A field experiment of a temperature-tolerant distributed acoustic sensor in deep geothermal reservoir prospecting

Abstract

Seismic exploration technologies with distributed acoustic sensing (DAS) are investigated to acquire seismic waves in high-temperature environments and locate supercritical geothermal reservoirs deeper than the brittle-ductile transition boundary in Japan. We conducted a seismic reflection survey in the Mori-Nigorikawa geothermal field in the Nigorikawa caldera basin. An optical fiber system and an array of geophones were installed to 2100 m depth in F-01, a little-deviated high-temperature geothermal production well, and along a road for about 1.2 km on the surface. Numerous reflectors appear to gather at a depth between 2000 and 3000 m, corresponding to the production zone of the well. Although the production zone is located mainly within the Nigorikawa volcanic conduit estimated in the past geothermal analysis, the reflectors are also distributed outside the conduit. The DAS waveforms show distinct traveltime discontinuity at 1060 m depth. Our traveltime tomography and forward-simulated synthetic DAS waveforms could indicate the existence of a water layer 250 m long and 50 m thick at 1060 m depth. The Mori Geothermal Power Plant reinjects wastewater at this depth through a reducing well. The use of high-temperature-tolerant fiber enables the deployment of seismic sensors much closer to the deep target and may investigate effective fluid circulation through production and reducing wells.