Observations of stress-induced fracturing in boreholes using geophysical logging

Posiva Oy is currently preparing to implement the final disposal of spent nuclear fuel. As part of the preparations, extensive studies have been conducted in the ONKALO® underground facility, Western Finland. This work focuses on the effects of thermal stress on the heating, temperature monitoring and acoustic emission monitoring holes around three experimental holes. Geophysical borehole logging was conducted in 14 drillholes varying in length between 7 and 13 metres. Measurements done in the holes include optical and acoustic imaging, gamma-gamma density logging and full waveform sonic logging. Same set of experiments was repeated in the holes twice after the heating: shortly after the test, and approximately one to two years after the tests. Heating patterns varied with the longest heating times being 64 days, greatest heating power 2000 W per hole, and highest observed temperature 585 °C. Optical and acoustic images were matched between runs and studied for new features. The locations of the new features were then studied in full waveform sonic logs to see if any could be predicted from the mechanical properties, such as showing up as a weakness zone prior to the heating. Due to the high competency of the rock at Olkiluoto, only very limited borehole breakout was observed. Formation of new horizontal fractures was not observed but opening of pre-existing tight and healed fracture was observed. This could be linked to a weakness zone observed in the geophysical logging prior to the heating experiment. The zone showed anomalously low P-wave velocities resulting in anomalously low Young’s modulus, Poisson’s ratio and Bulk modulus. More extensive studies are needed to verify the findings, but based on the results of this study, it would appear possible to pinpoint weak natural fractures prone to opening due to increased stress using borehole geophysics.