Surface heights of a fracture about 1 m in length were measured by using a large measuring system with a laser type displacement transducer to clarify the size effect on surface height and aperture distributions of a fracture. Based on the measured data, the size effect on the standard deviation and mean aperture of the fracture was studied. The results are summarized as follows:1. Standard deviations of both surface height and initial aperture increased with an increase in the fracture size. However, standard deviation of the initial aperture did not increase as that of the surface height did.2. When the wavelength exceeded 100 mm, the power spectral density function of the initial aperture became constant. Moreover, the standard deviation of the aperture was estimated to become almost constant when the fracture size is greater than 1000 mm. On the other hand, the mean aperture is still increases at fracture size of 1000 mm. This is because a few sharp asperities are greatly influenced by a certain projected contacting point.

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An injection test on Sumikawa well SD-1 was carried out to evaluate the effect of water temperature on injection-capacity. It was conclude that injecting low temperature water increases the permeability-thickness product and the injection flow rate owing to increase in fracture aperture and decrease in reservoir pressure around the injection well. To examine quantitatively this behavior, we have conducted numerical studies using a one-dimensional slab geometry model. Considering the relationship between porosity, permeability, and pressure/temperature, a modified Kozeny-Carman relation was applied to a vertical, one-dimensional slab geometry model, where the fracture-porosity and permeability should increase as a result of cooling in the reservoir. Consequently, we explain the improvement of transmissivity as a result of porosity increase due to low temperature water injection. We also explain that the decrease in reservoir pressure around the injection well is induced by the downward flow of low temperature water and the cooling reservoir where the fracture system has a strongly vertical orientation.

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Geochemical gains and losses for four types of hydrothermal alteration zones distributed in lacustrine sediments of the Quaternary Sunagohara Formation, all of which have been formed under presently active Okuaizu geothermal system, were studied. Least altered (LA) zone, characterized by fresh volcanic glass, is thought to have precursory composition. Smectite-zeolite (SZ) zone, characterized by smectite, clinoptilolite and mordenite, shows gains of Si, S, Cs, As and Sb, and losses of Mn, Mg, P, Cu, Zn V and Tb. Mixed-layered clay mineral (ML) zone, characterized by illite-smectite mixed layered minerals and K-feldspar, shows gains of K, S, Rb, Cs, As, Sb Cu and Tb, and losses of Fe, Mn, Mg, Ca, Na, P, Pb, Zn and V. Kaolinite (K) zone, characterized by abundant kaolinite, shows gains of S, As, Sb and Co, and losses of Mn, Mg, Ca, Na, K, Rb, Y and Zn. Gains and losses of these components basically correspond to the following observations on the alteration minerals in this area. Addition of hydrothermal silica minerals in the SZ and ML zones, decomposition of plagioclase in ML zone, replacement of volcanic glass to clay minerals and zeolites in the SZ zone, replacement of volcanic glass to clay minerals and K-feldspar in the ML zone, precipitation of pyrite and epithermal element-rich sulfide in the ML zone, and replacement of volcanic glass and plagioclase to kaolinite in the K zone.

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In this paper, optimum well completions and configurations of heat exchange wells in Ground-Coupled Heat Pump (GCHP) systems were studied under the existence of groundwater flow using numerical simulation models. For the performance prediction of an individual well, a 2-D radial model was developed and validated with analytical solutions and field test results. Case studies were made using the 2-D radial model to optimize the grouting material for heat exchange wells. The case studies showed the advantage of grouting with permeable material under high groundwater flow velocity. A 2-D Cartesian model was developed for the performance prediction of groups of heat exchange wells. In the optimization of well configurations, Genetic Algorithms were applied for maximizing the heat exchange rate in a fixed land space. The optimization calculations showed that regular well spacing could be modified to enhance the heat exchange rate under the existence of groundwater flow.

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Experimental study of amorphous Mg-SiO₂ scale was done to investigate the stability and chemical composition from 100 to 300°C in pH range from 3 to 7. MgCl₂ fluid (23 mg/kg in mixed water) and Na₂SiO₃ fluid (1500 mg/kg SiO₂) were mixed and sealed in quartz glass tube after adjusting each pH. The tube was packed into a SUS reaction vessel and heated up to 300°C for 3 days. The product was filtered out through 0.22μm filter and was analyzed by XRD and SEM-EDX for the chemical composition and mineral identification. The reaction product is in amorphous state and chemical composition is mainly composed of SiO₂ and a minor amount of Mg. The concentrations of Mg in fluids after reaction decreased with increasing pH at all temperature conditions. The ion activity product (Mg²⁺) (H₃SiO₄^-) (OH^-) of the amorphous Mg-SiO₂ scale was calculated thermodynamically and is decreased with increasing temperature up to 300°C. This tendency is almost accordant with the previous results reported in Icelandic geothermal fluids. However, the solubility of product calculated in this study is lower than that in Iceland (0.02 Mol/kg NaCl) and is depend on the salinity (0.05 and 0.5 Mol/kg NaCl in this study), that is, the solubility of product is higher in lower salinity.

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