Abstract The development of HDR reservoirs in the Carnmenellis granite has involved HFSM to depths of up to 2.6 km in several test series. The results of these and other stress measurements, and the operational experience gained, are reviewed in terms of proposed developments at 6 km. Potential borehole tangential stresses are examined in detail. HFSM will not be able to provide complete stress information because of geological and equipment limitations. However, in conjunction with other techniques, notably borehole breakout analysis, sufficiently comprehensive results appear possible to the extent that HDR systems can be designed and modelled.