Engineered geothermal systems (EGS) may utilise carbon dioxide as a heat extraction fluid instead of water. Nevertheless, water present in the geothermal reservoir will be extracted into the working fluid, affecting fluid flow behaviour and the required surface plant design for such a system. Dissolved water in a carbon dioxide-rich phase changes thermodynamic properties, and causes corrosion, and erosion where water droplets condense. The conditions for condensation of water in such a system have not been examined. We present condensation curves that predict conditions for water condensation, and bubble curves that predict carbonic acid concentration in a condensed H2O-rich phase. These diagrams predict concentration thresholds for condensation in the production wellbore and surface equipment. Predicted concentration thresholds for condensation do not change significantly in response to change in water content. The probable minimum CO2 concentration allowable for direct use of carbon dioxide as a working fluid is 95% for the turbine. (C) 2014 Elsevier Ltd. All rights reserved.