DETERMINING SOURCES OF BARIUM AND OTHER DISSOLVED SOLIDS IN OIL AND GAS PRODUCED WATER USING STABLE BARIUM ISOTOPES

Produced waters from conventional and unconventional hydrocarbon reservoirs, such as the hydraulically fractured Middle Devonian Marcellus Shale in the Appalachian Basin, USA, often contain high levels of total dissolved solids (TDS), including barium. The high Ba content in unconventional produced waters has been variously ascribed to drilling mud dissolution, interaction with pore fluids or shale exchangeable sites, or fluid migration through fractures. Identifying the source of Ba and processes involved in barite precipitation is critical to understanding scale-producing reactions and the geochemical evolution of fluids from reservoir rocks. Here we apply the non-traditional stable Ba isotope system to track TDS sources and water-rock interaction in hydrocarbon reservoirs.

Produced waters from multiple Marcellus Shale gas wells in Pennsylvania and the Marcellus Shale Energy and Environment Laboratory (MSEEL) site in West Virginia yield positive δ¹³⁸Ba values (permil deviation of the ¹³⁸Ba/¹³⁴Ba ratio from the NIST 3104a standard) that are distinct from the negative δ¹³⁸Ba values measured in produced waters from the overlying Upper Devonian/Lower Mississippian conventional reservoirs. None of the produced waters overlap with drilling mud barite used in the MSEEL well (δ¹³⁸Ba ≈ 0.0). Marcellus Shale produced water is also significantly offset from potential rock-based Ba reservoirs within the producing portion of the Marcellus Shale, including exchangeable sites and carbonate cement. Limited data from two flowback/produced water time series show little change in δ¹³⁸Ba with time, suggesting a constant source and minimal mass fractionation effects. Barium isotope data to date are consistent with derivation of unconventional well produced water from brines in adjacent formations via fractures. While the dataset for Upper Devonian/Lower Mississippian produced waters is limited, initial results suggest that the observed δ¹³⁸Ba relationships hold up across the Appalachian Basin (in contrast to ⁸⁷Sr/⁸⁶Sr), and could be a robust tool to differentiate between these sources in the event of a leak or spill. Barium isotopes show promise for tracking formation waters, for identifying the source of high-TDS fluids, and for understanding water-rock interaction in deep sedimentary basins.