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.

Maximum Removal Efficiency of Barium, Strontium, Radium, and Sulfate with Optimum AMD-Marcellus Flowback Mixing Ratios for Beneficial Use in the Northern Appalachian Basin

“…Bituminous AMD sites A–D had wide-ranging SO 4 concentrations from 293 to 3873 mg/L (Table ). From the previous work, four anthracite AMD sites were sampled from northeastern Pennsylvania and had lower SO 4 concentrations ranging from 53 to 175 mg/L. , Differences in SO 4 concentrations between anthracite and bituminous regions, important for co-treatment site selection, have been previously reported in Pennsylvania and are a function of season, mine hydrology, and time since mining operations …”

“…Bituminous AMD sites A−D had wide-ranging SO 4 concentrations from 293 to 3873 mg/L (Table 1). From the previous work, four anthracite AMD sites were sampled from northeastern Pennsylvania and had lower SO 4 concentrations ranging from 53 to 175 mg/ L. 46,47 Differences in SO 4 concentrations between anthracite and bituminous regions, important for co-treatment site selection, have been previously reported in Pennsylvania and are a function of season, mine hydrology, and time since mining operations. 23 Because the four anthracite AMD sites were below, or just above, the industry proposed 100 mg/L SO 4 hydraulic fracturing cutoff limit for water utilization, this study focused only on the bituminous AMD sites with SO 4 concentrations >200 mg/L (Table 1).…”

“…The four AMD sites were selected because, collectively, their geochemical characteristics represent the bituminous coal region of southwest Pennsylvania. Previous sampling data narrowed viable AMD discharges to the bituminous coal region due to higher SO 4 concentrations. , Specific conductance, pH, temperature, and ORP were measured in the field at each location. Water samples were field-filtered with 0.45 μm cellulose acetate membrane filters and preserved with nitric acid (pH ≤ 2) for cation and trace metal analyses.…”