HYDRAULIC FRACTURING AND RELEASE OF BARIUM IN PRODUCED WATER

Hydraulic fracturing is an important technological advance in the extraction of natural gas and petroleum from black shales, but the water used in the fracturing process returns with dangerously high concentrations of barium. A major barium-containing phase, barite (BaSO₄), is part of drilling mud as well as the black shale. However, the role of barite dissolution is unclear due to its relatively low solubility. We hypothesized that barium in produced water can be released by other phases in the shale and on a time scale relevant to fracking. We examined three drill cores from the Marcellus Shale in Pennsylvania and New York to determine the possible water-rock reactions that release barium during hydraulic fracturing. One sample contains macrocrystalline barite, which has been partially replaced by pyrite during diagenesis. All samples, with or without observable barite, contain elevated concentrations of Ba relative to the crustal average for shale rocks. µ-XRF measurements and SEM/EDS analysis combined with chemical sequential extraction methods revealed that a majority of the Ba in the rock (50-77%) is strongly partitioned into silicate clays and is only removed during silicate dissolution in hydrofluoric acid. We next studied how finely crushed shale reacts with pure water at T = 80 °C and under anoxic conditions to emulate the conditions that takes place at depth during hydraulic fracturing. Our experimental results indicate that the amount of Ba released from the bulk rock positively correlates with the ionic strength of the reacting fluid. A maximum of 33% of the total Ba in the shale can be leached from shale under ionic strength conditions typical of produced waters. We suggest that a reductive weathering of black shale is possible during hydraulic fracturing due to: 1) Ba²⁺ in clays exchanging with Na⁺ and Ca²⁺ ions that are present in high concentrations in produced water, and 2) increased solubility and dissolution kinetics of barite under high ionic strength conditions. At the low Eh conditions prevalent during hydraulic fracturing the sulfate deficient water allows Ba to be mobilized and come out with the produced water. Based on our Ba yields in simulated produced water, we suggest that all of the Ba can come from the rock based on reasonable estimates of the water:rock ratio during fracking.