16 Marcellus—Production and Disposal of Produced Water

Sunday, 20 March 2011
1:30 PM - 5:30 PM
Conference A

STRONTIUM ISOTOPIC COMPOSITION OF FLOWBACK WATERS ASSOCIATED WITH MARCELLUS SHALE NATURAL GAS EXTRACTION, BRADFORD COUNTY, PENNSYLVANIA

Paper number: 16-5
Program: Theme Sessions
Time: 2:30 PM – 2:45 PM

Authors

Abstract

The hydrofracturing technique used in natural gas extraction from the Middle Devonian Marcellus Shale produces large volumes of highly saline waters. It is not clear to what extent the dissolved load in flowback waters originates from connate Marcellus Formation waters, dissolution of salt by the fracturing fluids, or brines from adjacent formations. In part to address this issue, flowback waters from five hydraulically fractured Marcellus Shale gas wells in Bradford County, PA were analyzed for strontium isotope composition. TDS ranged from 110,000-211,000 mg/L, and elemental concentrations follow Cl > Na > Ca > Ba > Sr > Mg > Br. Two of the samples represent initial flowback fluids, and three represent waters that had been recycled multiple times as fracturing fluids. Isotopic analysis of the filtered flowback waters yielded a relatively restricted 87Sr/86Sr range of 0.71026-0.71076, corresponding to a Sr concentration range of 2.0-5.2 x 103 mg/L. The highest isotopic ratios are found in recycled fluids, suggesting that they most closely approach local Marcellus isotopic signatures. While samples of the initial freshwater fracturing fluids are not available, we speculate that the isotope ratios of the high-TDS flowback fluids are dominated by interaction with Marcellus (± adjacent units) solids and brines.

These and other preliminary strontium isotope results also suggest that Marcellus shale flowback water, brine from the Upper Devonian Venango Group sands, and mine drainage from Pennsylvanian Allegheny and Monongahela Group coal seams fall into isotopically distinct clusters. Ongoing sequential leaching studies of solid samples from the Marcellus Shale and surrounding units will provide tighter constraints on the isotopic endmembers of produced waters, and may provide insight into the source of the TDS in flowback fluids and the water-rock interaction processes involved in their geochemical evolution.