Estimating Radium Activity in Shale Gas Produced Brine

Shale gas reservoir-produced brines may contain elevated levels of naturally occurring radioactive material, including Ra-226 and Ra-228, which come from the decay of U-238 and Th-232 in shale. While the total Ra activity in shale gas wastewaters can vary by over 3 orders of magnitude, the parent radionuclides tend to only vary by 1 order of magnitude. The extent of Ra mobilization from the shale into produced brines is thought to be largely controlled by adsorption/desorption from the shale, which is influenced by shale cation exchange capacity (CEC) and reservoir brine salinity, often reported as the total dissolved solids (TDS). To determine how these factors lead to such large variation in Ra activity of produced brines, the U content and CEC of shale samples from the Antrim and Utica-Collingwood shales in Michigan and the Marcellus shale in Pennsylvania were evaluated. Analysis of produced brine from 17 Antrim shale gas wells was then used to develop an empirical relationship between Ra-226 activity and produced water TDS for a given U content of the shale. This correlation will provide an a priori estimate of the expected Ra activity of a produced brine from a given shale gas play when the brine salinity and U content of the shale are known. Such information can serve as a guide for optimal wastewater treatment and disposal strategies prior to any drilling activity, thereby reducing risks associated with elevated Ra activity in shale gas wastewaters.