Treating produced water from hydraulic fracturing: Composition effects on scale formation and desalination system selection

Produced water from unconventional gas and oil extraction may be hypersaline with uncommon combinations of dissolved ions. The aim of this analysis is to aid in the selection of produced water treatment technology by identifying the temperature, pH, and recovery ratio under which mineral solid formation from these produced waters is likely to occur. Eight samples of produced water from the Permian Basin and the Marcellus shale are discussed, with an average TDS of about 177 g/L but significant variability. Crystallization potential is quantified by the saturation index, and activity coefficients are calculated using the Pitzer model. The method is applied to estimate solid formation in the treatment of two design case samples: a 183 g/L sample representing the Permian Basin water and a 145 g/L sample representing the Marcellus. Without pretreatment, the most likely solids to form, defined by highest saturation index, are: CaCO3, FeCO3, MgCO3, MnCO3, SrCO3, BaSO4, CaSO4, MgSO4 and SrSO4. Some options for mitigating the formation of these scales are discussed. With appropriate pretreatment, it is estimated that recovery ratios of as high as 40-50% are achievable before NaCl, a major constituent, is likely to limit further concentration without significant crystallization.