A re-evaluation of gravity changes in the early stages of exploitation at the Yanaizu-Nishiyama geothermal field, Japan

Abstract

A 65 MWe power plant was built in 1995 at the Yanaizu-Nishiyama geothermal field, Japan, Annual gravity monitoring surveys in the field began in 1994. Another gravity monitoring survey program was started since 1997 to develop optimal gravity survey methods. The observed gravity change shows a rapid reduction after the beginning of production, and very small decrease after three years of production.
Now the method of Principal Component Analysis (PCA) was employed to expand all the observed gravity changes into the time-space domain. The result is as follows: (1) the 1st component of PCA is hundreds of times larger than the other components, (2) the trend of the temporal factor of the 1st component is obviously different between the former survey part and the latter survey part, (3) the spatial distribution of the 1st component has good correlation with the spatial distribution of gravity values. The result suggests that there is a problem in the calibration factors of the gravimeter that used at the surveys. Considering that calibration measurements were made every year during the latter survey, the calibration factor of the gravimeter used in the former survey should be corrected based on the result of PCA. Numerical simulation based on the observed gravity changes has been carried out to constrain reservoir models. The re-evaluated results suggest that a two-phase zone was expanding in the early stages of exploitation at the field and constrain to revise the reservoir model.