Prediction of mineral scale formation in geothermal and oilfield operations using the Extended UNIQUAC model: Part II. Carbonate-scaling minerals

Two parameters have been added to the Extended UNIQUAC model of Thomsen and Rasmussen [Thomsen, K., Rasmussen, P., 1999. Modeling of vapor–liquid–solid equilibrium in gas-aqueous electrolyte systems. Chem. Eng. Sci. 54, 1787–1802] to account for the pressure dependency of mineral solubility. The improved model has been used for correlating and predicting vapor–liquid–solid equilibrium for different carbonate systems (CaCO₃, MgCO₃, BaCO₃ and SrCO₃) causing mineral scaling problems. The solubility of NaCl and CO₂ in pure water and the solubility of CO₂ in NaCl and Na₂SO₄ solutions have also been correlated. The results show that the Extended UNIQUAC model, with the added pressure parameters, is able to represent binary (NaCl–H₂O, CaCO₃–H₂O, BaCO₃–H₂O, SrCO₃–H₂O, MgCO₃–H₂O, Mg(OH)₂–H₂O and CO₂–H₂O), ternary (CaCO₃–CO₂–H₂O, BaCO₃–CO₂–H₂O, SrCO₃–CO₂–H₂O, MgCO₃–CO₂–H₂O, CO₂–NaCl–H₂O and CO₂–Na₂SO₄–H₂O), and quaternary (CO₂–NaCl–Na₂SO₄–H₂O) solubility data within the experimental accuracy in the range of temperatures and pressures considered in the study, i.e. from 0 to 250 °C, and from 1 to 1000 bar, respectively.The modified Extended UNIQUAC model will be a useful tool for predicting and quantifying the scaling problems that may occur in wells and surface equipment during geothermal operations. This would allow adequate preventive measures to be taken before mineral deposition becomes troublesome.