The cap rocks in geothermal systems are divided into the primary type and the secondary one, and the latter is called the “alteration cap rock”. The alteration cap rock is impermeable altered layer in near-surface aquifer, formed by upflow of geothermal fluids. The alteration cap rocks are subdivided into porous sediment type, younger lava type and fracture type by the kind of near-surface aquifer. And they are subdivided into neutral type, H2S related acid type and SO2 related acid type by the kind of hydrothermal alteration. Many active geothermal areas explored for power station are classified by the cap rock type, in this paper. Porous sediment type; (Mori), Uenotai, Onikobe, (Oku-aizu), Takigami, Fushime Younger lava type; Otake, Hatchobaru, Oguni, Kirishima Fracture type; (Mori), Onuma (Sumikawa), Matsukawa, Kakkonda, (Oku-aizu) Neutral type; Mori, Onuma (Sumikawa), Kakkonda, Uenotai, Onikobe, Oku-aizu, Kirishima, Oguni, Fushime H2S related acid type; Takigami, Otake, Hatchobaru SO2 related acid type ; Matsukawa It appears that many active geothermal areas have alteration cap rocks, which are mainly com-posed of neutral alteration type. A alteration mineral is mainly montmorillonite. The alteration cap rocks have role of preventing deep hot waters from cooling, of keeping hot waters in the geothermal reservoirs, and of adsorbing of H2S and CO2 gases.
A simulator has been developed to study HDR (Hot Dry Rock)/hot water interaction. This system mainly consists of center autoclave, preheat autoclave and diaphragm tank. The center autoclave is the one of fluidized tank reactor, and is designed for modeling of geothermal reservoir. Preheat autoclave and diaphragm tank are used for preparing experimental solution of various condition. The highest operative temperature is 400°C, the highest operative pressure 4OMPa, and the flow rate range among 0.1-1.0ml/min. Five separate heaters around the center autoclave are controlled independently to achieve a desired temperature gradient. A preliminary test was carried out successfully with granite samples. The temperature distributions in the center autoclave along flow path could be controlled using the five heaters, and that hot water could pass through the granite samples in the center autoclave with given temperature range (275°C--350°C). The flow rate and pressure also could be controlled in planned range. Accordingly, the inter-action of rock and hot water in subsurface of 1000m to 4000m depth can be simulated in laboratory using this apparatus.
A simulation model for the closure of a mated fracture under nomal stress was developed by modifying the theory of Brown and Scholz. The initial aperture, instead of the height of composite topography, was used as a variable, and then the dependence of the radius at.a three-dimensional local minimum of aperture of the minimum value was considered. Furthermore, each term in a basic formu-la was evaluated independently. A chi-square distribution was fitted to the probability density function of the initial aperture determined experimentally for hydraulic fractures created in Inada granite in the laboratory. The relationships among normal stress, crack closure, and true contact area were simulated and compared to the experimental results. The simulation code reproduced the normal stress-crack closure diagrams obtained in the experi-ment above the stress levels of 0.07MPa except for the case where there were initially many secondary cracks open behind the main fracture surfaces. The true contact area was estimated to be less than 0.2010 of the nominal one even when the normal stress was 40MPa. The mechanical initial aperture esti-mated by the simulation was about double the hydraulic initial aperture determined in the experiments even if the mean value of the aperture minima was taken as the mechanical initial aperture.
Thermoluminescence of quartz in pyroclastic rocks of the Neogene to the Quaternary in the Minase geothermal area, Akita prefecture, was studied. This area is mainly composed of the Doroyu, Minasegawa, Sanzugawa Formations and the Kurikomayama volcanics in ascending order.. Quartz was picked out from nine boreholes cores. There is no clear relation between vertical variations of thermoluminescence (TL) intensity and the stratigraphical units. The TL intensity decreases in accordance with the present temperature in the bore-holes. These facts suggest that the storage process of paleodose is strongly influenced not only by the time and intensity of exposure but also by the paleotemperature of the formation. An original paleo-dose in quartz might be set to zero by the magma intrusion of the Kurikomayama volcanic group, resuming the storage of electrons in trap levels afterwards. TL intensities of quartz in the southern part of this area are smaller than those in the northern part. This suggests that the duration of hydrothermal activity in the south part has been relatively long. In conclusion, the accumulation of paleodose is affected by the temperature due to geothermal activity. These features show that the TL method may be applicable to geothermal exploration.
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