Predictions of boiling, scaling, and solubility of gases are important to the exploitation of geothermal resources. These chemical phenomena are strong functions of the composition, temperature and pressure of the working fluid and are therefore difficult to predict from prior experience or tabulated data. Thermodynamic models of geothermal brine behavior recently developed by the authors are introduced and used to study various problems involved in geothermal energy production. The models have been extensively tested. Results of comparisons with laboratory measurements indicate that predictions are very close to experimental data for a wide range of conditions. Comparisons with field data also show remarkable agreement. Here the models are also applied to the prediction of breakout and scale formation in geothermal wells. Good agreement is obtained with actual well performance.