Reducing Freshwater Consumption in the Marcellus Shale Play by Recycling Flowback With Acid Mine Drainage

Abstract Acid mine drainage (AMD), the largest source of stream pollution in the state of Pennsylvania, has the potential to be utilized as a make-up fluid for hydraulic fracturing in the Marcellus shale. Abandoned coal mines across the state are responsible for discharging AMD into nearby streams, imairing local ecosystems. If AMD could be used in hydraulic fracturing operations, gas companies can be responsible for a cleaner environment in Pennsylvania. To use AMD in hydraulic fracturing, the high sulfate (SO42-) concentration must be reduced (<100 mg/L). If not, there is a risk of scale formation in the wellbore. Sulfate concentrations can be reduced by mixing AMD with flowback, which has high concentrations of barium (Ba2+) and strontium (Sr2+). Sulfate will react with the barium and strontium to precipitate barite (BaSO4) and celestite (SrSO4) which can be removed from the fluid. The challenge is determining mix ratios of the two fluids, AMD and flowback, to reduce these three constituents to acceptable levels. Both AMD discharges and flowback vary from location to location making this challenge a difficult one. For this study four coal mine discharges and two wells located in southwestern Pennsylvania were sampled. These samples were mixed at various ratios to determine removal efficiencies and kinetics for sulfate, barium and strontium precipitation. Ideally, these reactions would occur quickly and result in a fluid free of sulfate, barium and strontium. The mixing tests showed that barium and sulfate could be removed to acceptable levels, while strontium could only be removed by a maximum of 50%. X-ray diffraction (XRD) analysis concluded that celestite does not form when the fluids are mixed, rather strontium co-precipitates with barium. Kinetic tests showed that precipitation reactions occur quickly, within one day of mixing and no further reactions occurred.