High-temperature measurements in well WD-1A and the thermal structure of the Kakkonda geothermal system, Japan

The New Energy and Industrial Technology Development Organization (NEDO) dr illed well WD-1a between 1994 and 1995 in the Kakkonda geothermal field as part of their "Deep Seated Geothermal Resources Survey" project. High-tempe rature measurements were carried out in WD-1a. Logging temperatures above 4 14 degrees C were confirmed at 3600 m and 3690 m depth after 82 h standing time. Simple Horner extrapolations based on observed temperatures up to 82 h after shut-in suggested a temperature of about 500 degrees C at 3500 m de pth. Temperatures between 500 degrees C and 510 degrees C were also confirm ed at 3720 m depth after 129-159 h standing time, using calibrated melting tablets. These are the highest temperatures measured in a geothermal well. These results suggest a thermal structure consisting of three layers. Layer one is a shallow permeable zone of the reservoir, at less than 1500 m dept h, at 230 degrees C to 260 degrees C. The second layer is a deep zone of th e reservoir, which is less permeable and has a temperature of 350 degrees C to 360 degrees C from 1500 m to about 3100 m depth. The third layer is a z one of heat conduction. The transition between the hydrothermal-convection zone and the deeper heat-conduction zone is at 3100 m depth in well WD-1a. (C) 1998 CNR. Published by Elsevier Science Ltd. All rights reserved.