TRACKING CBNG CO-PRODUCED WATER USING STABLE CARBON ISOTOPE OF DIC: AN INNOVATIVE APPROACH

Recovery of Coal Bed Natural Gas (CBNG) is commonly associated with large volumes of co-produced water. This water may be put to beneficial use or may be reinjected into subsurface aquifers. In either case, it would be helpful to establish a fingerprint for that co-produced water so that it may be tracked following discharge on the surface or reintroduction to geologic reservoirs. This study explores the potential of using δ¹³C of dissolved inorganic carbon (DIC) of coalbed natural gas (CBNG) co-produced water as a fingerprint of its origin and to trace its fate once it is disposed on surface. Our initial results for co-produced water samples from the Powder River Basin show that this water has distinct, strongly positive δ¹³C_DIC (12 to 22‰) that is readily distinguished from the negative δ¹³C_DIC of most surface and ground waters (-8 to -11‰). This contrasting δ¹³C_DIC allows us to identify surface waters that have incorporated CBNG co-produced water, including a tributary and parts of the main stem of the Powder River. Accordingly, we suggest that the δ¹³C_DIC of water can be used for long term monitoring of infiltration of CBNG co-produced water into ground water and streams. Our results also show that the δ¹³C_DIC of CBNG co-produced water from two different coal zones are distinct (12 to 14‰ for Lower Wyodak versus 18 to 22‰ for Upper Wyodak), leading to the possibility of using δ¹³C_DIC to distinguish different coal zones.