Scale prevention is one of the most important problems in the oil and gas industry. Due to the more aggressive production behavior recently, there are more chances to encounter high temperature, high pressure, and high TDS conditions. This study focuses on improving the scale prediction in the condition of high temperature (up to 210°C), and TDS (total dissolved solids, over 300,000 mg/L) with calcium concentration up to 2.0 molality (m). A hydrothermal autoclave reactor was developed for solubility measurement. The solubility of anhydrite was measured in the CaCl2-NaCl-H2O solution with constant ionic strength of 4 m. Results shows that the ionic strength effect and the Ca-SO4 association would increase the anhydrite solubility while the common ion effect decreased the anhydrite solubility. The measured solubility data can develop the virial coefficient for the ion interaction of Ca2+ and SO42. This virial coefficient can then be applied in Pitzer models to improve the calculation for the saturation index of scale. Quantifying the Ca-SO4 interaction parameters can make a better prediction of mineral solubility with high calcium concentration. The results can also improve not only anhydrite but all of the sulfate scale predictions at high temperature with high TDS conditions. This study offers a reliable and efficient method to obtain solubility under high temperature conditions and expands the scale prediction of the production brine with high calcium concentration at higher temperature and pressure limits.

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