In 1990, adding to the No.1 unit (55MWe) that started commercial operation in June 1977, the Hatchobaru No.2 unit (55MWe) owned and operated by Kyushu Electric Power Company started commercial operation. In order to monitor the mass change occurring in the reservoir due to the pressure changes that would be induced by the change of production and reinjection rates as output doubled from 55MWe to 11OMWe, gravity changes have been monitored since May 1990 at 46 stations in the Hatchobaru geothermal field. The reservoir pressures have been monitored using capillary tubes. Measurements of gravity and pressure indicate a close correlation between changes in these two parameters. Therefore, the gravity changes are considered to reflect the changes in reservoir mass behavior, and so a quasi-three-dimensional inversion of the gravity changes was carried out to delineate the reservoir behavior. According to the inversion results of gravity changes from 1994 to 2000, the reservoir pressure in the production area is estimated to have decreased by more than 1.0MPa. However the tendency toward decreasing pressure in the production area from 2000 to 2001 is relatively small compared with the previous years. This indicates that underground mass in the production area is likely to have stabilized in recent years.
The performances of U-tube ground heat exchangers (GHEs) in heterogeneous formations were predicted using a newly developed analytical modeling approach based on the cylindrical source function. The modeling procedure transforms the detailed vertical temperature profiles obtained with fiber optical temperature sensors to thermophysical properties of ground. The vertical distribution of thermal conductivities and heat exchange rates were estimated using a non-linear regression method. The estimated distribution of thermal conductivity correlated well with the geological and groundwater flow conditions at the well location, while the distribution agreed well with the interpretation results in an adjacent GHE completed with a co-axial heat exchanger.
The heat flow (conductive heat flux) in Japan has been measured extensively since 1957 to understand the heat flow distribution in and around the Japanese Archipelago. It is important to consider the effect of advection processes in the calculation of shallow crustal temperatures, for areas such as volcanic region. We calculated the total heat fluxes and fluid flow velocities from well data in Japan using a one-dimensional heat transfer model. We estimated that total heat flux is high-er than 0.2 W/m2 in volcanic regions and lower than 0.1 W/m2 in non-volcanic regions. This feature is consistent with heat flux distribution calculated using heat discharge from hot springs in previous study (Yuhara, 1973). However, the magnitude of heat flux calculated using our one-dimensional model is higher than that of the previous study in volcanic regions. These results indicate that advection process remarkably contributes to heat flux in volcanic regions.
Extensive geothermal activities occur in the Bajawa area, Flores Island, Indonesia. Chemical composition and sulfur isotopic ratio of sulfide were analyzed for cuttings from the exploration wells MT-1 and MT-2, which were drilled at the Mataloko geothermal field, one of the most active fields in the Bajawa area. Almost all of the sulfide minerals are pyrite with a little amount of sphalerite as small inclusions in pyrite. Pyrite contains As up to 2.5 wt.%, Co up to 0.8 wt.% and Mn up to 0.9 wt.%. High contents of As and Co are present in the core of pyrite crystals in the deeper samples, whereas Mn is enriched in the shallower samples. Sulfur isotopic ratios were analyzed for 17 samples. The δ34S values of the pyrite show the following two characteristics: (1) the isotopic values gradually change from about -8‰ at the shallower level to about -2‰ at the deeper level, (2) the values abruptly change to about -10‰ at the deepest part. The δ34Spy values concentrate around -5‰ which is not equilibrated with the δ34SH2S (-2.5 ‰) for the explosive gas of the well MT-1 at temperatures below 200°C. Combining these results with the previous results of geochemical and geophysical explorations, the fluid system can be divided into two zones at about 160-180 m: the deeper fluid which comes from the steam-dominated reservoir is oxidized and rich in CO2 and H2S of temperatures between 230°-250°C. This fluid led to the precipitation of wairakite and As- and Co-rich pyrite. The fluid of the shallower zone is made from the condensation of the deeper steam separated from the CO2- and H2S-rich fluids, and led to the re-equilibration of pyrite, and new alteration minerals of montmorillonite, kaolinite and alunite at temperatures less than 200°C. The deeper fluid retreated to deeper depth and the hydrothermal activity dropped below its peak. The present results of As, Co and sulfur isotopic anomalies of pyrite below 160 m can be possibly applied to detect an uppermost of the vapor-dominated geothermal reservoirs.
Groundwater temperature survey was widely conducted in Thailand in a view of applying geothermal heatpump systems in tropics. Generally in tropical climate regions, where the seasonal change of surface temperature is small, subsurface temperature is higher than atmospheric one through a year due to geothermal gradient so that underground may not be used as a cold heatsource. Nevertheless there may be possibilities of geothermal heatpump application, in tropical regions where seasonal atmospheric temperature variation is rather big as tropics and groundwater temperature is relatively low. Therefore, groundwater temperature profiles in boreholes were measured in the Chao Phraya plain, Thailand to compare with the climatic data. As a result, regional variation of subsurface temperature at depths from 20 to 50 m of 3.4K was observed in the whole plain. In some cities, subsurface temperature lower than surface one for 5K or more over four months was identified. Thus underground may be used as cold heat-source even in parts of tropical regions. Such detailed subsurface temperature survey is essential as base data for the promotion of underground thermal utilization in tropics.
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