Open AccessInferred role of natural fractures, veins, and breccias in development of the artificial geothermal reservoir at the Ogachi Hot Dry Rock site, Japan
Author(s) -
Ito Hisatoshi
Publication year - 2003
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2001jb001671
Subject(s) - geology , caldera , neogene , breccia , geothermal gradient , hydrothermal circulation , geochemistry , fracture (geology) , petrology , basement , andesite , geomorphology , seismology , volcano , volcanic rock , paleontology , structural basin , civil engineering , engineering
At the Ogachi Hot Dry Rock (HDR) test site, located in a Neogene caldera, northeast Japan, two artificial reservoirs, the upper one at 700 m depth and the lower one at 1000 m depth, were created in granitic basement by hydraulic stimulation. The new fracture sets, forming the reservoirs, propagated in different directions, the upper one to the east and the lower one to the NNE. Water recovery rate from a production well penetrating the two reservoirs was at best 25%, which is very low compared to other HDR test sites such as Fenton Hill, United States, and Soultz, France. In order to understand these phenomena, natural fractures were investigated using mainly core samples from the production well. It was found that natural fractures in the upper and lower reservoirs are remarkably different: the upper reservoir, at depths less than 900 m, is more densely fractured and contains far fewer thick hydrothermal veins than in the lower reservoir. It is believed that these features are due to hydrothermal brecciation associated with Neogene caldera formation and that they could greatly affect the HDR reservoir system. Thick hydrothermal veins and andesite dykes are expected to loci for reservoir fracture propagation and major flow paths because the veins and dykes occupy a prominent fracture system whose orientation coincides well with that of the reservoir propagation. On the other hand, the intensely brecciated zone is not associated with any major flow paths.