GEOCHEMICAL ANALYSIS OF TIME-LAPSE FLOWBACK WATER DATA FROM HYDRAULICALLY FRACTURED MARCELLUS SHALE

The chemistry of flowback waters produced by hydraulic fracturing of the Marcellus shale in Pennsylvania is a current topic of research because of the implications for treatment. This paper presents time-lapse inorganic water chemistry data sampled from three adjacent horizontal natural gas wells for ten days directly after hydraulic fracturing and again after forty-four days. These data are compared to historic well data from conventional Paleozoic oil and gas reservoirs in Pennsylvania. Similarities between the two data sets indicate that the flowback water sampled for this study is a mixture of in-situ brines and drilling fluids. Alkalinity (as CaCO₃) generally decreases in wells 1 and 2 (range: 152-410 mg/L ). In well 3, alkalinity decreases from 403 mg/L to 267 mg/L and then spikes to 903 mg/L. The concentration of most inorganic constituents (Ba, Br, Ca, Cl, K, Li, Mn, Na, S, Sr) increases steadily in all wells. Exceptions to this trend are Fe, Mg, and SO₄ . During the first ten days, Fe concentrations in wells 1 and 2, rise and fall twice. Concentrations range from 0.496 mg/L to 84.5 mg/L. After the second fall, Fe concentrations in the wells increase by day forty-four to 72.2 mg/L and 87.3 mg/L, respectively. Fe concentration in well 3 rises from 11.5 mg/L to 94.2 mg/L, with a rapid increase during day seven. Mg concentrations in all three wells rise and then fall (range: 165- 585 mg/L) within the first four days and then increase by day forty-four (range: 937-985 mg/L). SO₄ concentrations increase rapidly and then decrease by day forty-four (range: 8.10-42.5 mg/L). SO₄ M / S_total M ratios indicate that SO₄ accounts for 4.70 % to 54.8% of S_total . x/Br ratios show similar trends to concentration, suggesting that the reaction between in-situ brines and drilling fluids may not represent a simple dilution as previously believed.