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Workreport 2016-56



EDZ Study Area in ONK-TKU-3620: Discrete Fracture Network Based Modelling of Microcrack Systems in Drill Core Specimens and Comparisons with Petrophysical Measurements


Kovács, D., Dabi, G., M. Toth, T., Jacobsson, L., Kiuru, R.



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Microfractures in 16 specimens from drillholes ONK-SH32 and ONK-SH39 in the ONKALO EDZ study area in ONK-TKU3620 were analysed. Specimens included the dominant rock types in the study area: veined gneiss (VGN) and granitic pegmatoid (PGR). The specimens were prepared to thin sections, impregnated with fluorescent epoxy and photographed. The photographs were processed to create skeletonised binary images of the fracture network, from which geometric parameters (length, aperture, spatial density, strike and dip) were extracted via a series of MATLAB algorithms.

No systematic changes in the parameters were observed with respect to depth, while differences between VGN and PGR were observed. This suggests that excavation does not cause detectable perturbance of the intact crystalline 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.

Possible associations between physical properties and fracture network parameters were studied. No association with foliation direction was observed, but electrical properties showed varying levels of association. The gneiss and pegmatoid clearly showed different properties.

The geometric parameters were used as input for a fractal geometry based Discrete Fracture Network modelling. Models were built for five different scenarios, ranging from pure VGN to pure PGR. Microfractures in VGN were predominantly short and mostly arranged into one preferred orientation, with very few communicating clusters, and low number of fractures per cluster. PGR had relatively longer fractures with two preferred orientations, resulting in more communicating clusters, and more fractures per cluster. Largest percolation clusters constituted between 0.06 % (VGN) and 0.25 % (PGR) of all fractures. Composite models showed values between the two. The interconnectivity of microfractures seems to be negligible throughout the studied sections.


Discrete fracture network model, petrophysics, physical properties of rocks, laboratory testing, simulation, ONKALO, EDZ study area, excavation damage, percolation cluster


WR-2016-56 web (pdf) (15.5 MB)


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