(534e) Development of Saline Water Thermodynamic Model for Hydraulic Fracturing

The main problem with treating and reusing hydraulic fracturing flowback or produced water is precipitating followed by scaling. These fracturing products contain dissolved ions such as barium (Ba²⁺), strontium (Sr²⁺), calcium (Ca²⁺), carbonate (CO₃^2-), or sulfate (SO₄^2-) which can precipitate and produce scale. This can happen whether in high concentration saline system in case of disposing of these produced water to the environment or in the fractures underground because of high temperature in case of reusing them as a fracturing fluid.

The precipitation process is controlled by not only reaction kinetics, but also by thermodynamic properties of electrolytes in different physical conditions of the system. Thus, developing a comprehensive thermodynamic model which could be used to investigate the phase behavior of the saline water seems to be vital. The key thermodynamic and physical properties here include activity coefficient, enthalpy of solution, heat capacity, salt solubility, density, and viscosity. The model must accurately represent thermodynamic and physical properties of the saline water as functions of salt concentrations and temperatures.

Pitzer’s ion-interaction model has been the model of choice for modeling aqueous electrolyte solutions. However, Pitzer’s model requires up to three ion-specific interaction parameters for each binary system and up to eight temperature coefficients for each ion-specific interaction parameters in order to cover temperatures from 273 to 473 K. In other words, up to twenty-four parameters may be required for each aqueous electrolyte system. In this paper, we present a saline water model based on symmetric electrolyte NRTL model. The model requires two pair interaction parameters for each binary system and up to three temperature coefficients for each pair interaction parameter. That means six parameters for each aqueous single electrolyte binary system. We determine the model parameters from literature data such as osmotic pressure, mean ionic activity coefficient, vapor pressure, enthalpy of solution, heat capacity, salt solubility, density, and viscosity data sets at various temperatures and concentrations. We further show the model is capable of accurately representing thermodynamic and physical properties of various saline water binary systems and higher order systems with temperatures from 273 to 473 K and salt concentrations up to saturation.