Monofractality of the fracture networks in the Kurihashi granodiorite in the Kamaishi Mine was evaluated by using the box counting method. The fracture maps for fractal analysis were obtained by tracing fractures including open cracks, hydrothermal alteration veins and quartz veins at the scale mostly 2.0×4.0 m² on the wall rock of the E. L. 250 mL drift. The accuracy of the fractal dimension by the box counting was examined by taking account of the lower and upper cut off levels, so the fractal dimension could be higher quality enough to characterize the natural fracture networks. Distribution of fractures and alteration veins ranging from 1.5×1.5 m² to 2.0×2.0 m² were identified by monofractal, and their dimensions D were ranging from 1.04 to 1.09, however, the distribution of open cracks and quartz veins were non-fractal. The distribution of alteration veins in the E. L. 250 mL drift (2.5×360 m²) was also characte-rized. The long fracture map was divided into 12 units and piled up to compose virtual fracture maps at the scale 30×30 m². The distribution of alteration veins was characterized by monofractal and its dimension D ranges from 1.08 to 1.11. The distribution of alteration veins shows monofractality with similar dimensions (D from 1.04 to 1.11), which means that the distribution of alteration veins ranging from 1.5×1.5 m² to 2.5×360 m² could be characterized by an unique monofractal dimension.

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In this paper, calculation methods of single-phase and two-phase flows in the reservoir around the geothermal well have been discussed. In case of single-phase flow, the accuracy of the calculation method based on Forchheimer's equation is higher than the method based on Darcy's law. In twophase flow, the accuracy of a calculation method based on expanded Forchheimer's equation is also higher than the method based on Darcy's law. The methods based on Forchheimer's equation have been incorporated into a wellbore flow simulator WELCARD-IV, then a flow simulator WELCARD-V which can analyze a flow system connecting the reservoir around the geothermal well with the well-bore has been developed. When calculating the inflow performance, WELCARD-V has the advantages as follows:(1) Density and viscosity of fluid in the reservoir are not needed.(2) Relative permeability and reservoir temperature are not needed in case of two-phase flow.

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Silica scale deposition often causes serious problems in geothermal power stations. It has already been well recognized that silica scale deposition can be prevented by keeping the pH of waste geothermal brine acidic. On the other hand, several countries make regulations for mitigation of H₂S emission from geothermal power stations. From these backgrounds, H₂SO₄ production is proposed of using H₂S in the gas exhausted from geothermal power stations. The applicability is then investigated of the usage of thermo-acidophilic bacteria (Sulfolobus sp. Strain 7) for the H₂SO₄ production process for scale prevention. The Hs production experiments are conducted both in bench scale and pilot scale. The experimental results confirmed that the bioreactor with cultured Sulfolobus sp. Strain 7 can produce acidic solution containing H₂SO₄ continuously and the H₂SO₄ production rate of the bioreactor is 0.06 kg·m^-3·h^-1. For practical application of this methods to the Hatchobaru geothermal power station that discharges 1400 m³/h of waste geothermal brine, the following conditions are necessary to keep the pH of the waste brine acidic : 180 m³ of the bioreactor and 252 Nm³/h of the exhaust gas.

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In this study, the experiments on the polymerization of silicic acid were carried out at a fixed low temperature (35°C) . The effects of a little differences in both pH and initial silicic acid concentration at near neutral pH (6-8) are experimentally examined on the polymerization of silicic acid. The effects of metal cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) were also examined.
Even a little difference in both pH and initial silicic acid concentration affect clearly on the polymerization rate of silicic acid : the polymerization rate of silicic acid increased with increasing pH and initial silicic acid concentration. In all the metal cation solution, as the molarity of metal cation was higher, the polymerization rate was higher and the concentration of silicic acid after 2 hours was lower. These results are consistent with the tendency on decrease in equilibrium concentration of dissolved silica C_e due to salt out effect.Regardless of pH or initial silicic acid concentration C_o, the polymerization rate of silicic acid became maximum as the initial supersaturation concentration of silicic acid decreased by 25%. Then the maximum polymerization rate of silicic acid R_max is represented by the following equation.
log R_max=k₁·pH+k₂·log{(C_o-C_e)/C_e)}+K(T)At the range of the experimental condition (pH=6.4∼7.9 ; C_o-C_e=490∼650 mg/1 ; T=35±2°C), k₁=0.632±0.002, k₂=3.73±0.01, K(T=35°C)=-5.33±0.11.
The effects of adding each sodium, potassium and calcium ion on the polymerization rate of silicic acid are explained as the increase in supersaturation degree of silicic acid due to the salt out effect. On the other hand, the increase in polymerization rate of silicic acid in adding magnesium ions can not be explained as only the increase in supersaturation degree. From the fact that the concentration factor of magnesium between silica scale and geothermal brine is higher, magnesium ion is expected to be taken in the polysilicic acid produced.

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The Drift-Flux Model has been applied for simulating two-phase flows in geothermal wellbore. The calculated results were compared with those simulated by conventional homogeneous flow mod-els.
1) The Drift-Flux Model allows us to calculate void fractions in two-phase flow wellbore system without any empirical relation.
2) The method has been applied for two-phase flows through 2000m-deep wellbores with the bottom temperatures of 260°C, 280°C and 300°C. We get deeper understandings of variations in void fraction and drift velocity over vertical positions of the wellbores.
3) The Drift-Flux Model is found to be applicable to the flow simulation for geothermal wellbore.

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Concentrations and isotopic ratios of Sr were determined for waters and rocks in the Sumikawa and Ohnuma geothermal fields to provide constraints on modeling fluid movement and understanding water-rock interaction. The results are summarized as follows:
1) ⁸⁷Sr/⁸⁶Sr ratios of Quaternary volcanic rocks in the Sumikawa section (<0.7041) are significantly lower than those of Tertiary marine sediments (≤0.7075). As a result, ⁸⁷Sr/⁸⁶Sr ratios of rocks increase northward where marine sediments distribute predominantly.
2) The Sr isotopic ratios of the geothermal fluids and hot spring waters are affected significantly by their surrounding rocks. Particularly, geothermal fluids discharged from the Sumikawa wells have similar ⁸⁷Sr/⁸⁶Sr ratios to rocks around the feed zones, indicating an attainment of water-rock equilibrium in the vicinity of the wells.
3) A meaningful relationship between the concentration of Sr and Ca, and temperature in the reservoir fluid indicates that the fluid is saturated with respect to anhydrite ; this conclusion is supported by a chemical equilibrium model.
4) ⁸⁷Sr/⁸⁶Sr ratios of geothermal fluids discharged from the Sumikawa production well, SC-1, have been changing temporally over the past four years. This may be due to a boiling of fluid in reservoir, return of reinjection water, and change of flow path of fluid discharging into reservoir.

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The purpose of this paper is to verify the velocity structure model of Kakkonda field which was officially announced by NEDO. This model consists of six layers with different velocities each other. For this purpose, we carried out 2-D simulations by the staggered grid finite-difference method and compared the results with actual seismic waves observed by the pit explosives of the well KT-208. First, to verify simulation program, we compared velocities of surface waves extracted from the field data with those appearing in the simulation results. Second, we confirmed that the source function used in the simulations is suitable for representing the source signature of the pit explosives. Afterwards, we compared travel times of P-wave first motions and those of the simulation. The differences were small with about-1 to 4%. Therefore, we have concluded that NEDO's model appropriately delineates the shallow velocity structure (<1 km deep) of the area to the northwest of the well KT-208 in Kakkonda field.

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