This paper presents the results of heat extraction modelling for a prototype Hot Dry Rock (HDR) system at the Soultz-sous-Forets site, France. The significant data from the 1993 experimental HDR programme at the Soultz site are summarised. Geological field evidence indicates that two major fault structures are superimposed on the four natural fracture sets used in the fracture network model. The generation of independent fracture networks allows the creation of a picture of the intersection of fractures within the borehole, which is reasonably realistic. Numerical modelling of the hydraulic tests carried out in the deepest well (GPK1) is extrapolated to predict the hydraulic behaviour in a two-well system, incorporating the present deep well, and a second deep well (GPK2) which is to be drilled in 1994/95. This allows the thermal performance of the planned system to be modelled, and indicates excessive thermal drawdown in the first six months of circulation, possibly as a result of preferential flow paths developed along a main fault. Confidence in the model will only be improved when experimental data from a circulating system are available to test the predictions, and a better understanding of the hydrogeologic system has been obtained. At this stage, the extrapolations to a two-well system should be considered only as demonstrative guidelines for further investigations.