The Hengill Area is an important energy source for Reykjavik and surrounding area, both for electricity and district space heating. Two production fields are located in the area: Nesjavellir and Hellisheidi. Two other potential production fields are believed to be in the area. We present a new conceptual model supported by numerical calculations for the entire Hengill Area. Calculations were performed using the TOUGH2 software suite. The model contains nine layers consisting of 966 elements each (total of 8,964). Geological survey data, down-hole measurements, and production histories from the fields have been used to calibrate the model. The model has been used to predict how production will affect the geothermal fields. Information gathered throughout the production history, such as draw-down and changes in enthalpy, have been used to re-evaluate the size and the production capacity of the production fields. Different production scenarios, such as different energy throughput, have been simulated. The model simulations have also been used to estimate the capacity of potential future production fields.