Reductive Weathering of Black Shale during Hydraulic Fracturing and Release of Barium
Sharma, Mukul; Renock, Devon J.; Landis, Joshua D.
Hydraulic fracturing is an important technological advance in the extraction of natural gas and liquids 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 but this mineral cannot directly provide barium as it is rather insoluble in water and in strong mineral acids. This indicates that barium in produced/flowback water must be released by other phases present in the black shale. We examined three drill cores from the Marcellus Shale in Pennsylvania and New York to determine the possible water-rock reactions that take place during hydraulic fracturing that release barium. Some samples contain macrocrystalline barite, which has been replaced by framboidal pyrite, which itself coalesces at places to form massive pyrite. All samples, with or without visible barite, contain high concentrations of Ba. Laser Ablation ICP-MS measurements combined with SEM identified that barium from barite was likely released during diagenesis and partitioned onto clays and perhaps sulfides. We next studied how finely crushed shale reacts with pure water at 80 °C in the presence or absence of oxygen—the former models oxidative weathering of shale cuttings brought to the surface during drilling and the latter emulates the reductive weathering that takes place at depth during hydraulic fracturing. Our experiments revealed that under oxidizing conditions chemical reactions follow proton release from pyrite oxidation, which in turn releases Ca2+ ions through dissolution of calcite. Isovalence exchange by Ca2+ ions with Ba2+ residing on clay minerals then releases barium into solution. This barium however interacts with sulfate in the solution to precipitate barite. Thus oxidative weathering of black shale does not release significant quantities of barium. In comparison, reductive weathering of black shale takes place during hydraulic fracturing where exchange with Ca2+ ions released from the dissolution of calcite with dilute hydrochloric acid leads to the release of significant quantities of barium in solution. At low Eh conditions prevalent during hydraulic fracturing the dominant sulfur species is bisulfide and therefore no barium sulfate forms and bulk inventory of barium released during water-rock reaction comes out with produced/flowback water.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013