This work presents a numerical model for simulating the response of a geothermal reservoir to exploitation. The techniques developed are more efficient and in many ways superior to those of previous investigators. The model is capable of yielding a description of transient mass and heat flow in either a one- or two-dimensional reservoir defined by Cartesian or cylindrical coordinates. The techniques enable simulation of a geothermal flow in all three thermodynamic states ??? compressed water, two-phase and superheated steam regions ??? and transitions between these states. The model is able to simulate a geothermal system where the presence of carbon dioxide as a second component influences exploitation response. Results are presented for a range of reservoir states. The effects of different physical parameters are considered. The usefulness of the model for looking at real systems is demonstrated by simulating the development of Wairakei and Broadlands geothermal areas in New Zealand.