SOURCES OF BARIUM IN THE MARCELLUS SHALE AND ASSOCIATED PRODUCED WATERS

Produced waters from hydraulically fractured Marcellus Shale gas wells are Na-Ca-Cl brines with high concentrations of barium (up to 13,800 mg/L; [1]). At these levels, precipitation of barite can lead to scale and decreased permeability to gas flow. Previous studies have shown that the largest reservoir of available (readily extractable) Ba in the organic-rich Marcellus Shale is from cation exchange sites on shale minerals and organic matter [2-4], although very high rock/water ratios would be required to explain the observed concentrations in produced water. Alternate sources of Ba include preexisting formation water within or outside of the shale, barite nodules within the shale that may have formed in early diagenesis, or even drilling mud. To address the origin of Ba in the Marcellus Shale and its produced water, five rock samples were taken at various depths from a drilled core recovered from the Marcellus Shale Energy and Environmental Laboratory site in West Virginia, and five flowback/produced water samples from the same well were taken over a period of time from the beginning of flowback to stable high total dissolved solids. For the rock samples, exchangeable Ba was extracted with ammonium acetate, and a second leach with acetic acid was used to obtain Ba in carbonate cement. Separate dissolutions of silicate residue and drilling mud used in the well are ongoing. Analysis of geochemical and isotopic signatures, including stable Ba isotopes, are being used to (1) compare potential sources of Ba to that in the produced water; (2) identify the potential diagenetic and/or epigenetic redistribution of Ba in the shale; and (3) evaluate new isotopic tools for tracking post-depositional transport of cations in shale. References cited: [1] Barbot et al., 2013, Env. Sci. Technol. 47, 2562-2569; [2] Stewart et al., 2015, Appl. Geochem. 60, 78-88; [3] Phan et al., 2015, Appl. Geochem. 60, 89-103 [4] Renock et al., 2016, Appl. Geochem. 65, 73-86.