Seismic Time-lapse Approach to Monitor Temporal Changes in the Supercritical Water Reservoir

Abstract

We conducted a feasibility study in November 2018 in the Medipolis geothermal field, Kyushu, Japan. We installed a fiber-optic cable down to 977 m in the borehole. We measured the temperature and acquired seismic data in the borehole by distributed temperature sensor (DTS) and distributed acoustic sensor (DAS) modes, respectively. The measured highest temperature was 264°C at the 914 m depth. The DAS data were acquired at every 1 m-interval. We also installed 20 sets of seismometers on the ground surface in the study field. We obtained 4.5 days of the continuous seismic monitoring via DAS and surface seismometers. The seismic system observed seven natural earthquakes occurring in Kyushu region. One of the earthquakes with M5.2 at 100 km south of the DAS system occurred at 123 km depth and were observed by a whole depth of 977 m to 0 m. It is identified that less or no attenuation of seismic amplitudes at high temperature zone and Vp is approximately 4km/s at this zone. By semblance analysis of all earthquakes, we obtained Vp profile in the borehole. There was tremor like noises observed around the 350 m depth, but the cause is not known. Horizontal components of surface seismometers showed clear P to S converted phase, and it may suggest the presence of deep geothermal reservoirs in the Medipolis area.