Nobuo Doi

Deep geothermal resources survey program: igneous, metamorphic and hydrothermal processes in a well encountering 500°C at 3729 m depth, kakkonda, japan

Abstract The exploration well WD-1a was drilled to a depth of 3729 m in the Kakkonda geothermal field, northeast Japan, using efficient borehole cooling techniques. The well penetrated an entire shallow hydrothermal convection zone, an entire contact metamorphic aureole and part of a neo-granitic pluton. The recovered temperature of the well indicates a boiling point-controlled profile up to 380°C to a depth of 3100 m, and a conduction-controlled profile with a very high gradient from 3100 m to the bottom of the hole, where the temperature is 500°C. WD-1a may be the first geothermal well that encountered 500°C, which exceeds the conventional hydrostatic boiling-point curve. An inflection point of the temperature–depth profile at 3100 m and about 380°C reflects the brittle–plastic boundary. The brittle–plastic boundary constrains the maximum depth of fracture formation, and the fracture distribution constrains the maximum depth of hydrothermal convection.

Petrographic features of a high-temperature granite just newly solidified magma at the Kakkonda geothermal field, Japan

Abstract A 3729-m-deep geothermal research well, WD-1a, provides us with a unique opportunity to study initial petrographic features of a high-temperature granite just after solidification of magma. The well succeeded in collecting three spot-cores of the Kakkonda Granite that is a pluton emplaced at a shallow depth and regarded as a heat source of the active Kakkonda geothermal system. The core samples were collected at the present formation temperatures of 370, 410 and over 500°C. These samples are granodiorite to tonalite consisting mainly of plagioclase, quartz, hornblende, biotite and K-feldspar. A sample collected at a formation temperature of over 500°C possesses the following remarkable petrographic features compared to the other two samples. Interstitial spaces are not completely sealed. K-feldspar exhibits no perthite by the exsolution of albite lamella. Quartz includes glassy melt inclusions without devitrification. Hornblende is less intensively altered to actinolite, and biotite is not altered. This study directly confirmed that perthite in K-feldspar is a recrystallization texture formed at 410–500°C based on a comparison of the in situ temperatures of the samples. Chemical compositions of minerals were analyzed to compare temperatures determined from geothermometers in several publications to the in situ temperatures of the samples.

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 that exist beneath the already developed shallow reservoirs. A deep drill hole, WD-1, reached a depth of 3,729 m in July 1995 by applying the latest drilling techniques, such as a top-drive drilling system, enabling the collection of highly valuable information for understanding the characteristics of deep geothermal systems. Side-track drilling of WD-1 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.