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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黄鉄鉱中の微量元素の地熱探査への有効性を調べる目的で,霧島地熱地帯に産する黄鉄鉱をEPMAで分析した。分析した銅,ニッケル,コバルト,砒素のうち,コバルトと砒素が興味ある結果を示した。1)牧園溶岩の下部とえびの層中の黄鉄鉱はコバルトに富んでいるのに対し,その下部に位置する飯野溶岩の黄鉄鉱はほとんどコバルトを含まない。牧園溶岩と飯野溶岩はともに輝石安山岩で岩相がよく似ており,その識別は困難である。しかし,仮に両者の間に存在する泥岩と凝灰岩を主とするえびの層群が欠けていても,黄鉄鉱中のCo量を使うことにより,両溶岩を区別することができる。2)黄鉄鉱中のAs量は,中性の熱水を噴出するKT-7号井からの試料の方が,酸性の熱水を噴出するKT-8号井の試料より一般に高い。これは,熱水が酸化されることによって,そのpHが低下するとともに,黄鉄鉱に分配されるAs量が減少することによる。

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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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