COMPUTER MODELING FOR GEOTHERMAL SYSTEMS - PREDICTING CARBONATE AND SILICA SCALE FORMATION, CO2 BREAKOUT AND H2S EXCHANGE

This paper describes chemical equilibrium models for predicting carbon ate and silica scale formation, CO2 breakout and H2S gas exchange in g eothermal brine systems to high concentration and temperature. The equ ilibrium description is based on a minimization of the free energy of the system with solute activities described by the semiempirical equat ions of Fitter (1973; 1987). The carbonate model is parameterized by a ppropriate osmotic, electromotive force and solubility data(T less tha n or equal to 250 degrees C) available in binary and ternary solutions in the seawater Na-K-H-Ca-Cl-SO4-H2O system. The silica model is para meterized by solubility data to 320 degrees C in the Na-Mg-ClSO4-SiO2- H2O system. The H2S model is parameterized by solubility data in the H 2S-NaCl-H2O system to 320 degrees C. The predictive capabilities of th e models are demonstrated by comparison to both laboratory and field d ata. Examples have been given to illustrate the use of the carbonate m odel to predict downhole brine compositions in contact with specified formation minerals, temperature and pressure effects on carbonate scal ing, the effect of scale inhibitors and breakout characteristics. Appl ication of the silica model demonstrates the effect of temperature on silica scale formation. These illustrations show that the models can b e used to reliably predict important chemical behavior in geothermal o perations.