Hypersaline hydrofracturing brines host very high salt concentrations, as high as 120,000–330,000 mg/L total dissolved solids (TDS), corresponding to ionic strengths of 2.1–5.7 mol/kg. This is 4–10 times higher than for ocean water. At such high ionic strengths, the conventional equations for computing activity coefficients no longer apply; and the complex ion-interactive Pitzer model must be invoked. The authors herein have used the Pitzer-based PHREEQC computer program to compute the appropriate activity coefficients when forming such precipitates as BaSO 4 , CaSO 4 , MgSO 4 , SrSO 4 , CaCO 3 , SrCO 3 , and BaCO 3 in hydrofracturing waters. The divalent cation activity coefficients (γ M ) were computed in the 0.1 to 0.2 range at 2.1 mol/kg ionic strength, then by 5.7 mol/kg ionic strength, they rose to 0.2 for Ba 2+ , 0.6 for Sr 2+ , 0.8 for Ca 2+ , and 2.1 for Mg 2+ . Concurrently, the γ SO 4 2− was 0.02–0.03; and γ CO 3 2− was 0.01–0.02. While employing these Pitzer-derived activity coefficients, the authors then used the PHREEQC model to characterize precipitation of several of these sulfates and carbonates from actual hydrofracturing waters. Modeled precipitation matched quite well with actual laboratory experiments and full-scale operations. Also, the authors found that SrSO 4 effectively co-precipitated radium from hydrofracturing brines, as discerned when monitoring 228 Ra and other beta-emitting species via liquid scintillation; and also when monitoring gamma emissions from 226 Ra.
All Science Journal Classification (ASJC) codes
- Ecological Modeling
- Water Science and Technology
- Waste Management and Disposal