High Permeability of Quaternary Granites in the Kakkonda Geothermal Area, Northeast Japan

Abstract

Kakkonda geothermal area is one of the most active geothermal areas in Japan, accompanying a young and hot granitic intrusive underlying 2 km below the surface as a heat source. Deep geothermal well WD1a encounters the heat source granite below 2860m depth and recovered core samples from the hot region. The logging temperature of the Kakkonda granite in WD1a ranges from 375 to more than 500 oC. Core permeability, porosity and pore size of Quaternary unaltered granite from Kakkonda geothermal area were measured to evaluate the hydraulic properties of the heat source region. The measured permeability, when the Pe (effective confining pressure) is at about 2 MPa, ranges from 58 to 500 µd for unaltered Kakkonda granite whereas it is less than 10 µd for the altered samples. The core permeability of the Kakkonda granite is order of 100 µd, which is order of magnitude higher than the reported laboratory permeability of various granitic rocks. Mean crack-width, which ranges from 0.25 to 2.84 µm, has a positive correlation with the measured permeability. Geometry of the permeable pore can be simplified as a platy crack and permeability mostly depends on crack width rather than crack density. SEM observation revealed that the grain boundaries are open. The granite is considered permeable just after the solidification and subsequent alteration, and/or compaction reduces the permeability and crack-width. Grain boundary may play an important role as a migration of vapor phase in degassing process.