10:15 〜 10:30
[STT42-06] A feasibility study to identify supercritical geothermal reservoirs on southern Kyushu Island using DTS and DAS systems
★Invited Papers
キーワード：DAS、地熱フィールド、地熱貯留層、地震学的モニタリング、DTS、パッシブ地震波震源
Introduction: Supercritical water is drawing the attention of members of the global geothermal community as an important future renewable energy source for the world (e.g., Dobson et al., 2017). NEDO is promoting supercritical geothermal exploration as an important future energy source. There have been several efforts to use the fiber-optic distributed acoustic sensor (DAS). The DAS method is sensing strain or strain rate caused by seismic waves and the spatial resolution is a few meters. Because the temperature at the supercritical point is higher than 374 °C, ordinary geophones cannot be used in such supercritical conditions. In contrast, the optical fiber can be used at ~500 °C. We evaluated the usefulness of DAS technology on land and concluded that it is comparable to using geophones (Kasahara et al., 2018a). In order to study supercritical water reservoirs, we carried out a simulation using full-waveform inversion (Kasahara et al., 2018b). It was found that physical properties such as Vp, Vs, and density in the reservoir were well retrieved.
Field study: To evaluate the feasibility of our approach in a real geothermal field, we carried out a field study in the Medipolis geothermal field in Kyushu, Japan in fall, 2018. The fiber-optic cable was deployed down to a 977 m depth in the IK-4 borehole. We conducted the distributed temperature sensor (DTS) and DAS seismic measurements in the borehole using the same fiber-optic cable. We also installed 20 sets of three-component geophones along the 2 km long EW line at approximately 100 m spacing.
Results: The maximum temperature was measured as 264 °C at 914 m. The DAS data were obtained continuously at every 1 m for 4.5 days. Using the DAS system, we observed seven natural earthquakes between M0.8 and M5.2. The P-wave first arrival of the M5.2 earthquake was observed at the whole depth of 977 m to 0 m. Although the temperature at the 914 m depth was 264 °C, any evident seismic attenuation was not observed. In some earthquake records, some surface geophones show large amplitudes on horizontal components at 0.8 seconds after the P first arrivals in the vertical component. To estimate the vertical Vp profile of the surrounding area, we conducted a semblance analysis and obtained the Vp for every 100 m depth interval along the borehole. The Vp is estimated to be approximately 4 km/s and 3.3 km/s between 800–977 m, and between 500–800 m, respectively.
Discussion and conclusions: Although the temperature at a 914 m depth was as high as 264 °C, no significant attenuation of P arrivals was observed. The Vp profile in the borehole shows approximately 4 km/s between 800 m and 977 m. It seems no effects by the high-temperature zone. The reason for these measurements might be explained by the wavelengths of natural earthquakes being longer than the thickness of the high-temperature zone.
We observed seven natural earthquakes, but we did not see reflected phases from the deep reflectors. Surface geophones suggest the presence of P-to-S converted waves, and the conversion could be happening just below the study field. Although further studies are needed, the DAS system could supply a very dense vertical seismic array, and with the DAS seismic system and full-waveform inversion method, we could image the deep-seated supercritical geothermal reservoirs if they exist.
Acknowledgements: This study is supported by the New Energy and Industrial Technology Development Organization (NEDO). Medipolis Energy Co. kindly allowed the use of their IK-4 geothermal well for this study. Mr. Kimura provided us with the Schlumberger hDVS measurements. WELMA Co. provided us with the borehole fiber-optic system and measurement of temperatures in the borehole. The staff of West Japan Engineering Consultants, Inc., helped us during the field study.
Field study: To evaluate the feasibility of our approach in a real geothermal field, we carried out a field study in the Medipolis geothermal field in Kyushu, Japan in fall, 2018. The fiber-optic cable was deployed down to a 977 m depth in the IK-4 borehole. We conducted the distributed temperature sensor (DTS) and DAS seismic measurements in the borehole using the same fiber-optic cable. We also installed 20 sets of three-component geophones along the 2 km long EW line at approximately 100 m spacing.
Results: The maximum temperature was measured as 264 °C at 914 m. The DAS data were obtained continuously at every 1 m for 4.5 days. Using the DAS system, we observed seven natural earthquakes between M0.8 and M5.2. The P-wave first arrival of the M5.2 earthquake was observed at the whole depth of 977 m to 0 m. Although the temperature at the 914 m depth was 264 °C, any evident seismic attenuation was not observed. In some earthquake records, some surface geophones show large amplitudes on horizontal components at 0.8 seconds after the P first arrivals in the vertical component. To estimate the vertical Vp profile of the surrounding area, we conducted a semblance analysis and obtained the Vp for every 100 m depth interval along the borehole. The Vp is estimated to be approximately 4 km/s and 3.3 km/s between 800–977 m, and between 500–800 m, respectively.
Discussion and conclusions: Although the temperature at a 914 m depth was as high as 264 °C, no significant attenuation of P arrivals was observed. The Vp profile in the borehole shows approximately 4 km/s between 800 m and 977 m. It seems no effects by the high-temperature zone. The reason for these measurements might be explained by the wavelengths of natural earthquakes being longer than the thickness of the high-temperature zone.
We observed seven natural earthquakes, but we did not see reflected phases from the deep reflectors. Surface geophones suggest the presence of P-to-S converted waves, and the conversion could be happening just below the study field. Although further studies are needed, the DAS system could supply a very dense vertical seismic array, and with the DAS seismic system and full-waveform inversion method, we could image the deep-seated supercritical geothermal reservoirs if they exist.
Acknowledgements: This study is supported by the New Energy and Industrial Technology Development Organization (NEDO). Medipolis Energy Co. kindly allowed the use of their IK-4 geothermal well for this study. Mr. Kimura provided us with the Schlumberger hDVS measurements. WELMA Co. provided us with the borehole fiber-optic system and measurement of temperatures in the borehole. The staff of West Japan Engineering Consultants, Inc., helped us during the field study.