CO2 Mineralization in Produced Water: Transforming Waste Brines into a Carbon Sink

Abstract Produced brines from oil and gas fields are rich in divalent cations, offering the potential for CO2 sequestration as thermodynamically stable carbonates. This study investigates CO2 mineralization in produced water through yard testing the brine alkalization process in a continuously stirred tank reactor (CSTR) having a capacity of 600 L to demonstrate the kinetics and thermodynamics of magnesium and calcium hydroxide mineral precipitation at different pH regimes. Analytical characterization of the brine chemistry was done using ion chromatography (IC), inductively coupled plasma-atomic absorption spectroscopy (ICP-AES) and pH measurements. The results from the yard test demonstrate the selective removal of magnesium hydroxide after the initial alkalization step with a pH value between 8 to 9 followed by calcium hydroxide formation after the complete removal of magnesium at a pH above 10. A holistic understanding of the effect of process parameters like temperature, pressure, solid-to-liquid ratio, and the aqueous medium ionic strength is crucial for the rational design of large-scale ex-situ CO2 mineralization processes. Therefore, we investigated various parameters affecting the carbonation of the extracted calcium hydroxide minerals from the alkalization process using chemical thermodynamic modeling. The models were based on Pitzer's equations to account for the deviation from ideality in the electrolyte solution and compute the activities of its components whereas the gas phase fugacity was modeled using the Peng-Robenson (PR) equation of state. Increasing the temperature and reducing the pressure enhanced the CO2 mineralization thermodynamic yield. We also studied the stepwise addition of CO2 in a system of calcium hydroxide in water until a certain fugacity is reached (e.g., 1 atm). This simulates scenarios where CO2 mass transfer or the hydration/hydroxylation of CO2 is limiting. The amount of precipitated CaCO3 increases with adding CO2 at a stable pH, which can be between 11 and 13 depending on the temperature then CaCO3 partially dissolves as the pH drops, so the process should be terminated immediately after the pH starts plummeting to maximize yield. This work represents an advancement in CO2 mineralization in produced water, transitioning from laboratory-scale research to yard-scale testing using real brines. As no commercial CO2 mineralization technology currently exists, these findings provide valuable insights for designing large-scale CO2 mineralization processes in brine that can overcome operational challenges such as process rate, variability in feed brine chemistry, and techno-economic feasibility.