Anhydrite scale was recognized in Sumikawa production well SC-1, where it apparently caused a decrease of steam flow rate. In order to calculate scaling with time, assuming that anhydrite precipitated from geothermal fluid deposits inside of casing pipe with uniform thickness, mathematical model was developed using isothermally non-steady single liquid plug-flow model. The numerical simulation of anhydrite precipitation and production performance were made with scaling calculation coupled with wellbore simulation. The simulation of history matching with variable precipitation rate constants leaded to the rate constant, logk=-4.50. Prediction using the optimum rate constants showed that the slight difference in logk (±0.25) sensitively affected magnitude of decrease in steam rate. Consequently the developed model in this study approximately reproduced wellbore scaling. Under the situation of the local changes in fluid flow, such as with or without slits on 7″ slotted liner and boundary of the different diameter's casing, however, we failed to obtain the good matching between the measured and the calculated values.

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Experiments on the polymerization reaction of silicic acid were carried out at pH7 and the effect of temperature (nearly 313, 333 and 353 K) was experimentally examined on the polymerization of silicic acid, particularly on the maximum polymerization rate, in supersaturated silicic acid solutions (the initial silicic acid concentration was approximately 500, 700 and 900 mg/l). In the case of 700 and 900 mg/l of the initial silicic acid concentration, the polymerization rate increased with increasing temperature. In the case of 500 mg/l, the polymerization rate decreased with increasing temperature and the polymerization reaction did not occur at 353 K. At the fixed temperature, the polymerization rate of silicic acid was higher as the initial silicic acid concentration was higher. Regardless of both initial silicic acid concentration and temperature, the polymerization rate of silicic acid became maximum when the supersaturated silicic acid concentration decreased by 20∼30%. The maximum polymerization rate of silicic acid Rmax is represented by the following equation:log R_max=k₁·pH+k₂·log{(C₀-C_e)/C_e}+k₃T+k₄where R_max, pH and T are the maximum polymerization rate of silicic acid [mg 1^-1 min^-1], the pH and the temperature of solution [K], respectively. C₀ and C_e are the initial concentration and the equilibrium concentration of silicic acid [mg/l]. From the experimental results in this study and Sugita and Yamamoto (1999), the above equation will be able to be useful at the range of the following conditions : pH=6.4∼7.9, (C_o-C_e) /C_e=1.25∼4.59 and T=308∼353 K. Then, k₁=0.632±0.002, k₂=3.73±0.01, k₃=0.0385±0.0014, k₄=-17.1±0.5.

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Alteration halos of the Hachimantai Appi geothermal area is divided into alunite, kaolinite and smectite zones from 273 samples. Zonal arrangements are reformed by landslide and overlapping of more than two hydrothermal events are recognized. Thermoluminescence (TL) age determination of 12 altered rocks shows that the age of older event is expected around 0.9-0.5 Ma and that of younger is 0.3-0.1 Ma. Heat source volcanoes are identified by TL dating of 8 volcanic rocks. Ages of 4 samples from southern mountainous area are 0.31-0.37 Ma whereas those of 4 samples from northern slope are 0.58-0.72 Ma. It shows very good correlation with underground temperature profile. Lineaments identified from JERS-1 SAR images and colored aerial photographs show E-W directional structure and fractures for fluid flow. Regional geothermal model is constructed from age data and distribution of volcanoes. Promising geothermal areas can select by precise age determination of both alteration and volcanic rocks. Conditions for such potential drilling sites are as follows; (1) The occurrence of volcanism from 0.5-0.3 Ma, and extending to younger volcanoes around 0.1 Ma. (2) The presence of pyrophyllite and other high temperature minerals in the alteration zones. (3) The occurrence of more than two hydrothermal events, the most recent being younger than 0.3 Ma.

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Overheating experiments on fluid inclusions were carried out using calcite, anhydrite and quartz sampled from geothermal fields. Fluid inclusions in calcite, anhydrite and quartz started to stretch at 55°C, 35°C and 100°C by overheating above homogenization temperatures, respectively. Fluid inclusions in quartz are stable within 100°C overheating, therefore quartz is considered to be the most suitable mineral for measurement of homogenization temperatures among the three minerals. When calcite or anhydrite are used for homogenization temperature measurement, it is recommended to measure one fluid inclusion in one thin chip of mineral or to measure fluid inclusions in order from lower homogenization temperature up to the temperature at which the inclusion starts stretching.

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From November 27, 2000, a two-year circulation test (Long-term circulation test; Exp. 0002) was started with HDR-1 as an injection well, and HDR-2a and HDR-3 as production wells at the Hijiori HDR site in Yamagata prefecture, Japan. The purpose of test was estimation of the long-term production characteristics of the Hijiori HDR reservoir and to provide data to calibrate the numerical model of the system. Aiming to evaluate the characteristics of the Hijiori multi-reservoir system, we continue to measure the some data (Pressure, Temperature, flow rate) in the wellhead and monitoring the pressure data of the shallow reservoir. Using the Bed-of-Nails model at several fractures of the Hijiori multi-reservoir system and the results of measured data of Exp. 0002, we developed a numerical model using FEHM (Finite Element Heat and Mass Transfer) code.
The results obtained from this simulation are as follows.1) Variation of the downhole pressure of SKG-2 was expressed using this numerical model.2) We success to apply the Bed-of-Nails model at several fractures.3) Ratios of productivity from the shallow reservoir of both wells are different, with well of HDR-3 producing more from the shallow reservoir that well HDR-2a.

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