STRONTIUM ISOTOPES AS TRACERS OF WATER CO-PRODUCED WITH COAL BED NATURAL GAS IN THE POWDER RIVER BASIN, WYOMING

Coal bed natural gas (CBNG) is an emerging coal resource that provides clean energy but raises environmental concerns. Primary among these is the disposal of water that is co-produced with the gas during depressurization of the coal seam. It is beneficial to confine water production to the coal seam of interest and to avoid withdrawing water from hydraulically connected aquifers. This study evaluates the use of the 87Sr/86Sr ratio of groundwater to fingerprint the host aquifer and to identify coal seams that represent confined aquifers.

The Eocene coals of the Powder River Basin of northeastern Wyoming contain reserves estimated at over 25 trillion cubic feet of methane. More than 22,000 CBNG wells have been drilled. Water production from individual wells varies, but on average >4600 gallons of water per well per day are produced. The Sr isotopic ratio of groundwater in coal aquifers near Gillette, Wyoming, is positively correlated with residence time. Near the recharge zone the 87Sr/86Sr of the groundwater is ~0.71025. It increases 40 km basinward to ~0.7150 (Frost et al. 2002, Geology). CBNG wells located approximately 30 km basinward of the coal outcrops are completed in one of two coal seams. The 3-7 m thick Wyodak coal appears from well logs to be confined by shale, and the groundwater in these seams has 87Sr/86Sr > 0.714. The shallower Big George coal, which is generally 15 to 70 m thick, has more variable but less radiogenic 87Sr/86Sr than the Wyodak. Either the characteristic 87Sr/86Sr of water from this coal aquifer is different from that of the Wyodak, or that water from some parts of the Big George includes a component withdrawn from adjacent sandstones (which yield water with 87Sr/86Sr=0.71026). Hydraulic communication between some coal and sandstone aquifers is suggested by the elevated 87Sr/86Sr ratio (0.71389) of water from a well completed in sandstone within the coal zone. In addition, groundwater from the Big George in direct stratigraphic contact with a sandstone yielded water with unradiogenic 87Sr/86Sr of 0.71028. We observe no clear correlation between 87Sr/86Sr ratio of the produced water and gas production or gas/water production ratio. Instead, the volume of gas withdrawn appears to vary with geographic location, suggesting that structures may be an important control on CBNG production.