Chemical modeling for precipitation from hypersaline hydrofracturing brines

Hypersaline brines from hydrofracturing contain salt concentrations far exceeding those of seawater, reaching up to 330,000 mg/L TDS. Traditional methods for calculating activity coefficients fail at these extreme ionic strengths. Prior research has shown that conventional equations become unreliable above 1 mol/kg ionic strength. This gap motivated the need for alternative modeling approaches. The Pitzer model has been proposed for such high-salinity systems. However, its application in hydrofracturing brines remains limited. The Pitzer model allows for accurate activity coefficient estimation in complex ionic environments. No prior work had resolved how this model could predict precipitation behavior in hypersaline hydrofracturing brines. This uncertainty drove the current study's focus on Pitzer-based modeling.

The study aimed to assess the effectiveness of the Pitzer model in predicting precipitation in hypersaline hydrofracturing brines. The authors sought to compute accurate activity coefficients for various ions at extreme ionic strengths. They focused on sulfates and carbonates such as BaSO4, CaSO4, and SrCO3. The goal was to determine if modeled precipitation aligns with experimental and field data. This approach could improve brine management practices. The study also aimed to explore radium co-precipitation with sulfate minerals. The researchers wanted to validate the Pitzer model's utility in real-world conditions. Their findings could inform better handling of hypersaline brines in industrial settings.

The researchers employed the PHREEQC program, which uses the Pitzer model, to calculate activity coefficients. They selected several sulfates and carbonates for analysis, including BaSO4 and SrCO3. The ionic strengths ranged from 2.1 to 5.7 mol/kg. Activity coefficients for divalent cations were computed at these high concentrations. The team compared modeled results with laboratory experiments and field observations. They monitored radium co-precipitation using liquid scintillation and gamma emissions. The Pitzer-derived coefficients were used to simulate sulfate and carbonate precipitation. The model's predictions were validated against real-world data from hydrofracturing operations.

At 2.1 mol/kg ionic strength, divalent cation activity coefficients ranged from 0.1 to 0.2. By 5.7 mol/kg ionic strength, coefficients increased significantly, reaching up to 2.1 for Mg(2+). Sr(2+) and Ca(2+) showed moderate increases, with coefficients of 0.6 and 0.8 respectively. The sulfate activity coefficients were in the 0.02-0.03 range. Carbonate activity coefficients were slightly lower, between 0.01 and 0.02. Modeled precipitation matched well with laboratory and field data for sulfates and carbonates. SrSO4 was found to co-precipitate radium effectively. The PHREEQC model accurately predicted these precipitation behaviors.

The authors concluded that the Pitzer model, implemented via PHREEQC, effectively predicts precipitation in hypersaline brines. Modeled results aligned closely with experimental and field data for sulfates and carbonates. The model's accuracy at high ionic strengths supports its use in brine management. SrSO4's ability to co-precipitate radium suggests a practical application for radium removal. These findings suggest the Pitzer model is a reliable tool for high-salinity systems. The study did not propose new essential mechanisms but validated existing modeling approaches. The results support the use of Pitzer-based modeling in industrial brine treatment. The authors emphasized the importance of accurate activity coefficient estimation for effective brine management.