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Recent publications

Workreport 2017-56



EDZ Study Area in ONK-TKU-3620: Petrophysical, Rock Mechanics and Petrographic Testing and Analysis Conducted on Drill Core Specimens between 2014 and 2016


Kiuru, R., Heikkinen, E., Jacobsson, L., Kovacs, D.



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Physical and mechanical properties of drill core specimens were determined as a part of investigations into excavation damage in the dedicated study area in ONK-TKU-3620. Additionally, microfractures in 16 specimens from drillholes ONK-SH32 and ONK-SH39 were analysed and used as a basis for fractals based discrete fracture network modelling.

Best indicators for excavation damage, based on this study, would appear to be resistivity, S-wave velocity, and elastic parameters that are derived from S-wave velocity. The effects seemed to be mostly limited to pegmatoid specimens. Same properties also distinguished anomalous specimens, typically from shallow depths, some of which could be directly linked to visible EDZ features. 

Comparing the crack initiation stress values to prior data from specimens saturated with non-saline water, similar results were obtained. This suggests that whatever chemical corrosion the saline water may have inflicted on the specimens has happened prior to sampling in situ, and that the decrease in strength compared to dry specimens is mainly due to water content, rather than the type of water.

It was concluded that the difference in resistivity between pegmatoid and gneissic specimens of similar porosity was likely due to differences in the types of microfractures. This hypothesis was later confirmed from the thin sections and simulation: fractures in VGN were short and mostly in one preferred orientation, whereas the fractures in PGR were longer and had two preferred orientations. This suggests that VGN and PGR may suffer different types of excavation damage. Based on simulation results, the number of large fracture clusters and maximum number of fractures per cluster seem to increase with increasing volumetric PGR content. The interconnectivity of microfractures however seems to be negligible.

No depth dependencies in geometric parameters of the microfractures were observed, while differences between VGN and PGR were observed. This suggests that excavation does not cause detectable disturbance of the intact rocks’ microfracture porosity, and that the disturbed layer observed by geophysical methods may be caused by larger scale fractures not present in the relatively intact specimens.


ONKALO EDZ study area, petrophysics, rock mechanics, micro scale discrete fracture network model, laboratory testing, statistical analysis, simulation, ultrasonics, Brazilian test, UCS test


WR 2017-56_web (pdf) (2.3 MB)


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