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