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Workreport 2013-38



Analysis of Acoustic and Optical Televiewer Data from the Olkiluoto Site: Drillholes OL-KR10, OL-KR19, OL-KR40, OL-KR46 and OL-KR46B, 2010-2013


Ask, M., Ask, D.



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In this report, we first present the theory of stress-induced elongations in drillholes, i.e. borehole breakouts and drilling-induced fractures, logging tools and computer software that are used for identifying and analyzing these features. We have obtained acoustic televiewer data from five drillholes, OL-KR10, OL-KR19, OL-KR40, OL-KR46, and OL-KR46B from the Olkiluoto site. Borehole breakouts are formed in the direction of the minimum horizontal stress when the stress concentration of the in situ stress field around the drillhole exceeds the rock strength of the drillhole wall. Hence, the orientation of the maximum horizontal stress is 90º off. Drilling-induced fractures (DIF’s) are formed in the orthogonal direction to that of breakouts; hence, parallel to the direction of the maximum horizontal stress. The drillholes have been logged routinely with two geophysical logging tools, acoustic borehole televiewer (BHTV) and optical image method.

The most essential objective of this report is to investigate if borehole breakouts and DIF’s exist at the Olkiluoto site. Should these features be identified, a secondary objective is to roughly estimate the total length of breakouts and DIF’s, and their orientation, i.e. the orientation of minimum and maximum horizontal stress, respectively. In addition, these stress-induced features should be correlated with existing structures in the drillholes (fractures and fractures zones), as determined by the geological mapping. We emphasize that a full characterization of stress-induced features, i.e. the determination of magnitude of horizontal stresses, is outside the scope of this report, but may be undertaken at a later stage if borehole breakouts and DIF’s are found.

No large-scale, dog-ear shaped, breakouts or prominent DIF’s are observed in the investigated drillholes. Hence, the resulting stress concentration around the drillholes is not exceeding the strength of the rock at the Olkiluoto down to some 800 m vertical depth. However, long sections of grain-sized fallouts appear at diametrically opposite sides of the drillholes. These seem to be correlated to key seats, i.e. mechanical wear of the drill string, as they are observed near the top and bottom sides of the drillholes.

The analyses have been significantly hampered by the quality of the available data, of which orientation problems and artifacts are the most important data degraders. For this reason, we devote Chapter 6 to suggest improvements of quality assurance procedures. The orientation problems imply that the observed grain-sized fallouts may deviate from the top and bottom sides of the drillholes, and thus potentially be stress induced features.


Olkiluoto, stress-induced borehole failure, borehole breakouts, mechanically-induced borehole deformation, key seats.


Analysis of Acoustic and Optical Televiewer Data from the Olkiluoto Site: Drillholes OL-KR10, OL-KR19, OL-KR40, OL-KR46 and OL-KR46B, 2010-2013 (pdf) (13.8 MB)


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