ISOTOPE TRACERS TO IDENTIFY ORIGIN AND VERIFY SAFE DISPOSAL OF PRODUCED WATER FROM THE MIDDLE DEVONIAN MARCELLUS FORMATION, PENNSYLVANIA, USA

Extraction of natural gas, natural gas liquids, and oil from shales via hydraulic fracturing results in large volumes of co-produced water, usually with high levels of total dissolved solids (TDS). Increases in TDS of ground or surface waters in the drilling region can potentially be attributed to produced water leakage or improper disposal. However, the Appalachian Basin has a long history of fossil fuel exploration and production that predates the Marcellus Shale gas play, with potential impacts to streams and aquifers from coal mine drainage, fly ash disposal ponds, and brines and drilling-related fluids from shallow legacy gas wells. Multiple geochemical and isotopic tools, along with a detailed understanding of water-rock interaction in the subsurface environment, are needed to sort out the contributions from these different sources.

Isotope tracers can add an extra dimension to standard geochemical monitoring tools. For example, we carried out a strontium isotope study of Marcellus produced waters to aid in identification of leakage and verification of safe disposal. In order for the isotope ratio of strontium (⁸⁷Sr/⁸⁶Sr) to be used successfully as a natural tracer in ground and surface waters, the values of the potential endmembers must be distinct. Produced water samples from four counties in Pennsylvania spanning a distance of ~375 km yield a relatively restricted range of ⁸⁷Sr/⁸⁶Sr values from 0.7100 to 0.7121, consistent with leaching experiments on Marcellus Formation drill cuttings. Strontium isotope data from other potential TDS sources over a wide geographic and stratigraphic range, including produced waters from upper Devonian Venango and Bradford sands, indicate that most are isotopically distinct from Marcellus waters, and that influxes from these sources at any given location tend to fall within a fairly narrow range. This demonstrates that the Sr isotope ratio is likely to be a sensitive tracer for verification of safe produced water disposal. Ongoing work includes further development of the Sr, Li, U, and Nd isotope systems to gain insight into water-shale interaction and the geologic history of gas-producing shales, as well as baseline characterization of surface waters in the heart of the Marcellus gas producing region.