We made the machine testing the mechanical properties of rocks in wet conditions under confining pressure for estimating drillability of geothermal reservoir or stability analysis of underground structure in disposal of nuclear waste in deep rock mass becuase effect of temperature on mechanical properties of rocks might be different between in dry condition and in water circumstance. In this paper are described the results of triaxial compression tests of INADA granite in water circumstance at elevated temperature. Maximum temperature is 250°C and maximum confining pressure 300 kgf/cm2. Specimens were exposed to pressurized water without sealing in order to investigate the effect of both pore pressure, which is equal to confining pressure, and chemical reaction of hot water. Main results obtained in this study are summarized as follows: 1) Effect of confining pressure which is equal to pore pressure is negligible. Therefore, law of effective stress can explain the mechanical behavior of the granite. 2) In pressurized water, both stress of strength failure and Young's Modulus are almost identical with those at room temperature when temperature is below 100°C, remarkable decrease of both stress of strength failure and Young's modules ocurred with temperature. 3) In comparison with dry condition, decrease of stress of strength failure is much larger in wet condition of pressurized water especially when temperature is larger than 100°C. 4) As mechanism of deterioration of grante in pressurized water at elevated temperature, it is suggested that chemical reaction such as solution of minerals is important in addition to thermal stress caused by differential thermal expansion of different minerals or anisotropy of thermal expansion of each mineral.
Many primary liquid-rich inclusions are present along with a few vapor-rich inclusions in the calcite scale from DY-1 drill hole, Hohi geothermal area. They were studied microthermometrically, using a USGS type gas-flow heating/freezing stage. The data of the homogenization temperatures (Th) show that the calcite scale precipitated from the two-phase boiling fluid at 165°to 167°C. The NaCl equivalent salinity of the fluid is estimated to be 0.2 to 0.3 wt % from the data of melting point of ice (Tm, ice) in the liquid-rich inclusions. The PCO2 and pH of the fluid that produced the calcite scale are calculated to be 0.2 atm and 6.4 respectively from the data of Th and chemistry of the fluid discharged.
In order to clarify the relation between volcanic and geothermal activities in the Hohi geothermal area, fission track ages of the Kuju volcanic rocks, characterized by hornblende andesite, have been measured. The results show that the Kuju volcanism began about 0.3 Ma ago and continued to several tens thousand years ago. The most active Type A geothermal fields are associated with volcanoes younger than 0.20 Ma, the active Type B fields with those from 0.15 to 0.30 Ma in most cases. The volumes of these volcanoes range from 0.3 to 3.2 km3. Larger volcanoes tend to cool down more slowly than smaller ones.
Some equations to calculate temperature and pressure changes with time for a lumped-parameter reservoir model with water influx from an infinite aquifer are presented. The sensitivity study of the reservoir performance to V/Gp (reservoir volume/flow rate of discharge), V/B (reservoir volume/aquifer parameter) and φ(reservoir porosity) shows that V/Gp is much more influential than the others two, though the influences of these two have an increasing tendency as time passes.
ELF-MT surveys were carried out for the study of the geothermal structure in the Takenoyu Geo-thermal Area, Central Kyushu. The ELF-MT method measures natural electromagnetic field at three frequencies in Schumann reasonant band from 8 to 20 Hz, and the VLF band at 17.4 kHz are also measured. Magnetic and electric field intensities are processed with a handheld computer. Scalar apparent resistivity can thus be obtained immediately in the field. Apparent resistivity maps were compiled from 70 sites with about 0.5km interval. Major low apparent resistivity zone, less than 25 ohm-m, is associated with Takenoyu-Hagenoyu fumarous area where high underground temperatures are found at shallow depth. High underground temperatures at large depth are found at the foot of Mt. Waita and in an area 1 or 2 km north of Takenoyu-Hagenoyu. In these area, apparent resistivity values are not very low. Two dimensional resistivity model analyses were done by the finite element modeling technique. Those models indicate that low resistivity layers are associated with high underground temperature zones in shallow depth. Considering the resolution of the ELF-MT method, it was concluded that detectable depth of the bottom of the low resistivity layer is relatively shallow.
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