Seismic Approach Characterizing Geothermal Reservoirs Using DAS and FWI

Abstract

To image supercritical water reservoirs, we have proposed to use the distributed acoustic sensing (DAS) in the borehole, surface seismic array, active or passive seismic sources and full-waveform inversion (FWI) method. Through the comparison test of DAS and geophones in a field, we confirmed that the DAS system can be used as an array seismic sensor although it is less sensing the seismic waves perpendicular to the fiber elongation. The sensitivity is almost comparable to the surface seismometers. It can be used as dense seismic array(s). We have also examined the usefulness of full-waveform inversion (FWI) method for imaging of geothermal reservoirs. The FWI result suggests it can be used for geothermal reservoir imaging. To evaluate our approach, we carried out a feasibility study in e Medipolis geothermal field located on Kyushu Island, Japan. We deployed an optical fiber down to a 977 m depth in a borehole. Using distributed temperature sensing (DTS) mode, the measured temperature at the 914 m depth was 264 °C. We obtained four and half days of continuous seismic data via DAS and surface seismometers. The DAS data were obtained every 1 m from a 977 m depth to ground surface. We observed seven natural earthquakes. The DAS sensitivity is comparable to the surface seismometers. This suggest that the optical fiber deployment in the exiting borehole could provide reasonable coupling to the borehole casing. We obtained apparent interval Vp profile along the borehole. There was no distinct seismic attenuation observed, even in the high-temperature zone, and Vp in the high-temperature zone is estimated as 3.0 km/s. The P-to-S converted phase was evident on the surface seismometers, and this could indicate the presence of a conversion zone around the 4 km-depth beneath the Medipolis geothermal field. To image supercritical water reservoirs, we have proposed to use the distributed acoustic sensing (DAS) in the borehole, surface seismic array, active or passive seismic sources and full-waveform inversion (FWI) method. Through the comparison test of DAS and geophones in a field, we confirmed that the sensitivity is almost comparable to the surface seismometers. We have also examined the usefulness of full-waveform inversion (FWI) method for imaging of geothermal reservoirs. We carried out a field study in geothermal field. We deployed an optical fiber down to a 977 m depth in a borehole. Using distributed temperature sensing (DTS) mode, the measured temperature at the 914 m depth was 264 °C. We obtained 4.5 days of continuous seismic data The DAS data were obtained every 1 m from a 977 m depth to ground surface. We confirmed that the optical fiber deployment in the exiting borehole could provide reasonable coupling to the borehole casing. There was no distinct seismic attenuation observed, even in the high-temperature zone, and Vp in the high-temperature zone is estimated as 3.0 km/s. The P-to-S converted phase was evident on the surface seismometers, and this could indicate the presence of a conversion zone around the 4 km-depth.