Pressure Changes in Reinjection Wells and Gravity Changes in Otake Geothermal Field, Japan

Abstract

In the Otake geothermal field, gravity changes at reinjection well OR-15 have been monitored together with pressure changes using water level data for wells OR-15, OR-19 and O-16R to clarify the behavior of the reinjected water in the reservoir and its effect on the production zone. Using these data, the correlation between water level changes in the observation wells, changes in flow-rate of reinjected water and the observed gravity changes have been studied.

The water level changes in OR-15 are similar to the water level changes in O-16R. The reinjection area is subdivided into four blocks. Multiple regression analysis shows the pressure changes around these wells are affected not only by the reinjection rate changes in the immediate block but also by the reinjection changes in other blocks and the amount of rainfall (after a 3-month time lag). However, the pressure changes in OR-19 strongly correlate with the reinjection flow-rate in block D, and the presence of an impermeable structure between OR-15 and OR-19 is indicated.

The repeated gravity measurements show the gravity changes at each station have similar tendencies. However, a discrepancy was recognized between the values of the maximum value minus minimum value at each station. The general tendency of gravity changes at OR-15 is similar to the water level changes in OR-15. A quasi-three-dimensional inversion of the gravity changes was therefore applied to delineate the behavior of reinjected water in the reservoir and its effect in the production zone. From the inversion results, the geothermal reservoir in the Otake field is considered to be controlled by a NW-SE trending structure between OR-15 and OR-19. In addition, the reinjected water in the reservoir is considered to be flowing mainly in a northerly direction.