T215 Frontiers in Coal Science: From Basic Research to Applied Technology

Tuesday, 20 October 2009
1:30 PM - 6:00 PM
B114/115

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

Paper number: T215-9
Program: Technical Sessions
Time: 3:30 PM – 3:45 PM

Authors

Abstract

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 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 δ13Cdic differs between coal seams and other aquifers, 2) if δ13Cdic changes over with time 3) and if δ13Cdic 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.

δ13Cdic in this sample set varies from -24.7 to 24.5 ‰, spanning the range between most natural waters (δ13Cdic typically -9 to -15‰,) and positive values reflect methanogenic activity. Wells that have produced since 1994 have high δ13Cdic (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 δ13Cdic from 16.5 to 17.0‰. Wells with commingled open-hole completions have a highly variable δ13Cdic (-24 to 9 ‰). The variable δ13Cdic 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.