[SVC44-08] 九州メディポリス地熱地帯におけるDTSとDASを用いた第二回地震計測・温度計測の結果
キーワード：地熱地帯、分布型地震計測センター、地熱貯留層、超臨界水、オフセットVSP、分布型温度センサー
To obtain highly efficient geothermal sources, great efforts to use supercritical water reservoirs have been done by many countries including Japan. To investigate presence of any supercritical water zones, we deployed optical fiber systems in the borehole at the Medipolis geothermal field in the southern Kyushu, Japan. We measured temperature and seismic waves by DTS and DAS modes using optical fiber system, respectively. The optical fiber system was deployed to the depth of 977 m and 1,545 m in 2018 and 2019, respectively. We also deployed 3C surface seismometers in the test field. Using DTSs, the temperature was measured as 272.8°C at 918-m deep (Fig.1).
In 2018, seven natural earthquakes were observed by DASs and surface seismometers for four and a half days (Kasahara et al. 2019). Through the comparison of the DASs and 3C seismometers records, we recognized the presence of the P-to-S conversion phases on the NS and EW components of surface seismometers. The P-to-S conversion phases can be explained by the structure model with a zone of Vp/Vs~3 at approximately 4-km deep (Kasahara et al., 2020b).
In 2019, we conducted DAS-VSP measurements for five source locations along the 2-km long EW line using a MiniVib seismic source (Kasahara et al., 2020a). We used 10-m gauge length for the DAS measurements and the seismic data were obtained with the 1-m spacing. We used the MiniVib for 10 to 75 Hz frequency sweep during 30 s at every minute. Because the power of the MiniVib seismic source was not powerful enough to obtain deep penetration of seismic waves, we stacked the seismic data for 480-960 times to improve the S/N. By the 960 times stacking, the S/N ratio could be improved as 30 times.
Analyzing the DAS waveforms, we obtained the velocity structure models of Vp and Vs down to 4-km deep (Fig. 2). Using migration of reflected arrivals of DAS records, we obtained the reflection image near the well. The reflection image is superposed on Vp/Vs profile as seen on Fig. 2. The results suggest the presence of high Vp/Vs zone at the depth of approximately 3.6 km (Kasahara et al., 2020a).
This high Vp/Vs zone is probably identical as that suggested by the interpretation of the P-to-S conversion phase. The Vp/Vs value corresponds to Poisson’s ratio being equals to 0.467, and it might be caused by high water saturation zone, possibly supercritical water.
Acknowledgements
This study was supported by the New Energy and Industrial Technology Development Organization (NEDO). Medipolis Energy Co. kindly allowed us to use their IK-4 geothermal well. Mr. Kimura provided us with the Schlumberger hDVS measurements. WELMA Co. provided us with the borehole fiber-optic system and assisted in the measurement of temperature in the borehole. The staff of West Japan Engineering Consultants, Inc. helped us during the field study. The MiniVib source was operated with assistance from Dia Consultants Co., Ltd.
References
Kasahara, J., Hasada, Y., Kuzume, H., Fujise, Y. and Yamaguchi, T.: Seismic feasibility study to identify supercritical geothermal reservoirs in a geothermal borehole using DTS and DAS, EAGE extended abstract, EAGE 2019 Annual meeting, London. (2019).
Kasahara, J., Hasada, Y., Kuzume, H., Mikada, H., and Fujise, Y.: The second seismic study at the geothermal field in southern Kyushu, Japan using an optical fiber system and surface geophones, Proceedings, 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA (2020a).
Kasahara, J., Hasada, Y., and Kuzume, Y.: Possibility of high Vp/Vs zone in the geothermal filed suggested by the P-to-S conversion, Proceedings, 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA. (2020b).
In 2018, seven natural earthquakes were observed by DASs and surface seismometers for four and a half days (Kasahara et al. 2019). Through the comparison of the DASs and 3C seismometers records, we recognized the presence of the P-to-S conversion phases on the NS and EW components of surface seismometers. The P-to-S conversion phases can be explained by the structure model with a zone of Vp/Vs~3 at approximately 4-km deep (Kasahara et al., 2020b).
In 2019, we conducted DAS-VSP measurements for five source locations along the 2-km long EW line using a MiniVib seismic source (Kasahara et al., 2020a). We used 10-m gauge length for the DAS measurements and the seismic data were obtained with the 1-m spacing. We used the MiniVib for 10 to 75 Hz frequency sweep during 30 s at every minute. Because the power of the MiniVib seismic source was not powerful enough to obtain deep penetration of seismic waves, we stacked the seismic data for 480-960 times to improve the S/N. By the 960 times stacking, the S/N ratio could be improved as 30 times.
Analyzing the DAS waveforms, we obtained the velocity structure models of Vp and Vs down to 4-km deep (Fig. 2). Using migration of reflected arrivals of DAS records, we obtained the reflection image near the well. The reflection image is superposed on Vp/Vs profile as seen on Fig. 2. The results suggest the presence of high Vp/Vs zone at the depth of approximately 3.6 km (Kasahara et al., 2020a).
This high Vp/Vs zone is probably identical as that suggested by the interpretation of the P-to-S conversion phase. The Vp/Vs value corresponds to Poisson’s ratio being equals to 0.467, and it might be caused by high water saturation zone, possibly supercritical water.
Acknowledgements
This study was supported by the New Energy and Industrial Technology Development Organization (NEDO). Medipolis Energy Co. kindly allowed us to use their IK-4 geothermal well. Mr. Kimura provided us with the Schlumberger hDVS measurements. WELMA Co. provided us with the borehole fiber-optic system and assisted in the measurement of temperature in the borehole. The staff of West Japan Engineering Consultants, Inc. helped us during the field study. The MiniVib source was operated with assistance from Dia Consultants Co., Ltd.
References
Kasahara, J., Hasada, Y., Kuzume, H., Fujise, Y. and Yamaguchi, T.: Seismic feasibility study to identify supercritical geothermal reservoirs in a geothermal borehole using DTS and DAS, EAGE extended abstract, EAGE 2019 Annual meeting, London. (2019).
Kasahara, J., Hasada, Y., Kuzume, H., Mikada, H., and Fujise, Y.: The second seismic study at the geothermal field in southern Kyushu, Japan using an optical fiber system and surface geophones, Proceedings, 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA (2020a).
Kasahara, J., Hasada, Y., and Kuzume, Y.: Possibility of high Vp/Vs zone in the geothermal filed suggested by the P-to-S conversion, Proceedings, 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA. (2020b).