(386d) Thermodynamic Modeling of Produced Water with Electrolyte NRTL Model: Aqueous Sr2+- Na+ - so42-- Cl- Quaternary System

Oil and gas activities especially hydraulic fracturing lead to production of significant amount of highly saline flowback produced water. The environmental concerns related to disposing the produced water as well as fresh water conservation strategy, encourage the reuse of the flowback produced water especially the direct reuse for hydraulic fracturing. However, scaling of some minerals at fracturing temperature and pressure is a serious concern for using saline water for fracturing. One of the leading cations in sulfate type scaling is strontium (Sr²⁺) which indicates the importance of comprehensive thermodynamic modeling of aqueous system including Na⁺-Cl^- ions which cause more than 90% of salinity of brine solution, and Sr²⁺- SO₄^2-ions.

In prior studies, Pitzer model has been mostly used for saline water modeling. However, the model results have been shown to be accurate only for dilute solutions with ionic strength up to 6 molal. Further, the need for ternary interaction parameters in addition to binary interaction parameters makes the application of this model for multi-component produced water cumbersome. In most cases, there are not enough available experimental data to correlate numerous Pitzer model parameters.

Therefore in this study symmetric eNRTL activity coefficient model is used to model the aqueous Sr²⁺-Na⁺- SO₄^2--Cl^- quaternary system. The model requires only molecule-electrolyte and electrolyte-electrolyte binary interaction parameters. The temperature dependency of these interaction parameters are described via Gibbs-Helmholtz equation including up to three temperature coefficients. Experimental data such as osmotic coefficient, mean ionic activity coefficient, vapor pressure, heat capacity, excess enthalpy, and salt solubility at different temperature up to 473.15 K and also at different electrolyte concentration up to saturation are used to identify the model parameters. The model provides accurate representation for all thermodynamic properties of the quaternary system.