INORGANIC GEOCHEMISTRY OF MARCELLUS SHALE NATURAL GAS HYDROFRACTURING 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. The injection water includes addition of numerous inorganic and organic chemical compounds in proprietary mixtures. 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 85 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). Flowback water usually shows increased salinity with time. Titrations lacking distinct inflection points suggest, as in oil-field brines, that much of the alkalinity is due to naturally-occurring organic acids rather than HCO₃^-. Ba and Sr concentrations range widely, with concentrations as high 26,800 and 5,230 mg/L, respectively. Based on r² values for linear fits, the positive correlations with TDS follow the order: Cl > Na > Ca > Br > Mg ≈ Spec Cond ≈ Sr > Hardness > K ≈ Li. SO₄, acidity, and alkalinity range from 5 to 2920, -210 to 1230, and 24 to 800 mg/L, respectively and show no correlation with TDS. Although few data are available, these waters have a wide range of radioactive constituents, with gross alpha and ²²⁶Ra as high as 19,200 and 4,180 pCi/L. Durov diagrams comparing major cation/anion constituents, pH, and TDS are presented, as are spatial distribution maps of 15 parameters for flowback water collected at > 90 days after hydrofracturing.

Geochemical modeling using PHREEQC can reasonably reproduce flowback water major element chemistry using the standard PHREEQC database, but Ba and Sr cannot be explained. Database modification suggests that Ba and Sr complexation by organics could explain the extremely high Ba and Sr concentrations observed in flowback water.