Deep Geothermal Resources Survey Project in the Kakkonda Geothermal Field

Abstract

The New Energy and Industrial Technology Development Organization (NEDO) has been conducting a research project named "Deep-Seated Geothermal Resources Survey" since 1992 in order to establish a desirable direction for development of deep geothermal resources which exist beneath the alreadydeveloped shallow reservoirs. A deep drillhole, WD-l, has been drilled in the Kakkonda geothermal field in northern Japan. WD-l reached a depth of 3,729 m in July 1995 by applying the latest drilling techniques such as top-drive drilling system, enabling the collection of highly valuable information for understanding the characteristics of deep geothermal system.

Quaternary granite, considered to be a possible heat source, was encountered at depths of over 2,860m. The borehole was drilled into the granite for a length of 870 m to determine the thermal structure and deep fracture systems. Although we did not encounter major lost-circulations during drilling in the granite, we confrrmed a temperature greater than 500°C at the bottom of the hole. The recovered temperature profile suggests a drastic change from hydrothermally convective zone to thermally conductive zone at a depth of approximately 3,100 m. This implies the existence of the bottom of hydrothermal convection system at this depth. Borehole fluid sampling was carried out for geochemical investigation after the temperature recovery measurement, and extremely saline hydrothermal fluids were collected near the bottom of WD-l.

Side-track drilling of WD-l was started from a depth of 2,200 m in September 1996, targeting productive fractures expected near the boundary of the granite in a depth range from 2,800 to 3,000 m. We successfully encountered large lost-circulation at some depths, and the side-track drilling was terminated at a depth of 2,963 m in January 1997. The well is now in a process of various loggings, and we expect a production test in early summer 1997.