Deepwater oil and gas production plays a more important role in global energy support today in which high temperature and high pressure (HTHP) conditions usually occur. Mineral solubility predictions at HTHP with mixed electrolytes is thus getting more attention since it is critical for getting rid of scaling risks under such extreme conditions. In this study, Pitzer theory was applied to predict the solubility of gypsum, anhydrite and calcite over wide ranges of temperature, pressure, and ionic strength with mixed electrolytes. Solubility of gypsum was measured from 0 to 40 °C, from 14.7 to 20,000 psi, with 0 to 4 mol NaCl/kg H2O. Anhydrite solubility reported in literature was confirmed and adopted in this study. The equilibrium constants of gypsum and anhydrite are incorporated by the temperature dependent part reported in SOLMINEQ.88, and Atkinson and Mecik’s pressure dependent part. Based on these solubility data, equilibrium constants, and other virial coefficients, virial coefficients for Ca2+ and SO42- interactions with pressure dependence (i.e. βCaSo4(0), βCaSo4(2), CCaSo4(0)) are fitted over wide ranges of temperature and pressure (i.e. 0 to 250°C and 14.7 to 20000 psi). The solubility of gypsum/anhydrite can be precisely predicted based on the derived virial coefficients. More importantly, they can also be applied to accurately predict calcite solubility with mixed electrolytes from 0 to 250 °C (except for 100 °C) and up to 21000 psi. Based on the βCaSo4(2) values derived in the calcium sulfates system, the association constant of CaSO(0) at 14.7 psi and 25 °C matches very well with that reported in literature derived based on other methods.

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