­­­­­USING INSAR AND GROUNDWATER PUMPING DATA TO MODEL LAND SUBSIDENCE FROM COALBED METHANE PRODUCTION IN THE POWDER RIVER BASIN, WYOMING

Groundwater pumping during coalbed methane (CBM) production releases methane gas by reducing pore-water pressure in the source rock. In Wyoming’s Powder River Basin (PRB), groundwater has been extracted for CBM at rates greater than 94 million gallons per day, or 65 thousand gallons per minute. Land subsidence can result from aquifer compaction as groundwater is removed, as is observed in the PRB. Synthetic Aperture Radar Interferometery (InSAR) data indicates several centimeters of subsidence in the Powder River Basin near clusters of coalbed methane pumping wells. From July 3, 1997 to July 27, 2000, up to 4.7 centimeters of subsidence were observed; from August 5, 2004 to July 26, 2007, up to 8.3 centimeters of subsidence were observed. For the study area, coalbed methane pumping well data and monitoring well data were also available through 2010. Using the USGS-2000 flow engine in Visual MODFLOW 2010.1, groundwater flow models were developed using monitoring well data and coalbed methane pumping well data for each study period. Model outputs show that the areas of greatest groundwater drawdown of 100 to 170 meters (328 to 558 feet) are spatially correlated with the major subsidence signals in each interferogram. The analytical solution to calculate aquifer storage from groundwater drawdown in the coal seams (monitoring well data) and ground compaction (InSAR line-of-sight change data) was applied to both study periods; the aquifer storage coefficient decreased between the study periods, from 4.94 x 10^-3 to 3.33 x 10^-3. The correlation of land subsidence to drawdown from coalbed methane wells, and the decrease in aquifer storage over time, is sufficient evidence that subsidence is associated with coalbed methane production in the Powder River Basin.