Removal of Ca2+, Mg2+, Ba2+ and Sr2+ from shale gas flowback water from the Sichuan Basin in China by chemical softening under the guidance of OLI Stream Analyzer: precipitation behaviors and optimization study

Abstract

The disposal of flowback water is recognized as a key issue for the sustainable shale gas development and discharge after reasonable treatment is considered as a feasible pathway. One of the challenges during treatment is the severe mineral scaling potential in reverse osmosis desalination, especially with high amounts of Ca²⁺, Mg²⁺, Ba²⁺ and Sr²⁺ in flowback water. In this study, precipitation behaviors of Ca²⁺, Mg²⁺, Ba²⁺ and Sr²⁺ during traditional chemical softening was evaluated so as to achieve optimal chemical dosage. Both jar tests and OLI Stream Analyzer simulation revealed that the main precipitates were CaCO₃, SrCO₃ and BaSO₄ during Na₂CO₃ addition, and Ba²⁺ could not be removed efficiently by Na₂CO₃ unless a high dosage was applied since Ba²⁺ would react after the precipitation of Ca²⁺ and Sr²⁺. Reverse Osmosis System Analysis simulation indicated that Ba²⁺ was a concern because Ba²⁺ would form tenacious BaSO₄ scale on the reverse osmosis membranes. Finally, the Na₂SO₄-NaOH-Na₂CO₃ process was proposed for chemical softening as it has a high removal efficiency and low chemical cost. Overall, this study presents an effective chemical softening method and OLI Stream Analyzer could serve as a reliable tool for the calculation, which would finally improve the design and operation of shale gas flowback water treatment.