Isotope Fractionation of Barium in Shale and Produced Water from the Appalachian Basin, USA

Waters co-produced with oil and gas are often rich in barium (Ba), which can cause scale formation and fouling of wells, especially in unconventional Marcellus Shale gas wells. To address the source of barium in these produced waters, Ba isotope ratios were determined on returned water samples from a Marcellus Shale gas well in West Virginia and other oil- and gas-producing units in Pennsylvania, as well as on exchangeable and carbonate Ba from core samples. This study presents the first known measurements of barium isotopes in produced waters from conventional and unconventional wells and from shale core material. Previous studies have shown that the lighter isotopes of Ba are preferentially fractionated into solid aqueous Ba-rich minerals (e.g., barite, BaSO4, and witherite, BaCO3), and that Ba isotopes can be fractionated by biological and cation exchange processes.. A methodology for measuring stable barium isotope ratios is presented, including separation of Ba from the matrix and using a double spike to correct for mass fractionation. Barium isotope ratios are reported as δ138Ba, the permil (‰) deviation of the 138Ba/134Ba ratio from NIST standard SRM 3104a. A time series of produced water from the hydraulically fractured Marcellus well yielded values ranging from δ138Ba = 0.81‰ to 1.01‰ (typical uncertainty ±0.06‰ or better). In contrast, the barium found in the exchangeable sites of Marcellus rock, the largest source of labile Ba, varied from 0.45‰ to 0.60‰, significantly lower than the produced water, suggesting that Ba in produced water is not sourced from shale exchange sites. The carbonate fractions from the same rocks had similar values but lower Ba concentrations by a factor of 102. Produced water from Marcellus Shale wells in different geographic locations (Greene, Westmoreland and Tioga Counties, PA) yield similar δ138Ba values, although the Westmoreland Co. sample had a higher value (1.50‰). In contrast, waters derived from units both below and above the Marcellus shale yielded significantly lower δ138Ba values, ranging from 0.81‰ to 0.09‰. This system shows promise as a means of differentiating fluids migrating from Marcellus Shale wells from those of nearby conventional oil and gas wells