Groundwater levels are impacted by coalbed methane (CBM) production, decreasing as production increases, and vice versa, according to a Bureau of Land Management (BLM) report that analyzes data from 62 groundwater monitoring wells in Wyoming’s Powder River Basin (PRB).
Water level monitoring in the PRB has been ongoing for nearly two decades.
The latest report used data through 2012 and found that the Big George CBM play showed a continued decline in groundwater levels as gas production increased, while the Upper Wyodak showed areas of groundwater recovery where gas production had ceased or was declining.
Big George (Wyodak Rider) and Upper Wyodak are two of five coal zones in which BLM monitors groundwater levels. Water levels are also monitored in two sand zones. Data from the Wyoming Oil and Gas Conservation Commission (WOGCC) and BLM's Buffalo, WY, field office were used in the latest report.
Natural gas production overall in the PRB has been declining, according to the BLM report. It peaked at 49.4 Bcf in January 2009 and based on October 2012 data it was 31.7 Bcf.
PBR production since 1989 cumulatively represents about 5.22 Tcf of coalbed natural gas (CBNG).
Correspondingly, water production has decreased since reaching a peak of 68.07 barrels in October 2006, BLM noted. In October 2012, the last month for complete statistics, water levels had dropped to 28.2 million barrels. Since 1987, nearly 7.4 billion barrels of water were co-produced as a result of CBNG production.
"Monitoring wells in the Wyodak Rider (Big George) coal zone showed the most overall drawdown in the PRB during 2010-12, while also showing the highest amounts of water production," BLM said.
The report concluded that with the expected continued decline in the gas production from coal, groundwater declines were seen as stabilizing or being reversed.
"Rates of [decline] have decreased in most of the monitored coal zones over the past three years with the Upper and Lower Wyodak coal zones, generally, showing overall recovery."
Monthly CBM production in the Powder River Basin has been on the decline for several years, reaching just 24.54 Bcf in October, compared to a high of 44.87 Bcf in April 2009, according to WOGCC data.