Thermodynamic Model Improvements for Common Minerals at High Temperature, High Pressure and High TDS with Mixed Salts

Abstract

Deep water oil and gas production has been challenged due to limited knowledge of thermodynamic properties of minerals under high temperature (150 to 200 o C), high pressure (1000 to 1500 bar), and high total dissolved solids (more than 300,000 mg/L). Scale prediction models are limited by lack of experimental data and inadequate understanding of modeling parameters. With a new apparatus, solubilities of barite and calcite were measured at temperatures up to 250 o C, pressures up to 22000 psi, and ionic strength up to 6 m NaCl and in the presence of elevated concentrations of mixed electrolytes representing the nealy maximum range of interferences expected in brine. Preliminary measurement of gypsum solubility under high pressures were also conducted. Based on solubility measurements, the temperature and pressure dependence of stability constants and Pitzer coefficients were validated and adjusted. In addition, specific ion interactions are validated and constructed through solubility measurement in synthetic brine with mixed electrolytes. On these bases, our thermodynamic model was improved and can be applied to scale prediction for barite and calcite under extreme conditions usually encountered in the oil field.