Evaluation of Geothermal Reservoir Mass Change from the Gravity Change at the Takigami Geothermal Area, Oita Prefecture, Japan

Abstract

In Takigami geothermal area, we have continued the geothermal reservoir monitoring by using gravity change from 1991, in order to monitor the mass movement caused by production and reinjection of geothermal fluid. We, however, had measured only relative gravity measurements by using relative gravimeters (SCINTREX CG-3, CG-3M and CG-5), so we haven’t been able to evaluate the gravity change at the reference station of the relative gravity measurements. Therefore, we introduced A10 absolute gravimeter (Micro-g LaCoste, Inc.) in 2008. We used A10 gravimeter for not only the assessment of the gravity changes at the reference station, but also the detection of the absolute gravity change caused by the subsurface fluid mass changes at some other measurement stations. As a result of absolute gravity measurements, the gravity change at the reference station of the relative gravity measurements is small enough for this evaluation, within about 10 µgal. So we estimated that this reference station is proper for the relative gravity measurements. Because we judged that the gravity change detected by the relative and absolute gravity measurements illustrated the mass movement in the geothermal reservoir, we can divide 3 areas from the pattern of the gravity change after the commencement of the Takigami geothermal power plant. And, we can estimate the 4 stages of geothermal fluid flow pattern from temporal gravity change. Based on these classifications, we led the conceptual reservoir model of the Takigami geothermal area.