SPATIAL DISTRIBUTION OF INORGANIC GEOCHEMISTRY OF MARCELLUS SHALE FLOWBACK WATERS

The Marcellus Formation shale likely hosts the largest natural gas field in the US. A resource boom using hydrofracturing for exploration and production of natural gas from the Marcellus Formation in PA, OH, WV and NY requires large volumes of water for the drilling and injection fluids, and it produces large volumes of very saline “flowback” or “frac” water that requires either deep well injection for disposal or non-conventional wastewater treatment and sludge collection before discharge into the environment, although reuse of water can cut the water demand. The injected water mixes with oil-field brines and/or dissolves salts from the shale.

We analyzed inorganic chemicals in 5 flowback water samples, and we gathered 100 chemical analyses (from the PA Department of Environmental Protection and private sources) of flowback water. The flowback waters are pH 5 to 8 Na/Ca/Cl dominated brines with TDS ranging from 1,850 to 345,000 mg/L (10x more saline than seawater). Durov diagrams comparing major cation/anion constituents, pH, and TDS are presented for all samples. For samples that reflect more mature (late stage) flowback, which is presumably most like the formation waters, we prepared inverse-distance weighted interpolation maps (concentration contours) for numerous inorganic constituents including Ca, Na, Cl, Br, Mg, Ba, Sr, Ra, Rn, Gross Alpha, Gross Beta. Concentrations range widely in their spatial distribution, displaying regions of high and low concentrations as well as “hot spots” of high concentration. Such “hot spots” do not correlate geographically for ions that might be expected to behave similarly according to Goldschmidt’s classification, i.e., ions of similar charge and size do not track spatially. These observations are assumed to correlate with spatial variation in the host rock and fluids, although little is published about spatial variation of the Marcellus Formation.