Impact of Water Chemistry on Element Mobilization from Eagle Ford Shale

Rapid expansion of hydraulic fracturing operations for natural gas and oil production can impact water quality. Water that flows back to the surface as part of the hydraulic fracturing process and during well production can contain trace elements, including regulated metals and metalloids, mobilized by interactions of the fracturing fluid with the formation. The rate and extent of mobilization depend on the geochemistry of the formation, the composition of the fracturing fluid, and the contact time. Laboratory experiments detailed here examined the influence of water chemistry on element mobilization from core samples taken from the Eagle Ford Formation, which is currently producing natural gas from hydraulically fractured zones. Fluid properties were varied with regard to pH, oxidant level, and solid:water ratio. Release of elements (Ca, Mg, Fe, Ba, As) from the Eagle Ford samples strongly depended on pH, which in turn was primarily controlled by calcite dissolution. Presence of oxygen and the addition of H2O2 led to pyrite oxidation and resulted in an elevated amount of sulfate. Barium concentrations were largely controlled by the amount of sulfate present through solubility equilibrium of barite that formed as a secondary phase. The effect of increasing solid:water ratio on the extent of mobilization varied widely for different elements. Taken together, these findings demonstrate the need to understand both the aqueous and geochemistries of a hydraulically fractured formation with regard to elemental mobilization in produced and flowback waters.