Measuring Solubility of Halite in Water and Calcium Chloride Solution to 200°C, 1200 Bar

Abstract Accurate halite solubility data in water and in CaCl2 solutions under HPHT conditions is critical for halite scale prevention or treatment strategies, such as water wash or temperature/pressure control. However, the importance of halite solubility in the system NaCl-CaCl2-H2O is contrasted by the scarce literature data, which covers only low-temperature and low-pressure conditions. Measurements of NaCl solubility at HPHT conditions have not been reported previously, mainly due to experimental challenges such as instrumentation and sample analysis. This paper describes the methodology and results of halite solubility measurement under HPHT and high Ca conditions. The new data is compared with model predictions from the current version of PHREEQC, it is also incorporated into thermodynamic database of ScaleSoftPitzer (SSP2025) to improve its prediction accuracy at extreme temperature, pressure, and composition (T/P/x) conditions. A purpose-built flow-through apparatus was designed to determine halite solubility in both water and CaCl2 solution, up to 200°C and 1200 bar. Halite crystals was packed inside a salt column, which was connected to a high-pressure pump to deliver undersaturated mixture of NaCl solution and CaCl2 solution (0-3 m), the effluent was mixed with a dilution solution, which is either LiCl tracer solution when measuring NaCl solubility in water, or pure water when measuring NaCl solubility in CaCl2 solutions. The diluted effluent is further diluted for ICP-OES analysis of Na, Ca and Li concentrations. Solubility has been determined at 25°C, 100°C and 200°C, pressure to 1200 bar, CaCl2 from 0 m to 3 m, with the temperature controlled to be within 0.5°C and pressure within 30 bar. The measured halite solubility has an estimated uncertainty of about 0.5% in CaCl2 solutions and about 1.2% in pure water. An empirical equation for halite solubility with temperature, pressure, and CaCl2 concentration has been formulated based on the newly measured halite solubility data, together with data from selected literature sources. Our solubility data of halite in water is consistent with experimental results from Sawamura et al. (2007) at 25°C. The solubility of halite in water to 300°C and 1000 bar can also be calculated from Pitzer model and halite Ksp model based on parameters given by Archer et al. (1992, 1997, 2000), the modeling results agree with our results to be within 0.5% at 25°C and 100°C, and within 1.5% at 200°C, demonstrating thermodynamic consistency. In general halite solubility increases with temperature and pressure and decreases with CaCl2 concentration. The positive temperature dependence of halite solubility is much larger than pressure dependence. Pressure dependence would decrease when [CaCl2] increases from 0 m to 3 m. PHREEQC (v3.73, pitzer.dat) predictions show good agreement with our measurement at 25°C and 100°C, but the prediction error is striking large at 200°C. This work is the first to measure halite solubility at both high-temperature and high-pressure conditions, the method would also be useful to determine halite solubility in complex brines with major compositions such as KCl, MgCl2, and CaCl2. The new data would be valuable to calibrate thermodynamic models to extreme conditions with applications not only limited to mineral scale prediction in oil & gas production.