ABSTRACT

Kyushu Electric Power Co. Inc., has conducted geothermal development in the Kujyu volcanic area of central Kyushu since the 1950s, and three power stations are now in operation, including Hatchobaru I and II (55 MW each) and Otake (12.5 MW). The geothermal fluid in the Otake-Hatchobaru system is recharged by meteoric water from the southeast, is heated at depth, and is subsequently discharged to the northwest along fractures in the Otake-Hatchobaru system. In the Hatchobaru area, reinjection of waste hot water began in 1973 for environmental reasons. However, soon after, this reinjection began to affect the reservoir, with the temperature gradually decreasing due to interference between the reinjection zone and steam production zone. As a result, the distance between production wells and new reinjection wells is now kept as large as possible. This has stopped the trend of falling reservoir temperature. However, it is also apparent that reinjection of hot water is necessary to help recharge the reservoir, and maintain pressures in the production zone.