The north-western area of Kuju Volcano is one of the most active geothermal fields in Japan, where the seismic activity including seismic swarms is very active. The Vp/Vs ratio study for earthquakes occurred at this area from November 1995 to May 1998 was carried out. At the shallow part (above 2 km below sea level), Vp/Vs ratios are wide ranging 1.65 to 1.85. On the contrary, almost all of Vp/Vs ratios are limited at the values less than 1.73 at the deeper part (deeper than 2 km below sea level). The Vp/Vs ratio changes with time and increases before the seismic swarm. These characteristics are inferred to be affected by the presence of water and fractures in the geothermal field.
The purpose of the paper is to clarify the application of cathodoluminescence (CL) to geothermal exploration such as detection of a productive fracture. The CL of anhydrites and calcites in the different origins or stages in the Kakkonda and Mori geothermal fields has been studied using the Luminoscope (ELM-3R) and the CL mode of a scanning electron microscope (CL-SEM). The CL images of natural hydrothermal anhydrite have brown, pale green colors and colorless. The CL images with brown and pale green colors result from both Sm3+ and Dy3+ emissions, and Mn2+ emission, respectively. Calcite in the three different origins or stages such as natural hydrothermal calcite, calcite scale and limestone shows CL images with red color, resulting from Mn2+ emission. Intensity of the CL spectrum of Mn2+ in the calcites suggests that Mn content in limestone is lower than that in other calcites. Based on these investigations, it is concluded that the CL of anhydrite and calcite is a useful method to discern the different origins or stages of these minerals, and to detect productive fractures in geothermal fields. Moreover, this method provides us important information for mixing degree of magmatic fluid with geothermal fluid in reservoir.
In European and North American countries, Ground-Coupled Heat Pump (GCHP) systems have been widely used for space heating and cooling purposes of houses and buildings. In Japan, however, the rate of increase in the number of GCHP system installation is still slow mainly due to the high drilling cost of ground heat exchange wells. The cost-effective design of GCHP systems, therefore, will be of crucial importance for promoting the application of GCHP systems in Japan. In this paper, an approach in optimizing the well designs and operating conditions of heat exchange wells in GCHP systems was proposed. In the optimization process, the net present value of expenditures for the constructions and operations of GCHP systems were minimized as the profitbased objective function. As optimization methods, quasi-Newton method, the Polytope method and Genetic Algorithms (GA) were applied and their efficiency and stability were investigated. Among the optimization methods, GA showed their advantage in optimizing discrete variables, i.e., well numbers. GA also showed the high efficiency and stability in simultaneously optimizing well designs and GCHP operating conditions. The Polytope method optimized well depth and GCHP operating conditions efficiently though the method could not optimize well numbers. The performances of quasi-Newton method were unstable since the method required appropriate initial guess for convergence to the optimum solution.
In a Hot Dry Rock (HDR) geothermal reservoir, water/rock interactions such as dissolution and precipitation of rocks, may significantly influence the long-term reservoir performance of the artificial water circulation system. In order to predict the long-term reservoir performance of the HDR system, it is first essential to understand the kinetics of the water/rock interactions at various fluid velocities, in addition to their temperature dependence. In this work, the dissolution kinetics of a granite, in pressurized high temperature (250-350°C) water at 0.05×10-3-2.0×10-3 m/s, has been investigated experimentally using an once-through type tubular flow reactor in which hot water flows through the passage between the granite samples and the inner wall of the reactor. It has been shown that the velocity of hot water significantly affects the dissolution rate of the granite as well as temperature. The dissolution rate of the granite is shown to increase with temperature and fluid velocity. It is suggested that the apparent dissolution rate constant of the granite is affected by both the surface reaction an diffusion in the boundary layer, within the range of temperature and fluid velocity used for this study. The apparent dissolution rate constant of the granite has been determined as a function of temperature and Reynolds number. Based on analysis of the apparent dissolution rate constant, the rate constant obtained from this study, can also be regarded as the mass transfer coefficient.