NEWS RELEASE: Wyoming Geological Survey Publishes Report on Groundwater Level Changes due to CBNG Production in the Powder River Basin

WSGS Publishes Report on Groundwater Level Changes due to CBNG Production in the Powder River Basin

A new study by the Wyoming State Geological Survey (WSGS) examines groundwater level responses in the Tertiary sandstone aquifers associated with coalbed natural gas production in the Powder River Basin.

The report—Groundwater Level Recovery in the Sandstones of the Lower Tertiary Aquifer System of the Powder River Basin, Wyoming—is available as a free download from the WSGS website.

“Changes in coalbed natural gas production and produced water provide a unique opportunity to study long-term groundwater changes,” says Dr. Erin Campbell, State Geologist and Director of the WSGS. “Understanding how subsurface systems relate to groundwater recovery allow us to best plan future development.”

Between 2001 and 2019, the Powder River Basin (PRB) in Wyoming produced more than 6.1 trillion cubic feet of coalbed natural gas (CBNG) and nearly one million acre-feet of groundwater from coal seams in the Fort Union and Wasatch formations that form the lower Tertiary aquifer system. Annual CBNG production in the PRB peaked in 2009 at more than 556 billion cubic feet, or 2.1 percent of all U.S. natural gas production for that year. Since then, annual gas production has declined by 83 percent to 92.6 billion cubic feet during 2019.

CBNG is produced by pumping large volumes of groundwater from a targeted coal seam, reducing both the groundwater level and water pressure. This allows microscopic films of natural gas within the pores and fractures of the coal to coalesce into bubbles much like carbon dioxide effervesces from a newly opened bottle of seltzer.

During this dewatering phase, groundwater levels typically decline by several hundred feet in the coal seam aquifers. As the pumping rate declines or ceases, water levels in the targeted coal seam frequently rise (recover); however, in some cases water levels may remain static or continue to decline. Moreover, these groundwater level fluctuations are not restricted to the producing coal seam but frequently extend to adjacent sandstone aquifers.

“This report examines and analyzes groundwater level data collected by the Bureau of Land Management over the last three decades from nearly 100 coal seam and sandstone wells located on 39 monitoring sites,” says WSGS hydrologist and lead author, Karl Taboga. “Specifically, we looked at how water levels in the sandstone aquifers have fluctuated over time in response to declining water production from nearby CBNG wells.”

The study shows that groundwater levels have declined by more than 100 feet in some deep sandstone aquifers located more than 600 feet below the surface that are separated from a producing coal seam by less than 200 feet. Smaller declines were observed in shallower sandstones located more than 200 feet from a developed coal seam. Typically, groundwater levels in the affected sandstone aquifers briefly rise by several feet for a few months after CBNG production ceases, but this rapid recovery frequently decreases to one foot or less annually after a year or two.

The report supersedes the 2017 Report of Investigations 74, which also examined groundwater level changes in the lower Tertiary aquifer system resulting from CBNG production in the PRB. The new report contains updated data from the BLM and Wyoming Oil and Gas Conservation Commission collected during 2017–2020. It also includes two analyses not provided in the previous investigation—estimations of the duration of groundwater recovery in some affected sandstone aquifers, and an examination of the relations between groundwater levels in the sandstone and associated coal seam aquifers.