Chemical and isotopic evolution of flowback fluids from the Utica Gas Shale Play, Eastern Ohio USA

Abstract

Hydraulic fracturing flowback fluids were collected from two Utica/Point Pleasant well pads in eastern Ohio. One site was in the wet gas zone (UPPW), while the site ∼50 km south consisted of four wells on the same pad in the dry gas zone (UPPS). Samples of input fluids also were collected before and during hydraulic fracturing. Flowback fluids are Na-Ca-Cl brines with total dissolved salt (TDS) concentrations that increase over several months from ∼100 to 200 g/L. The slightly higher TDS of the dry gas fluids are in part due to recycled flowback used as input fluids, and in part due to lower volume of water used in hydraulic fracturing. Concentrations of most major ions (Ca2+, Mg2+, Na+, Sr2+, Fe, Mn, Cl−, Br−) are similar for the five wells sampled, although small but systematic changes occur in the major element ratios over time.Most notably, an increase in the Sr/Cl ratio corresponds to an increase in the 87Sr/86Sr ratio in the fluids, suggesting interactions with a more radiogenic Sr source in the subsurface. Dissolved Ba concentrations and Ra activities were different between the two sites, reflecting a high SO42− fluid used at the UPPW site, and water-rock reactions occurring during hydraulic fracturing at the UPPS site.Water oxygen (δ18O) and hydrogen (δD) isotopes for input fluids used in the UPPW4 well and fresh water used for the UPPS wells fall on the Global Meteoric waterline (GMWL). Flowback fluids from both sites are relatively enriched in δ18O and δD compared to the input, but do not appear to follow a simple mixing trend, suggesting reaction and isotopic exchange with carbonates and fractionation due to imbibition in the rock. Cl isotopes, δ37Cl in the FP fluids varied from ∼ −0.43 to +0.13‰, the largest variation was observed in the earlier stages of flowback, while in the later stages δ37Cl exhibited a small but systematic increase over time, suggesting diffusion control of isotopic composition.Some of the trace species measured (dissolved Fe, Mn, thiosulfate and organic acids) do not follow the same trends as the major ions, suggesting contributions from input fluids, or microbially mediated reactions are exerting control on their concentrations. Input water chemistry exerts an important control on the Sr and Ba concentrations in flowback water. High SO42− in the input fluids used for hydraulically fracturing the wet gas well leads to precipitation of barite-celestite in flowback fluids.