TRACKING THE IMPACT OF CBNG PRODUCTION ON GROUNDWATER RESOURCES FROM 2006-2009, POWDER RIVER BASIN, WYOMING; THE RESERVOIR RESPONSE TO DECREASING PRODUCTION IN MATURE CBNG FIELDS

From 2006-2009, coalbed natural gas (CBNG) production in Wyoming’s Powder River Basin (PRB) produced 1.9 trillion cubic feet of gas and 2.9 trillion barrels of groundwater (Reference). The Bureau of Land Management (BLM) operates 132 monitoring wells in the PRB that are designed to track the impact of CBNG production on groundwater resources in coal seams and sandstone reservoirs. We analyzed the groundwater data from 2006-2009 and incorporated localized CBNG production data relative to defined coal zones. The focus of this study is to determine the extent to which water production associated with CBNG production effects groundwater resources in associated coal zones and sandstone aquifers Monitoring well data showed groundwater level drawdown in coal seams directly relates to the extent of local CBNG production. The data also shows that groundwater in overlying sandstone reservoirs are also influenced by local CBNG production.

Geographic Information System (GIS) was employed to determine a buffer around BLM monitoring wells. A buffer was employed to determine which CBNG wells are within a 1.5 mile radius of each groundwater monitoring well. Production data from associated CBNG wells was compared to groundwater monitoring well data. The coal zone wells that produced the most water effectively had the largest groundwater drawdowns. For example, the Wyodak-Rider coal zone experienced the most water production, one monitoring well, Pumpkin Creek, saw a groundwater level change of -681 ft. from 2006-2009. CBNG water production data reveals a rapid increase over the same period, with a peak of over 275,000 barrels per month of water produced in late 2006. Eighteen monitored sandstone aquifers showed some groundwater recovery, but mostly along the periphery of the monitoring well network. Production data shows average water to gas ratios increase as coal zone age increases, from 1.3 for the youngest coal zone and 12.7 for the oldest coal zone. During the study period, selected CBNG wells in the Upper Wyodak Coal Zone began to decrease production. Monitoring well data indicates groundwater levels are stabilizing or even recording recovery of up to 120 feet. These conclusions show that consistent groundwater monitoring is needed to understand the long and short term effects of CBNG production on groundwater levels.