Abstract The behavior of gases in geothermal fluids was studied at Republic Geothermal, Inc.'s (RGI's) East mesa test site. Specifically, the solubility of carbon dioxide (CO2) and nitrogen (N2) as a function of temperature and-pressure was investigated in detail by chemical analyses and theoretical evaluations. Carbon dioxide determinations in the field were made using a newly designed sampling device. The analytical data give reliable CO2 mass balances in the system H2O - CO2. An attempt was made to generate a numerical model capable of describing and predicting the complicated phase behavior of CO2 and N2 in geothermal fluids. phase behavior of CO2 and N2 in geothermal fluids. The thermodynamic model developed can be applied to predict the CO2 and N2 gas behavior in reservoirs, predict the CO2 and N2 gas behavior in reservoirs, well bores and surface equipment. Although intended for East Mesa, it can easily be expanded to almost any geothermal system and to other gases. Fundamentally, the model is based on Henry's Law and takes into account the nonideal behavior of CO2 and its solubility in waters having any ionic strength. Initial comparisons of sodium bicarbonate (NaHCO3) content of the fluid resulted in large errors. Thus, the new model was revised to take the NaHCO3 content into account with subsequent good agreement between predicted and measured behavior. predicted and measured behavior. Relative to other geothermal fields the CO2 concentration in the fluids at East mesa was found to be very low (1000 ppm). Nonetheless, it will still require a significant investment in power plant extraction equipment to handle the noncondensables conventionally. The new model predicts a disproportionately high CO2 concentration in the first stage flash and very low concentration in the second stage flash. Consequently, a simple vent process is proposed that will remove most of the CO2 from the proposed that will remove most of the CO2 from the water before any major steam flash is taken. Introduction A geothermal reservoir can contain fluids in either a single or two phase condition. The fluid in RGI's East Mesa geothermal reservoir consists of a single phase liquid, i.e., water, with no apparent gas phase. Gases dissolved in the liquid phase under reservoir conditions may, however, "break out" of solution as this liquid phase is produced. This gas evolution from the liquid phase is experienced due to temperature and pressure decreases during production; with the various types of gases (CO2, N2, AR, hydrocarbons, etc.) evolving at different rates. The amount and composition of this gas phase may change continuously due to temperature and/or pressure drop during the process of producing and utilizing the geothermal fluid. The behavior of gases in the reservoir, well bores, and surface equipment during geothermal production operations is important and must be understood in detail. The objectives of the present work were to:Determine the CO2 and other gas concentrationsin the geothermal fluid at various pressures and temperatures in the field.Develop a computer model to calculate gas concentrations for various thermodynamic conditions.Compare measured with calculated data and define the reliability of the model.Suggest handling processes for noncondensablesin RGI's East Mesa field and power plant operations based on model results. CO2 DATA AS A VITAL GEOTHERMAL PARAMETER Carbon dioxide is a major component of most gases observed in geothermal systems and deserves special attention for the following reasons:1) Its phase behavior often determines the pH value of the geothermal liquid, and with that the detrimental formation of carbonate scale, particularly CaCO3 and FeCO3.