Hot Dry Rock Geothermal Heat Extraction Experiments

Abstract

A man made geothermal reservoir has been created at a depth of 2.7 km in hot, dry granite by hydraulic fracturing. The system was completed by directionally drilling a second well in close proximity with the top of the vertical fracture. In early 1978 heat was extracted from this reservoir for a period of 75 days. during this period thermal power was produced at an average rate of 4 MWt. Theoretical analysis of the measured drawdown suggests a total fracture heat transfer area of 16,000 m2. Viscous impedance to through flow declined continuously so that at the end of the experiment this impedance was only one fifth its initial value. Water losses to the surrounding rock formation also decreased continuously, and eventually this loss rate was less than 1% of the circulated flow rate. Geochemical analyses suggest that, hotter reservoirs, hot dry rock reservoirs can ultimately produce level of power on a commercial scale.