Stable Isotope Techniques for Coalbed Aquifer Characterization; Powder River Basin, Wyoming
Positive ratios of 13C to 12C of dissolved inorganic carbon expressed as δ13Cdic can identify water originating in a coalbed hosting microbial production of natural gas (Sharma and Frost 2008). We undertook carbon, oxygen and hydrogen stable isotopic analyses of coalbed natural gas (CBNG) co-produced waters from the Powder River Basin (PRB) of northeastern Wyoming in order to determine 1) whether δ13Cdic differs between coal seams and other aquifers, and 2) whether δD and δ18O are also useful in coal seam identification. Water samples were collected from 20 CBNG wells near the town of Wright in the PRB. The CBNG wells in this sample set were drilled from 2000 to 2006. Fifteen wells have commingled completions in the Anderson (40-60 ft in thickness at depths of 200-400 ft) and Canyon coalbeds (30-40 ft in thickness at depths of 400-1200 ft.). Five are completed solely in the Canyon. Water-to-gas ratios for these wells range from 1 to 36 Bbls/Mcf. The coalbed water samples were analyzed for dissolved inorganic carbon, hydrogen and oxygen isotopic compositions. This investigation is conducted in cooperation with Anadarko Petroleum Corporation, Wyoming State Geological Survey, University of Wyoming, and the Wyoming Water Development Commission.
δ13Cdic appears to identify water produced solely from coal seams versus water from coal and other aquifers: produced water from wells with single completions in the Canyon coal has distinct positive δ13Cdic. Wells with commingled completion intervals have a highly variable δ13Cdic. The variable δ13Cdic indicates a fresh water component in the co-produced water. Enriched δ13Cdic correlate with low water-to-gas ratios (<10 Bbls/Mcf). Most natural waters (non coalbed) have values of δ13Cdic that range from -9 to -15‰. Thus coalbed water δ13Cdic decreases with increased freshwater introduced by dewatering strata in addition to coalbeds. δD and δ18O also appear to be useful as additional proxies in identifying water from different coal horizons.
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.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009