EXPERIMENTAL INVESTIGATIONS OF MARCELLUS SHALE INTERACTION WITH FLUIDS AT DOWNHOLE CONDITIONS USING BA ISOTOPES

Produced waters from unconventional Marcellus Shale gas wells have anomalously high barium (Ba) concentrations and are some of the isotopically heaviest (δ¹³⁸Ba up to +1.49‰) fluids. Experiments were conducted to identify the primary source of Ba in these fluids and the controls on barite (BaSO₄) precipitation and dissolution in oil and gas wells: (1) Powdered Marcellus Shale core and barite-bearing drilling mud were reacted with synthetic low-Ba fracturing fluid using static autoclaves under simulated down-hole conditions (65.5° C, 20.7 MPa). This resulted in decreased Ba concentrations in the fluid, with the largest decrease in the shale-only run, likely due to barite precipitation aided by sulfate release from shale pyrite. δ¹³⁸Ba values of the fluid increased from +0.06‰ to +0.55‰ as Ba concentrations decreased, consistent with closed-system Rayleigh fractionation. (2) Core flood experiments, in which synthetic fracturing fluid was pumped through a cut surface within Marcellus shale cores, were conducted at 65.5° C and 20.7 MPa for 28 days. Effluent Ba concentrations were an order of magnitude lower than influent concentrations, while effluent sulfate concentrations increased over time. Effluent δ¹³⁸Ba values increased over the first 12 days from approximately +0.5‰ to a plateau of about +1.3‰. Modeling suggests that a combination of barite precipitation and release of Ba from the shale can explain the data. (3) Barite dissolution experiments evaluated barite solubility in 2 N HCl at 80° C for periods of 2, 6, and 48 hours to simulate the HCl slug added at the beginning of hydraulic fracturing. This resulted in <1% barite dissolution, and no appreciable change in δ¹³⁸Ba. Taken together, these experiments suggest that barite precipitation in fractures and in the well bore, while capable of producing high δ¹³⁸Ba fluids, is unlikely to cause high-δ¹³⁸Ba Marcellus produced waters due to the resulting low Ba concentrations in the residual fluid. We find that release of sulfate from oxidation of shale sulfide rapidly catalyzes barite precipitation, and that dissolution of drilling mud barite or natural barite in the shale is unlikely to be the major source of Ba in Marcellus produced waters.