Comparative Geochemistry of Flowback Fluids from the Utica and Marcellus Formations
Flowback fluid chemistry and isotopic compositions provide “windows” into the kinds of rock-water interactions occurring during and after hydraulic fracturing. In turn, quantifying these interactions allow us to estimate the extent of the stimulated reservoir volume affect by frac fluids. Flowback/Produced fluid samples from several wells were collected from two Utica-Point Pleasant (UPP) sites (UPPW and UPPS) in Ohio, and one Marcellus (Marcellus Shale Energy and Environment Laboratory (MSEEL)) site in West Virginia over a period of approximately two years. Although these formations have different depositional environment, diagenetic history, and geochemical and mineralogical composition (the UPP is carbonate rich and the Marcellus is more siliceous), analysis of trends in fluid species over time shows that, overall, the TDS and major solubilized elements in the UPP and Marcellus brines are remarkably similar (Na, Ca, Cl). Total dissolved solute (TDS) in these brines ranged from approximately 40 to 250 g/L salt and in general increased over time. The behavior of Na, Br, and Cl suggests that the produced water signatures from these formations are derived from the native rock brines derived from evaporated seawater. There is a strong correlation between Cl and Br, indicating that they behave conservatively, and the similarity among brines from all wells shows no appreciable contribution of salt from halite dissolution because Br is excluded from the halite structure. Other elements, such as K, which readily reacts between fluids and ion exchange sites on clays, generally exhibit conservative behavior for an individual site, but show large variations among well pads. The concentrations of Sr and Ba vary dramatically among well sites, and increase with respect to Cl-, suggesting solubilization from the formation(s). The UPPW well site has very low Ba due to high-sulfate input fluid, which resulted in precipitation of barite/celestite in the brines, while the UPPS well site had elevated Sr, presumably due to the use of Sr-rich recycled brine used in hydraulic fracturing. The Marcellus site has the highest Ba concentrations (up to 10 g/l) and Ba/Sr ratios in the fluids, due to the high concentration of barium in the Marcellus target (~ 1000 ppm, as compared to ~200 ppm in the UPP). These observations suggest that solutes are derived from native brines and water-rock interaction, and that input fluid chemistry can influence flowback fluid chemistry.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019