Identification and Regulation of Coal Bed Natural Gas Co-Produced Water In Surface Waters, Powder River Basin, Wyoming and Montana

Recovery of coalbed natural gas (CBNG) in the Powder River Basin of Montana and Wyoming has increased dramatically over the past decade. The production of this energy resource has been accompanied by environmental concerns regarding the disposal of co-produced water. In 2006 the state of Montana issued numeric standards for electrical conductivity (EC) and sodium absorption ratio (SAR) of surface water that enters Montana from Wyoming. This study assesses the natural variability of water quality in the Powder River and the extent to which natural variations have been affected by the input of CBNG co-produced water by comparing historical data with data collected at high and low flow in 2007.

Carbon isotopes are a sensitive indicator of the presence of CBNG co-produced water because that water has a distinctively positive delta13C value of +12 to +22 per mil. The presence of CBNG co-produced water is clearly indicated by elevated carbon isotopic values between Beaver Creek and Clear Creek in Wyoming, and up to 20% of the dissolved inorganic carbon in the Powder River in Montana may also be of CBNG origin.

During spring high flow all samples, both historic and modern, meet the Montana standards for EC, but during low flow most samples in both Wyoming and Montana exceed the standards. SAR of Powder River water collected in the spring meets the Montana standard except for some historic samples affected by discharge of oilfield brine from the Salt Creek area. In fall 2007 most samples exceed the limit, including samples collected in Montana from tributaries with no CBNG development.

Some CBNG produced water is present in the Powder River. However, the current Montana numeric standards are not well-suited to identify this component because they do not account for the natural seasonal variability of water quality.