Hot Dry Rock (HDR) technology started from an idea to help ful®ll future energy needs as the availability of cheap fossil and other known fuels slowly reduces. The HDR concept itself is very simple but the development of the associated technology has taken signi®cantly longer than anticipated. Anyone with experience of natural materials such as rocks knows that there are always imponderables that have not been really understood and indeed cannot at present be dealt with in a fully satisfactory manner. Furthermore, geology always has a habit of presenting us with new problems. Under these auspices and considering the limited funds that have been made available, it is encouraging to note that at long last light is visible at the end of the long tunnel of uncertainty. The results of the 1997 circulation test at Soultz-sous-Forets (France) certainly seem to show that we may have come up with a type of concept and an appropriate set of background site conditions to advance the technology.
The concept of an HDR reservoir has evolved from that of a single penny-shaped fracture borrowed from the oil industry to the present graben or HWR (Hot Wet Rock) concept. International co-operation has been a key issue so far, and the expensive nature of this research demands that co-operation of this type continues to break new ground in the future. The necessary supporting technology has also evolved and the time appears to be ripe for taking advantage of this new and exciting development. Not all the answers are known, but at least we know now which questions to ask. It is worth remembering that