Gravity Monitoring of Geothermal Reservoirs - A Case Study of the Production and Reinjection Test at the Takigami Geothermal Field

Abstract

Takigami geothermal field is a hot water dominated geothermal system in central Kyushu, Japan. A new 25MW geothermal power plant is expected to commence the operation in 1996 at this field. A short period production and reinjection test was conducted fro November 1991 to February 1992. We conducted repeated gravity surveys at specified intervals (one to three months) for two years including the test period to detect gravity changes caused by production and reinjection. Gravity changes of up to 150 microgal have been observed during the two years. Most of such large changes are found to be due to seasonal variations of shallow waterlevel. After correction of such shallow water level effect, we detected gravity increases of up to 30 microgal in the reinjection zone. This change is reasonable if we assume that the reinjected water stays for a while beneath the reinjection zone.