PRODUCED WATER GEOCHEMISTRY OF THE ORDOVICIAN UTICA SHALE AND ADJACENT FORMATIONS IN THE APPALACHIAN BASIN

The organic-rich Ordovician Utica Shale in the Appalachian Basin has large technically recoverable volumes of oil (1,960 MMBO) and gas (780 TCF) that rival the Devonian Marcellus Shale up section. The Utica and underlying Point Pleasant shale interval have been extensively drilled in western Pennsylvania, eastern Ohio, and northern West Virginia. Production from large shale plays is currently most economic using hydraulic fracturing, a process that both requires and produces large volumes of water. Analyzing the chemistry of the produced water is important for disposal plans and understanding subsurface fluid migration within and between reservoirs. Produced waters can be mixtures of injected water and natural formation water from the host-shale or adjacent reservoirs if there is fracture connectivity. Parsing out the contributions of these components can be difficult, but recent time-series studies of the Marcellus have shown that though initial produced waters dominantly consist of injected fluids, mature wells produce mostly natural Marcellus formation brines.

Our goal is to determine baseline geochemical brine compositions of the Utica and adjacent formations using produced water samples from mature wells to fingerprint different reservoirs and help identify fluid movement between reservoirs during hydraulic fracturing. Utica and conventional reservoir brine samples from western Pennsylvania and eastern Ohio were sampled and analyzed for major, trace, and isotopic compositions. Utica samples have calculated TDS concentrations of ~200,000 mg/L, within the range of Marcellus shale salinities yet orders of magnitude higher than previously analyzed Utica flowback waters. This supports the assumption that Utica produced waters from mature wells, like those from the Marcellus, represent natural formation water. The δ¹⁸O and δD isotopic results reveal brines that have remained isolated from meteoric surface waters yet are similar to other deep conventional reservoirs and may not be useful for uniquely identifying the brines. Strontium concentrations (> 5,000 mg/L), however, are higher in the Utica than nearly all other sampled formations in the basin, and plot in a restricted region of a ⁸⁷Sr/⁸⁶Sr vs 1/Sr mixing diagram, suggesting that Sr isotopes may be useful for fingerprinting Utica brines.