Solubility Measurements and Predictions of Gypsum, Anhydrite, and Calcite Over Wide Ranges of Temperature, Pressure, and Ionic Strength with Mixed Electrolytes

Today’s oil and gas production from deep reservoirs permits exploitation of more oil and gas reserves but increases risks due to conditions of high temperature and high pressure. Predicting mineral solubility under such extreme conditions is critical for mitigating scaling risks, a common and costly problem. Solubility predictions use solubility products and activity coefficients, commonly from Pitzer theory virial coefficients. However, inaccurate activity coefficients and solubility data have limited accurate mineral solubility predictions and applications of the Pitzer theory. This study measured gypsum solubility under its stable phase conditions up to 1400 bar; it also confirmed the anhydrite solubility reported in the literature. Using a novel method, the virial coefficients for Ca²⁺ and \({\text{SO}}_{4}^{2 - }\) (i.e., \(\beta_{{{\text{CaSO}}_{4} }}^{(0)} ,\beta_{{{\text{CaSO}}_{4} }}^{(2)} ,C_{{{\text{CaSO}}_{4} }}^{\phi }\)) were calculated over wide ranges of temperature and pressure (0–250 °C and 1–1400 bar). The determination of this set of virial coefficients widely extends the applicable temperature and pressure ranges of the Pitzer theory in Ca²⁺ and SO ₄ ²⁻ systems. These coefficients can be applied to improve the prediction of calcite solubility in the presence of high concentrations of Ca²⁺ and SO ₄ ²⁻ ions. These new virial coefficients can also be used to predict the solubilities of gypsum and anhydrite accurately. Moreover, based on the derived \(\beta_{{{\text{CaSO}}_{4} }}^{(2)}\) values in this study, the association constants of \({\text{CaSO}}_{4}^{\left( 0 \right)}\) at 1 bar and 25 °C can be estimated by \(K_{\text{assoc}} = - 2\beta_{{{\text{CaSO}}_{4} }}^{(2)}\). These values match very well with those reported in the literature based on other methods.