Mitsubishi Materials Corporation (MMC) initiated an exploration program in the northern part of the Hachimantai volcanic region in 1965. After the completion of the Ohnuma geothermal power station in 1974, a decision was reached to direct further exploration towards the Sumikawa geothermal field. The Sumikawa geothermal power station has been developed jointly by MMC, which is in charge of the steam supply, and Tohoku Electric Power Company Inc., which is in charge of generating electricity, and was completed in 1995. It has been producing electricity at range of 42 MWe and 50 MWe. This paper mainly presents a review of the developing history, development strategy and reservoir behavior of the Sumikawa geothermal field. Furthermore, we would like to show some subjects in order to promote a future geothermal development.

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The Kakkonda geothermal field is located 600 km northeast of Tokyo. The first power plant, Kakkonda Unit 1, 50 MW, has been in operation since 1978 by Touhoku Electric Power Inc. (TEP), where Japan Metals and Chemicals Co., Ltd. (JMC) is steam supplier. The development of Kakkonda Unit 2 was started in 1982 by JMC, wascontinued since 1985 by Tohoku Geothermal Energy Co., Ltd. (TGE), a joint venture led by JMC. Before 1988, the shallower reservoir than 1, 500m depth has been developed. However, due to the limited horizontal extent of the reservoir, it has become necessary to develop a deeper reservoir than 1, 500m depth for Unit 2. Thus, a deep exploration well was drilled in 1985 and revealed geological and thermal characteristics of the deep reservoir. In result, there are two reservoirs with different temperature and permeability in the Kakkonda hydrothermal system. The shallow reservoir is permeable and 230 to 260°C, while the deep reservoir is less permeable and 300 to 350°C. However, they are hydraulically connected each other. Then, the first deep production well was drilled in 1989 and confirmed that the Kakkonda deep reservoir was very productive. Geological information from these deep wells revealed existence of Pre-Tertiary formation below Tertiary formations, neo-granitic pluton. There is a permeable horizon at the top of the pluton, and is very productive. The second power plant, Kakkonda Unit 2, 30 MW, was put into operation in March1996 with TGE to supply steam for TEP. This paper describes the outline of the development history, the characteristics of geothermal reservoir and the plant operation system at the Kakkonda geothermal power plant Unit 2.

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The area in which the Uenotai geothermal power station is located was surveyed originally by owa Mining Co., Ltd. (DOWA) in 1971. Akita Geothermal Energy Co., Ltd. (AGECO), an affiliate company of DOWA, was established in 1981, and all of the technical information and data on the Uenotai field were transferred to AGECO. In this report, I cover the following three categories;1. A summary of the 23 years of investigation and development in the Uenotai area.2. The operation conditions during the four and half years since this power station commenced its operations.3. The problems to be settled in the future.

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The Yanaizu-Nishiyama geothermal field is located on the western part of the Aizu district, Fukushima prefecture, northeast Japan. Geothermal investigation in the area were commenced by Mitsui Mining and Smelting Co., Ltd. (MMS) in 1974. Advanced surveys were also carried out by Ministry of International Trade and Industry and New Energy and Industrial Technology Development Organization (NEDO). In the earliest model, the geothermal reservoir was thought as a horizontally spread porous media beneath the Miyashita mudstone member which acts as a cap rock. But later surveys, such as structural well drilling in NEDO's survey during 1982-1983, indicated that the reservoir was strongly controlled by vertical faultzones. And Okuaizu Geothermal Co., Ltd. examined the detail fissure structure. Okuaizu Geothermal Co., Ltd. planned the steam production facilities with next characteristics. (1) A single flash method has been applied. (2) Two phase fluid from each production well is transported to two steam separation facilities. (3) In order to prevent silica scaling in the re-injection wells, brine is dilutedwith waste water from the power station and river water. (4) Concentrated operation monitoring system using DCS (Distributed Controll System) has been applied. Construction work has been undertaken between 1993-1994, and steam production has been started in December 1994 for rehearsing plant. Commercial operation beganon May 25, 1995. Large amount of steam decline was observed, mainly because of carbonate scalinginside of several production wells. Although this trouble was dissolved using scale inhibitor injecting system, steam decline problem still remains. It is necessary to investigate the problem.

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Hachijojima is a gourd shaped volcanic island in the Pacific Ocean, comprisingNishiyama volcano and Higashiyama volcano. From the results of geothermal exploration since 1984, a promising geothermal resource was found on the island. Three exploration wells were drilled in northern part of Nakanogo in 1997, andconfirmed high temperature and high pressure vapor dominated geothermal resources which was sealed by cap rock of hydrothermal alteration. At present, a 3.3 MW geothermal power plant is under construction. Commercial operation will be started at the end of March 1999.

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Kyushu Electric Power Co., Inc. operates six units at five geothermal power stations. Their total capacity is 207.5 MW, accounting for 39% of the installed geothermal power plants in Japan. Hatchoubaru Power Station started operation about 21 years ago with the first unit. Now stable operation on the output of 110 MW (55 MW×2) goes on. On the way to stable operation, we confronted output decline due to reservoir interference by reinjected hot water. In order to recover the output, both steam producing system and power generating system were checked up as a consequent system. The idea is based on our company's fundamental strategy for geothermal development: “consistent development” from upstream to downstream. We carried out optimum reservoir management with more accurate reservoir monitoring data. Also we improved the generation facilities such as turbine and two-phase flow line (NISOURYUUTAIRENRAKUKAN), to utilize existing wells. As a result, these efforts recovered the output successfully and reduced generation costs. We learned that reservoir management is essential to keep stable output of the Hatchoubaru Power Station. We shall make continuous efforts to keep economically stable output by advanced reservoir management, applying the newest simulation technology to balance reinjection with production.

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The geothermal exploration at the Takigami area started in 1979 with various surveys and drill holes. The 25 MW Takigami power plant was opened in November 1996. Since the production rate of steam is very stable, the power plant continues the cost effective generation. On the other hand, injection rate is still declining with 10∼25% a year probably due to the silica-scaling. To keep the effective generation, much effort has been conducted to solve the problems as follows.
1) Engineering study of scale inhibition of injection in wells and also in fracture with reservoir simulation including chemical components.
2) High-angle directional-drilling with MWD system to reduce the drilling cost and site area.
3) Utilization of separated hot water.
4) Improvement of operation system and technique.

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Nittetsu Kagoshima Geothermal Co., Ltd. is supplying geothermal steam for the Ogiri Power Plant, which commenced commercial operation on 1st March 1996. The utilization factor of the plant reaches up to 95.6% through 2 years and 5 months from the beginning of commercial operation and there has been non stop-page of operation caused by the steam supply facilities. It took about 17 years from the beginning of Kirishima geothermal development survey to the commercial operation. This Paper reports the progress of the development, reservoir evaluation, construction and the steam production activities. Future prospect, 2nd stage planning will also be described.

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The Yamagawa gethermal power plant started on the 30 MW commercial operation in March 1995. Since December 1997, steam production has been falling down to about 60% of full scale production by some serious troubles of production wells and injection wells. Major problems of production wells are precipitation of sulfide scale inside the well and decline of discharge enthalpy, and major problem of injection wells is decline of injection capacity because of precipitation of silica scale. It seems that those problems are origenated from the characteristics of Yamagawa geothermal reservoir, those are high reservoir temperature, high sanility, high discharge enthalpy from the well and existence of cool zone at the depth of 1, 000 m. Present status of the Yamagawa power station is making up the programfor solving the serious problems by drilling new wells for recovering the generation power.

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