上の岱地熱地帯の探査とその開発

The Uenotai geothermal field, currently developed by the Akita Geothermal Energy Co. (AGECO) which succeeded the extensive exploration by Dowa Mining Co. (DOWA) in 1986, is located approximately 25 kilometers north of the Onikobe geothermal power plant, on the northern heights of the Quaternary volcanic mountains(Oyasu-dake and Takamatsudake)at the southeastern corner of Akita Prefecture, Japan. No surface manifestation is found in the project area itself which is totally covered with the Quaternary acidic pyroclastics but several hot springs and fumaroles are known in the surrounding area.<BR>The field is one of the most promising areas which had been found from the surface and subsurface exploration by DOWA in the area covering 200 equare kilometers since 1971. Based on the data obtained during the exploration and because of the geographic advantages, the exploitation had been concentrated to the Uenotai field since 1975 as the first priority area to be developed, including the drilling of the large exploratory holes and the joint investigation on the potential for the commercial power generation with the Tohoku Electric Power Co.. After the simultaneous flow test using 8 discharge wells in 1988, the geothermal reservoir modeling and simulation studies on a significant amount of the data which were provided by DOWA, AGECO and the government, it was concluded that the field had enough potential for a long-term 27.5 MW power generation. The project is scheduled to be on the commercial operation in 1994 and the construction of the steam supply system has already been started.<BR>The subsurface geology which was revealed by a large number of wells shows that the Uenotai project area is underlain by a NW trending horst block which is considered to be a northern part of the major Oyasu-dake uplift, bounded by a NW-SE fault system. It has also been found that the production in the area is primarily from the microfractures in the lower Doroyu and pre-Tertiary formations, which were created by the uplifting movements, a large number of the intrusive rocks and the faults.<BR>The temperatures of the Uenotai geothermal reservoir at the elevation of 500m below sea level are more than 220°C with a very similar trend of the contours to the geological structure of the horst block and the highest temperatures(>300°C)which are found at the SE corner of the project area are still extending toward Oyasu-dake. From the subsurface temperature distribution and the geochemical studies on the discharged fluids, the heat source of the Uenotai geothermal reservoir is suggested to be located beneath the Oyasu-dake uplift.<BR>During the simultaneous production test in 1988, it was observed that the stabilized flow rate of the steam from 8 wells was 272tons/hour (WHP=6KSCA) and the most of the wells are the steam dominated with a few exceptions from which a minor amount of a typical NaCl-type hot water were produced. The overall steam/hot water ratio is at 3/1.