SR ISOTOPIC CHARACTERIZATION OF CBNG CO-PRODUCED WATER IN THE POWDER RIVER BASIN, WY AND MT

Coalbed natural gas (CBNG) is produced in the Powder River Basin by withdrawing ground water to depressurize the coal seams and recover methane. This study examines the strontium isotopic ratio of co-produced water to determine if the water is being withdrawn solely from the targeted coal seam or whether hydraulic connections between sandstone and coal aquifers exist. Preliminary studies indicated that sandstone and coal aquifers have distinct 87Sr/86Sr ratios (Frost et al. 2002). This study evaluates if waters from different coal zones and different parts of the basin have distinctive Sr isotopic characteristics.

In the Buffalo area of the Powder River Basin, different coal zones appear to be characterized by different 87Sr/86Sr. Water produced from the Lake De Smet coal zone (0.71256-0.71414) is more radiogenic than water from the Felix coal zone (0.71149 to 0.71239).

In the area of Gillette and Wright, the Sr isotopic composition of ground water produced from the Upper Wyodak (UW) coal zone increases with increasing distance from the recharge zone. The radiogenic 87Sr/86Sr of water from sandstone immediately above the UW coal zone suggests that coals and sandstones form a single aquifer system. Northeast of Gillette the 87Sr/86Sr of ground waters from the Upper and Lower Wyodak coal zones are geographically controlled. A group of samples close to the east side of the basin has high 87Sr/86Sr (0.71335 to 0.71481) and TDS values typical of water from coal aquifers (290 ppm to 1,000 ppm). These coal seams appear to be well-confined aquifers. A group of samples from in the northern part of the area has lower 87Sr/86Sr (0.71065 to 0.71312) and higher TDS (1,240 ppm to 1,741 ppm). This suggests interaction with water from sandstone aquifers that have high TDS (avg: 1,475 ppm) and low 87Sr/86Sr (avg: 0.71261). However, these wells produce more gas than the wells in the eastern group.

The Wyodak Rider coal zone in the Gillette and Schoonover areas is characterized by large variation in 87Sr/86Sr (0.71100 to 0.71540) and TDS (571 ppm to 3,828 ppm) and elevated Cl contents. Wells in this area have higher than average water production.

Although the system is complex, 87Sr/86Sr is a useful tracer in identifying areas and coal zones where water production will be minimized during CBNG production.