EXPERIMENTAL INVESTIGATION OF BARIUM SOURCES AND FLUID-ROCK INTERACTION IN UNCONVENTIONAL MARCELLUS SHALE WELLS USING BA ISOTOPES

Anomalously high barium (Ba) concentrations in Marcellus Shale produced waters are accompanied by the formation of barite (BaSO₄) scale which decreases well productivity. To understand the primary source of Ba in these waters and the conditions under which barite precipitation and dissolution occur, we carried out experiments on the interaction of simulated hydraulic fracturing fluid with drilling mud and Marcellus Shale core material at downhole temperatures and pressures using stable Ba isotopes. Marcellus Shale produced waters are some of the isotopically heaviest (highest ¹³⁸Ba/¹³⁴Ba, expressed as δ¹³⁸Ba) natural fluids measured to date. Various mixtures of powdered Marcellus Shale core material and drilling mud (including barite) were mixed with synthetic fracturing fluid and brought to 65.5°C and 20.7 MPa using static autoclaves for a total of 6 hours, and the fluid analyzed for δ¹³⁸Ba. Under these conditions, Ba was observed to decrease in concentration over the duration of all experiments, with the greatest decrease in the shale-only run; this is likely due to barite precipitation, aided by release of sulfate from shale pyrite. The δ¹³⁸Ba values of the fluid increased from +0.06‰ to +0.55‰ as the concentration decreased by a factor of 3. The data are consistent with Rayleigh fractionation using published barite fractionation factors. Benchtop dissolution tests using natural hydrothermal barite and drilling mud samples in 2 N HCl at 80°C for periods of 2, 6, and 48 hours to recreate the HCl slug added at the beginning of hydraulic fracturing yielded <1% dissolution of barite, and no change in δ¹³⁸Ba outside of measurement uncertainty. These experiments together suggest that (1) barite precipitation, while capable of producing high δ¹³⁸Ba fluids, is unlikely to be the cause of high-δ¹³⁸Ba Marcellus produced waters because of the resulting low Ba concentrations in the residual fluid; and (2) dissolution of drilling mud or natural barite in the shale is not likely the major source of Ba in Marcellus produced waters. Preliminary data from Marcellus Shale core flow-through experiments under downhole conditions also suggest barite precipitation and isotopic fractionation during hydraulic fracturing, with minimal contribution from exchangeable Ba in the shale.