The feasibility study of HDR system in Japan started at 1974 under Sunshine-Project. Field tests have been made at Yakedake Area in middle Japan since 1977, contributing to improve some techniques such as Hydraulic-Fracturing, Fracture-Mapping and so on. This paper presents details of the recent HDR project up to 1982 at Yakedake.
The temperature distribution in the porous geothermal reservoir, in which the reinjection water flows linearly, is calculated by means of the method of shifting particles proposed by Jinno et al. in order to solve the solute transport equation. In the method instead of solving the heat and mass transport equation directly, the characteristic equations are determined and solved. Although there are some complexities in designing influence element meshes and locating points, numerical solutions obtained by the method show fairly good agreements with analytical ones for the two dimensional steady state heat and mass transport equation. The method is superior in accuracy to the conventional finite difference method.
The author calculated the amount of heat supply from the upper mantle and estimated the temperature distribution at the Moho, in and around the Japanese Islands (33°-47°N, 127°-146°E). Although the depth of the Moho is different from place to place, there exists some characteristic tendencies in temperature distribution at the Moho:(A) Low temperature distribution is seen in southwest Japan (except the Green-Tuff region), which corresponds to the geologically non-active area, (B) Low temperature distribution is seen in outer side (Pacific Ocean side) of northeast Japan, which corresponds to the geologically active island arc, (C) High temperature distribution is seen in inner side (Japan Sea side) of northwest Japan, (D) High temperature distribution is seen in the Green-Tuff region, (E) High temperature distribution is seen in the Japan Sea and the Philippine Sea, which are marginal seas of southeastern Japanese Islands arc and the Izu-Bonin arc, respectively. Comparing the Japan Sea with Philippine Sea, large difference in temperature distribution pattern at the Moho can be seen. In the Japan Sea, fluctuation of temperature distribution is relatively small comparing with other regions. However, in the Philippine Sea, very large (about 500°C) fluctuation of temperature at the Moho can be comfirmed in some parts in this region. Obviously, this pattern of thermal state of the Moho in both regions may be caused by the difference of thermal structure in the upper mantle. In the Green-Tuff region, the amount of heat supply from the upper mantle would be estimated between 1.0 HFU and 1.5 HFU. In the areas including west of off Hokkaido, southern part of the Japan Sea, San-in district (Japan Sea side of southwestern part of Honshu), a part of Philippine Sea and west of off Kyushu regions, the temper-ature of the Moho will be exceeded 1000°C. Almost of these regions are located in high heat flow area (Higher than 2.0 HFU) and inner area from the volcanic front. Estimating from temperature distribution pattern in the Moho, the uppermost depth of the partial melting zone will be reached the shallower depth of the upper mantle in the Japan Sea and the Philippine Sea regions than the other regions.
A new method is proposed for the measurement of in situ stress in the deep earth's crust. The method utlizes hydraulic fracturing technique and measurements of AE signals emerging from the cracks created during the hydraulic fracturing. Its basic concepts are derived through considerations on crack initiation and crack curving in a rock mass under three dimensional stress states, so that the method is available to the case that the direction of the principal axes of stress is totally unknown; this is generally the case of most geothermal fields. Whereas the conventional hydraulic fracturing stress measurement, which is based on the two dimensional theory of elasticity, is not necessarily adequate to the above mentioned case, since the conventional method assumes that one of the principal axes of stress is vertical and the magnitude of the principal stress in the vertical direction is equal to the overburden pressure.
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