High temperature reinjection scheme is applied to many recently constructed geothermal power stations to prevent wells and reservoirs from silica scaling and to make an effective thermal recovery. At Sumikawa, Northeast Japan, production and reinjection started in November 1994 to supply steam for the geothermal power station. The injectivity has progressively decreased with time. To clear the effect of water temperature on the injectivity, injection tests were carried out for SD-1 at Sumikawa in 1995, where borehole pressures were monitored with changes in injection temperature and flow rate. The temperature of the injected water was controlled to be approximately 50°C and 150°C at wellhead. The results show that the pressure differences between different injection flow rates and the borehole pressures at the depth a pressure gage was located are smaller and lower respectively, for low temperature water than those for high temperature water. This means that the transmissivity and the injection flow rate at low injection temperature become larger than those at high injection temperature. In conclusion, it can be said that water temperature affects the permeability-thickness and the injection flow rate, due to the changes in fracture aperture and reservoir pressure around the injection well, changing the injectivity.
Three-dimensional imaging of oriented granite cores from the borehole WD-1a, located at the Kakkonda geothermal field (Northeast Japan), was performed by a medical X-ray CT scanner. This paper confirms that X-ray CT is powerful tool for core observation because of (1) non-destructive observation, (2) rapid three-dimensional imaging and (3) easy operation. CT images display miarolitic cavities, felsic minerals, mafic minerals and en-claves inside granite cores. Three-dimensional distribution of miarolitic cavities can be constructed from CT im-ages and shows spatially heterogeneous development with the boundary which strikes 20°, and dips 55°to the NW. Because this trend is coincident with that of hypocenter distribution of microearthquakes, distribution of isothermal contour and resistivity gradient, these cavities may be related to the development of the Kakkonda geothermal reservoir.
The microthermometric measurements of fluid inclusions in quartz in granodiorite, and hydrothermal anhydrite and adularia were examined to determine the timing of fracture development in the Hijiori hot dry rock test site, Yamagata Prefecture, Japan. Two stages of fracturing are distinguished by the type of host mineral. The first stage is recorded as healed microcracks (fluid inclusion planes) in quartz of the granodiorite, and in anhydrite veins cutting the granodiorite. This stage is characterized by (1) homogenizationtemperatures inconsistent with the formation temperatures measured by borehole logging at samplingdepths, (2) salinities ranging from 3.5 to 5.0 wt.% eq. NaCl. The minimum salinity close to that of sea water suggests that this stage was related to Miocene submarine volcanic activity. The second stage is recorded in hydrothermal anhydrite and adularia. This stage is characterized by (1) the temperature-depth profile obtained from vein fillings close to profile measured after drilling, and by (2) salinities ranging from 0.0 to 2.9 wt.% eq. NaCl similar to modern brine. These characters possibly indicate that this stage is related to Holocene volcanic activity.
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