Hot dry rock, an alternate geothermal energy resource: a challenge for instrumentation

The natural internal heat from the Earth is one of the cleanest, nearly inexhaustible energy sources. The hot dry rock that composes most of the Earth's crust has the potential of becoming one of the largest reservoirs of energy economically available in the near future. The Los Alamos Scientific Laboratory, for the past five years, has been working toward exploiting this very abundant, clean energy source. The LASL technique to demonstrate the technical and economic feasibility of extracting heat from the hot dry rock source depends upon the connection of two deep boreholes drilled into impermeable precambrian granite where the temperature approaches 200/sup 0/C. The boreholes are connected by a system of hydraulic-produced fractures. Cold water flowing down the deeper hole will be heated by the hot rock and will be brought to the surface through the second hole. The hot water will be circulated through a closed-loop heat exchanger. The first phase of operating the 10 MW(t) heat extraction experiment will be conducted to determine the mechanical, physical and chemical properties of the reservoir and heat exchange system. Environmental effects will be carefully monitored. For optimum extraction of energy from this man-made geothermal reservoir, it is vital that the absolute location of the fractured system be well established. Fracture mapping techniques include an array of downhole instrumentation that must survive the hostile environment of the deep hot boreholes