A mathematical model to simulate permeability decrease caused by the depositionof silica in a reservoir near an injection well has been developed. Isothermal and steady state water flow is assumed in the reservoir. Sensitivity study has been carried out with the model under injecting geothermal water of 90 degree C and of 522 mg/l mono-silicic acid at a constant injection rate, 60m³/h. Reservoir porosity gives serious effect both on the injectivity decrease of the well and on the extention of permeability damaged area due to the silica deposition. Reservoir thickness also gives the effect on the injectivity decrease with time.

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Electron Spin Resonance (ESR) dating has been tried on quartz separated from hydrothermally altered rocks (silicified rocks, sinter and quartz veins) from someold and active geothermal fields in Kyushu. Three different ages are usually obtained from E', Ge centers and OHC of quartz, some of which are too weak in thermoluminescence signals for the dating. The age obtained from E' centers is always very young compared with that from the other centers, indicating that the E' centers are much less thermo-stable. The Ge centers and OHC are of the almost same age, although the Ge age tends to be slightly older than the OHC one. These differences in ESR age from one sample can be applied to trace the thermal history of geothermal systems.

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To examine the availability of trace elements in pyrite for exploration, elevencore samples from the Kirishima geothermal field were analyzed for copper, nickel, cobalt and arsenic by electron microprobe. Among them, cobalt and arsenic gave interest informations. Cobalt contents of pyrite are relatively high in the lower Makizono lava of pyroxene andesite and the underlying Ebino formation of tuff breccia and tuffaceous shale, while they are remarkably low in the underlying Iino lava of pyroxen andesite. This suggests that, even in the well lacking the Ebino formation, cobalt contents give a key to identify the Makizono lava and the Iino one, both of which are petrographically similar to each other. Arsenic contents of pyrite are low in the KT-7 well of neutral hydrothermal solution, as compared with the KT-8 well of acidic solution. This implies that the increase ofoxygen activity resulted in the concentration of arsenic into pyrite as well as the decrease of pH.

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Characteristics of geothermal heat extraction have been studied for an artificial reservoir crack which is surrounded by thermally induced secondary crack network. To this end, numerical simulation was performed by using a model consisting of three parts: a discrete crack model for the artificial reservoir crack, a double porosity model for the region of the secondary crack network and a porous media model for the region outside the secondary crack network. The results show that the temperature at the outlet is highly dependent on the permeability of the secondary crack network in the direction parallel to the main reservoir crack, whereas, the permeability in the direction normal to the main reservoir crack is not so effective. The temperature at the outlet is also sensitive to the width of the region of the secondary crack network. Furthermore, the area of the region of intense temperature drop increases with the permeability of the secondary crack network in the direction parallel to the main reservoir crack.

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Onikobe geothermal area, situated in northeastern Japan, is characterized by the large hydrothermal alteration zones named Katayama, Okunoin and Arayu. “Fingerprint” geochemical survey at 134 stations was conducted in Okunoin area, where faults had been detected by other previous studies. In this survey the soil-gas absorbed by activated charcoal was analyzed by mass spectrometry. Thus obtained geochemical data show that there are two types of soil-gas anomalies indicating fault/ fracture, one of which is high gas emission and the other is low gas emission. Among of two types of soil-gas anomalies, anomaly of the former type is found commonly in other fields. On the other hand, anomaly of the latter type has not been found in previous studies. Moreover distribution map of the classified types of soil-gas anomalies shows that several anomalous zones of the latter type are sandwiched by anomalous zones of the former type, forming a sequence of anomalous zones as high gas emission-low gas emission-high gas emission. Considering all geological and geophysical information, the two types of soil-gas anomalies and the sequence of anomalous zones were interpreted as bellow. (1) Anomaly of high gas emission indicates fracture zone which plays as a passage for geothermal fluid. (2) Anomaly of low gas emission indicates highly altered zone which corresponds to the top of fracture zone sealed with clay, minerals formed by hydrothermal alteration. (3) The sequence of the anomalous zones indicates migration of the passage for geothermal fluid from the center of alteration zone to the outside.

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