X-Ray CT Imaging of Pores and Fractures in the Kakkonda Granite, NE Japan

Abstract

Three-dimensional imaging of the Kakkonda granite from the borehole WD-1a was performed to clarify meso- and microscopic structures of pores and fractures in the geothermal reservoir by medical and industrial X-ray computerized tomography (CT) scanners. The granite was recovered from WD-1a drilled at the Kakkonda geothermal field, NE Japan, by the New Energy and Industrial Technological Development Organization (NEDO).

A third-generation Hitachi Medical Corporation CT-W2000 medical CT scanner enabled us to visualize millimeter-scale rock textures of the granite inside cores without any destruction. Miarolitic cavities, enclaves, felsic minerals (quartz and feldspar) and mafic minerals (biotite and hornblende) were identified from CT images. Three dimensional distribution of miarolitic cavities was reconstructed from contiguous CT images and showed spatially heterogeneous distribution with the boundary that strikes N 20? E. Proportion of miarolitic cavities larger than 2 mm3 was 0.33 /cm3 between 2936.722 and 2936.921 m. Porosity calculated from X-ray CT images was 0.9 % from 2936.722 to 2936.821 m. We fitted ellipsoids with three arbitrary axes to the miarolitic cavities extracted from the contiguous CT images and examined aspect ratio and shape preferred orientations of ellipsoids. Fitted ellipsoids exhibit ellipsoidal shape and shortest axis trends to E-W. These indicate that these cavities may be deformed by the same regional stress as present one, which is E-W compression. The shape analysis of cavities would clarify the past deformational event.

A third-generation Nittetsu Elex Corporation ELE SCAN industrial CT scanner enabled us to visualize micron-scale microcracks in the granite. Microcracks are developed along grain boundary. These would be important to evaluate percolation and storage of the hydrothermal fluid in the granite.