Seismic Feasibility Study to Identify Supercritical Geothermal Reservoirs in a Geothermal Borehole Using DTS and DAS

Abstract

Summary To image supercritical water reservoirs, we propose the use of the distributed acoustic sensing (DAS) in the borehole, surface seismic array, active or passive seismic sources and full-waveform inversion (FWI) method. The result of the simulation shows clear imaging of reservoirs. To evaluate our approach, we carried out a feasibility study in the 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 4.5 days of continuous seismic data via DAS and surface geophones. The DAS data were obtained every 1 m from a 977 m depth to ground surface. We observed seven natural earthquakes. We used semblance analysis for the DAS datasets, and the Vp profile along the borehole was obtained. There was no distinct seismic attenuation observed, even in the high-temperature zone, and Vp in the high-temperature zone is estimated as 4 km/s. The P-to-S converted phase was evident on the surface geophones, and this could indicate the presence of a conversion zone just beneath the test field.