbituminous coal 40 days of desorption data is sufficient to predict a well's first year of production. Using the 40-day graph, we can predict that if the measured gas content of coal from a well was 45 standard cubic feet per ton and the depth was 800 ft, the well would have a 12-month production of about 31 million cubic feet of gas.