Carbon dioxide can have a significant effect on the production performance of geothermal reservoirs. The main impact is on the flashing point of water - carbon dioxide mixtures. Even small amounts of carbon dioxide can significantly increase the flashing point considerably. Hence at relatively high values of pressure, a gas phase could form during production either in the well or sometimes in the reservoir. When modeling geothermal systems with carbon dioxide it becomes crucial to include the effects of carbon dioxide in the model. Therefore it is very important to be able to keep track of the inventory of carbon dioxide. During production/re-injection operations the amount of carbon dioxide could change. This change in carbon dioxide should be modeled accurately to be able to make accurate future performance predictions. Even if two phase conditions never occur in the reservoir, it is still very important to account for the change in carbon dioxide due to the wellbore. The changes in carbon dioxide significantly affect the flashing point depth and the wellhead pressure. High wellhead pressures are necessary for keeping power plants operational and maintaining high production flow rates.