2006 Volume 28 Issue 2 Pages 181-197
In 1990, adding to the No.1 unit (55MWe) that started commercial operation in June 1977, the Hatchobaru No.2 unit (55MWe) owned and operated by Kyushu Electric Power Company started commercial operation. In order to monitor the mass change occurring in the reservoir due to the pressure changes that would be induced by the change of production and reinjection rates as output doubled from 55MWe to 11OMWe, gravity changes have been monitored since May 1990 at 46 stations in the Hatchobaru geothermal field. The reservoir pressures have been monitored using capillary tubes. Measurements of gravity and pressure indicate a close correlation between changes in these two parameters. Therefore, the gravity changes are considered to reflect the changes in reservoir mass behavior, and so a quasi-three-dimensional inversion of the gravity changes was carried out to delineate the reservoir behavior. According to the inversion results of gravity changes from 1994 to 2000, the reservoir pressure in the production area is estimated to have decreased by more than 1.0MPa. However the tendency toward decreasing pressure in the production area from 2000 to 2001 is relatively small compared with the previous years. This indicates that underground mass in the production area is likely to have stabilized in recent years.