Self-potential Measurements for Reservoir Monitoring at the Okuaizu Geothermal Field

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  • NISHI Yuji
    Institute for Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology (AIST)
  • ISHIDO Tsuneo
    Institute for Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology (AIST)

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Other Title
  • 奥会津地熱地域における自然電位モニタリング
  • オウアイズ チネツ チイキ ニ オケル シゼン デンイ モニタリング

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Abstract

In the Okuaizu geothermal field, we carried out continuous self-potential (SP) measurements in 2000, 2002 and 2004 to monitor field-wide shut-ins associated with regularly-scheduled maintenance of the Yanaizu-Nishiyama geothermal power plant. Thirteen stations for SP measurements were distributed in the central area of the Okuaizu field, at each of which two or three non-polarizing electrodes were set at one meter depth to check the station reliability. SP changes of 5 to 10 mV associated with the shut-in and re-start of production wells were detected at a few stations located in a low resistivity area in the vicinity of the Sarukurazawa fault. In order to evaluate the observed SP variations, numerical simulations of changes in reservoir conditions due to fluid production and successive “SP-postprocessor” calculations were performed based upon a simplified reservoir model. The reservoir region of the model is divided into deeper and shallower parts, both of which are represented as MINC double-porosity media. In the final model, the matrix permeability of the deeper reservoir needs to be smaller than 10-20 m2 to reproduce the rapid increase of the produced fluid enthalpy as observed in the first a few years of the power plant operation. In contrast, it is required to assign the matrix permeability to be 10-18 m2 or more for the shallower reservoir to reproduce the observed SP changes associated with the short-term shut-ins. This combination of deeper and shallower reservoir properties also reproduces long-term SP changes revealed by repeated SP measurements in 1996 and 1998, and long-wavelength micro-gravity decline between 1997 and 2000. In conclusion, continuous SP measurement to detect short-term SP changes such as associated with field-wide shut-ins is thought to provide additional useful constraints against which reservoir models can be calibrated.

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