A GIS APPROACH TO THE IMPACT OF CBNG DEVELOPMENT ON TARGETED COALBED GROUNDWATER LEVELS IN THE POWDER RIVER BASIN, WYOMING 1993-2006

By 2006, coalbed natural gas (CBNG) production in Wyoming’s Powder River Basin (PRB) (Figure 1) had produced 2.2 trillion cubic feet of gas and 4.13 billion gallons of groundwater from approximately 24,000 CBNG wells. The Bureau of Land Management (BLM) operates a network of 111 monitoring wells in the PRB (Figure 1). Since 1993, BLM monitoring wells have recorded groundwater levels and gas pressures in CBNG-targeted coal beds and adjacent horizons. Data from these wells through 1993-2006 were analyzed by the Wyoming State Geological Survey.

This project used Geographic Information System (GIS) and monitoring well data to determine the amount of groundwater drawdown within CBNG targeted coal seams and adjacent and overlying strata. Golden Software Surfer 8 was used to illustrate changes in groundwater levels from 1993-2006. These data recorded drawdown in most coal horizons (e.g. average drawdown for Wyodak is 282 ft.), with the exception of some local recharge events (1.7 ft., Big George coalbed at Sec. 31, T. 45 N., R. 75 W.). Drawdown in the overlying Wasatch formation is also identified (268 ft at Sec. 26, T. 47 N., R. 74 W.), suggesting an unconfined aquifer system. Interpolated surface modeling of groundwater potentiometric surfaces shows groundwater drawdown where development progresses. By applying dimensional graphing, water level drawdown increases as CBNG development progresses through time.

Data from overlying strata suggests that the majority of the targeted PRB coalbeds are confined, and that CBNG production is effectively dewatering target coal beds. Data analyzed through GIS processes suggests some local coal and sandstone aquifers are unconfined.