Numerical simulation techniques are used to study the effects of noncondensable gases (CO 2) on geothermal reservoir behavior in the natural state and during exploitation. It is shown that the presence of CO 2 has a large effect on the thermodynamic conditions of a reservoir in the natural state, especially on temperature distributions and phase compositions. The gas will expand two-phase zones and increase gas saturations to enable flow of CO 2 through the system. During exploitation, the early pressure drop primarily results from" degassing" of the system. This process can cause a very rapid initial pressure drop, on the order of megapascals, depending on the initial partial pressure of CO 2. The flowing gas content from wells can provide information on in-place gas saturations and relative permeability curves that apply at a given geothermal resource. Site-specific studies are made for the gas-rich, two-phase reservoir at the Ohaaki geothermal field in New Zealand. A simple lumped-parameter model and a vertical column model are applied to the field data. The results obtained agree well with the natural thermodynamic state of the Ohaaki field (pressure and temperature profiles) and a partial pressure of 1.5 to 2.5 MPa [217 to 363 psi] is calculated in the primary reservoirs. The models also agree reasonably well with field data obtained during exploitation of the field. The treatment of thermophysical properties of H 2 O/CO 2 mixtures for different phase compositions is summarized.