In the transportation pipe of the hot water at geothermal power station, the silica scale is precipitated as amorphous silica. It is aggregate of the opal ranging 0.05-1.0 μm in size, accompanied by the small amounts of quartz, pyrite, colloform magnetite and others. The chemical compositions of the hot water and chemical and mineralogical properties of the silica scale are studied to clarify the precipitation process of the silica. Also, the experiments of the artificial silica formation from the hot water using autoclave and glass loop were carried out in the laboratory. From these studies the amount of silica scale is independent of the colloidal silica in the hot water. The amount of silica scale has the close relation with the concentration of supersaturated soluble silica in the hot water. The amount of silica scale from the hot solution supersaturated with the soluble silica reaches the maximum at pH 8 in the range of 6-9.
An opened thermal convection model is applied to the actual geothermal system of the Isawa hot spring, Yamanashi prefecture. The area is underlain by thick diluvial and alluvial formations, and the discharge rate and the temperature of hot water are respective maxima at the No. 24 well and decrease gradually with distance from it. A cylindrical reservoir model is used to simulate the temperature distribution of the hot spring aquifer of the area. In the simulation a try and error technique is used to decide the most suitable model reservoir, which produces the smallest difference between the calculated and observed values. From the results of simulation, it is estimated that: (1) a circular heat source with radius of 410.5 m and the maximum temperature of 336°C lies at the 1368 m deep, and thermal convective current results from the heating in the hot spring aquifer with the permeability of 10-3μm2, (2) hot water of 0.76 m3/s in total flows out from the discharge area with the radius of 1300 m.
The Tamagawa Welded Tuffs are distributed in and around the Hachimantai geothermal field, northeast Japan (Figs. 1 and 2). The total volume of the tuffs is estimated over 500km3, and the magma reservoir can be expected as a heat source of the geothermal field still now . The Tamagawa Welded Tuffs are devided into Dacite, Upper Rhyolite, Middle Rhyolite and Lower Rhyolite Welded Tuffs in descending order (Table 1). And some part of the underlying Tertiary sediments are composed of dacite and rhyolite tuffs (Fig. 3). Twenty potassium-argon ages (Table 2) established the age range of the Tamagawa Welded Tuffs and the underlying tuffs as from about 2.0 Ma to 1.0 Ma and from about 8.2 Ma to 2.4 Ma respectively. The K-Ar dates and magnetic polarities of the volcanic units agree with previously published geomagnetic-reversal time scale (Fig. 4). The Dacite (0.9-1 .2 Ma), the Upper Rhyolite (1.2-1.7 Ma) and the Lower Rhyolite (1.7-2.0 Ma) Welded Tuffs are correlated to the Matsuyama epoch and the Middle Rhyolite Welded Tuff (1.6-1.7 Ma) to the Olduvai event.
This paper presents a three-dimensional model for geothermal reservoir simulation to show some fluid behaviors under production and reinjection. A mathematical relationship in the developed simulator describes a three-dimensional flow of hot water, steam and heat. Equations for mass and energy are solved simultaneously by use of an implicit pressureexplicit saturation (IMPES) method. As for the calculated results, distributions of pressure, temperature and velocity vector under steam-hot water production and hot water reinjection for various type of reservoirs are clarified, and several informations are obtained from these sample calculations.