Mass Balance from Gravity in the Takigami Geothermal Reservoir, Oita Prefecture, Japan

In order to utilize the geothermal resources sustainably, it is necessary to monitor and recognize the behavior of geothermal reservoirs. We have continued the geothermal reservoir monitoring by using gravity measurement since 1991 in Takigami geothermal area. We, however, had measured only relative gravity and we assumed the gravity at the reference station doesn’t change. Therefore, we introduced an A10 absolute gravimeter (Micro-g LaCoste, Inc.) since 2008.As a result of absolute gravity measurements, the gravity change at the reference station (T1), which is within about 10 μgal, is small enough to judge relative gravity measurements appropriate. Therefore, we judged that the gravity change detected by the relative gravity measurements illustrated the mass transfer in the geothermal reservoir. We have calculated the mass balance in the Takigami geothermal system. And, we can divide study areainto 3 areas from the pattern of the loci of the gravity change after starting production and reinjection of geothermal fluid in the Takigami geothermal power plant. Moreover, we can estimate the 4 stages of geothermal fluid flow pattern from temporal gravity change. Based on these classifi cations, we led a conceptual model of the Takigami geothermal system.