Understanding the distribution of the permeable zones in the geothermal field is important for the exploration and assessment of the reservoir. The distribution of the permeable zones such as pores and fractures is observed in the rock samples. These structures of the permeable zones would influence storage and percolation of the hydrothermal fluids.
The borehole WD-1a was drilled at the Kakkonda geothermal field as a part of the 'Deep-Seated Geothermal Resources Survey' project by the New Energy and Industrial Technological Development Organization (NEDO). Miarolitic cavities were found in the granite core recovered from WD-1a. These cavities may constitute a part of the geothermal reservoir in the granite, because several boreholes penetrating the granite encountered major lost circulation zones (Kato and Sato, 1995). Therefore, it is important to know the cavity characteristics such as three-dimensional distribution, size distribution, and shape characteristics.
We describe here the result of three-dimensional imaging of millimeter-scale miarolitic cavities in a granite core from WD-1a and discuss the relationship between the structure of cavity and the geothermal reservoir. We also tried to visualize micron-order microcracks by X-ray CT, because permeable structures consist of not only millimeter-scale cavities but also micron-order microcracks.
| previous slide | next slide | return to title slide | graphics |