PROJECT BACKGROUND: Many significant expenses encountered by the geothermal energy industry are related to chemical effects. When the composition, temperature or pressure of the fluids in the geological formation are changed, during reservoir evolution, well production, energy extraction or injection processes, the fluids that were originally at equilibrium with the formation minerals come to a new equilibrium composition, temperature and pressure. As a result, solid material can be precipitated, dissolved gases released and/or heat lost. Most geothermal energy operations experience these phenomena. For some resources, they create only minor problems. For others, they can have serious results, such as major scaling or corrosion of wells and plant equipment, reservoir permeability losses and toxic gas emission, that can significantly increase the costs of energy production and sometimes lead to site abandonment. In future operations that exploit deep heat sources and low permeability reservoirs, new chemical problems involving very high T, P rock/water interactions and unknown injection effects will arise.The ability to predict chemical behavior and heat content for wide ranges of composition, temperature and pressure would enable the design of optimal operating strategies to minimize or avoid these adverse chemical effects. Predicting reservoir and energy production chemistry will become even more important as deeper, higher temperature geothermal resources, with very high development costs, are utilized to meet future energy needs. Unfortunately, predicting the chemistry of geothermal systems is difficult because of the variability and complexity of the solidliquid-gas equilibria controlling chemical behavior. These equilibria are complicated functions of the system's composition, temperature and pressure (XTP). These variables are different for different resources and can vary over time and from well to well in a single resource. Marked changes in XTP variables also occur during energy extraction from a specific site. Reliance on experimental data for estimating behavior is generally inadequate because the data pertain to a given set of variables and are often for relatively low temperature (˜100°C) and pressure (≈1 bar). The chemical complexity of geothermal systems makes the extrapolation of data to different conditions problematic. New laboratory simulations are time-consuming and costly, particularly at high temperature and pressure. Moreover, the very high temperatures and pressures of deep crustal and magma resources, where the bulk of the earth's heat is stored, are difficult or impossible to duplicate by current experimental methods.Computer modeling technologies present the means to resolve these difficulties. Wellconstructed models, based on sound thermodynamic principles, can handle the complicated chemical reactions determining the behavior of geothermal energy production processes. They provide a means of succinctly summarizing large amounts of experimental data and of With support from the DOE Ge...