Study on Reservoir Behavior from Micro-Gravity Changes Monitored in the Hatchobaru Geothermal Field, Japan

Abstract

In the Hatchobam geothermal field, gravity changes at 46 benchmarks have been monitored since May I990 to stu& the correlation between gravity and reservoir pressure changes that have been measured using three observation wells with capillary tubes. From the data monitored during I990 to 1993, a close correlation between these two changes was recognized, and the area was divided into three zones, such as northern, central and southern zone, j?om the pattern of gravity changes. In the northern and the central zones, the value of gravity at each point showed a steep increase just alter the commercial operation of the Hatchobaru N0.2 unit (55 was commenced, ajler that, it turned to decrease to the initial level in the northern zone and to lower level than the initial level in the central zone. In the southern zone, graviv has decreased without any initiai increase tendency. From a two-dimensional modeling anaIysis, graviQ changes i;i each zone are ciinsiJered to reflect the chanp Gf reservoir mass behavior that had been caused by production and reinjection of geothennaljluids in the reservoir.