Unlocking the potential of hydraulic fracturing flowback and produced water for CO2 removal via mineral carbonation

Flowback and produced water (FPW) generated by hydraulic fracturing operations is highly saline and contains elevated concentrations of ions including calcium and magnesium. Here, we investigate the use of FPW as a source of calcium and magnesium for carbon dioxide (CO2) removal and storage in carbonate minerals. We performed pH titration experiments to precipitate calcium and magnesium from three FPW samples from Alberta and Saskatchewan, Canada. Depending on the pH (8.5–12), calcite (CaCO3) and brucite [Mg(OH)2], with occasional aragonite (CaCO3) and portlandite [Ca(OH)2] precipitate from FPW within 24 h of exposure to atmospheric pCO2 at ∼18 °C. Our experiments demonstrate that pH adjustments are an effective means of precipitating calcium and magnesium carbonates and hydroxides from FPW, but that relying solely upon passive removal of CO2 from air is limiting to carbonation. We estimate that carbonation of all calcium and magnesium from brines produced in conventional and hydraulic fracturing operations in Canada could store up to 1,010 Mt CO2 annually. Carbonation rates could be improved by using higher pCO2 gases, such as flue gases from fossil energy generation (via CO2 capture), gas streams produced by Direct Air Capture plants or using novel looping technologies.