Study on Reservoir Behavior based on Gravity Changes in the Hatchobaru Geothermal Field

Abstract

In the Hatchobaru geothermal area, gravity changes at 46 stations have been monitored since May 1990 to study the effectiveness of gravity monitoring technique whether it has good correlation to reservoir behavior or not. A clear coincidence can be recognized between gravity and pressure changes monitored by capilary tubes which set into several observation wells. From the tendency of gravity changes, the Hatchobaru geothermal area can be divided into three zones such as northern, central and southern zones. In the northern and central zones, the gravity changes showed steep increase just after when Hatchobaru No.2 unit commenced its commercial operation, which might be caused by sudden rein jection due to No.2 unit operation. In the southern part, gravity has decreased without any initial increase like northern and central zones, which indicates that reinjected water has not circulated upto the production layer located around southern zone. After 3 years, the tendency of gravity changes looks to be settled which may show that the reservoir is mostly balanced after about 90 MW production and recovers its quantity balance year by year.