CARBON ISOTOPE TECHNIQUE FOR COALBED AQUIFER CHARACTERIZATION; POWDER RIVER BASIN, WYOMING

Positive ratios of ¹³C to ¹²C of dissolved inorganic carbon expressed as δ¹³C_dic can identify water originating in a coalbed hosting microbial production of natural gas (Sharma and Frost 2008). We undertook isotopic analyses of dissolved inorganic carbon in coalbed natural gas (CBNG) co-produced waters from the Powder River Basin (PRB) of northeastern Wyoming in order to determine 1) whether δ¹³C_dic differs between coal seams and other aquifers, 2) if δ¹³C_dic changes over with time 3) and if δ¹³C_dic can determine methanogenic natural gas production. Water samples were collected from 176 CBNG wells in the PRB. The CBNG wells in this sample set were drilled from 1994 to 2008 and are completed in 6 different coal seams (Anderson, Canyon, Big George, Wall, Werner, WYODAK), at varying depths (400 to 2600 ft), using different completion techniques. This investigation is conducted in cooperation with Anadarko Petroleum Corporation, Wyoming State Geological Survey, University of Wyoming, and the Wyoming Water Development Commission.

δ¹³C_dic in this sample set varies from -24.7 to 24.5 ‰, spanning the range between most natural waters (δ¹³C_dic typically -9 to -15‰,) and positive values reflect methanogenic activity. Wells that have produced since 1994 have high δ¹³C_dic (17.5‰) suggesting little change with time. The ratio identifies water produced solely from coal seams versus water from coal and other aquifers: produced water from wells with single completions in the WYODAK coal has distinct δ¹³C_dic from 16.5 to 17.0‰. Wells with commingled open-hole completions have a highly variable δ¹³C_dic (-24 to 9 ‰). The variable δ¹³C_dic indicates a fresh water component in the co-produced water. This non-coalbed water may be introduced during dewatering from strata above or below coalbeds.

Our preliminary results suggest that stable isotope ratio compositions of water coproduced with coalbed natural gas may provide a relatively simple and inexpensive way to determine whether coal seams are confined and identify those coals associated with abundant microbial production of natural gas. This information is useful in minimizing water production, increasing gas production efficiency and lowering environmental footprints in CBNG plays.