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POSIVA Report 2017-4



Quality Assessment of PFL Transmissivity Data Identified in Subsurface Drillholes of ONKALO


Tammisto, E., Ahokas, H., Komulainen, J., Pekkanen, J.



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As part of the Posiva’s investigation programme for the final disposal of the spent nuclear fuel, Posiva Oy investigates the prevailing hydrogeological conditions at Olkiluoto Island. The location and hydraulic properties of fractures are of interest for example in the groundwater flow modelling.

This report assesses the applicability of the PFL DIFF (and PFL MFD) methods to characterise flow of rock fractures intersecting subsurface holes drilled from ONKALO, which is an underground rock characterisation facility that extends approximately to the depth of 450 m. So-called PFL transmissivity can be determined at flowing fracture intersections in drillholes with the aid of Posiva Flow Log (PFL DIFF/MFD) measurements. 

Representativeness of the PFL flow logging data measured under large drawdown in ONKALO drillholes has been a topic for discussions, especially due to a doubt that large drawdowns in the subsurface drillholes could lead to turbulent flow.

Functionality of the PFL DIFF tool has been tested at laboratory and also at field to verify functionality at authentic conditions in ONKALO. The tests in ONKALO included, for example, PFL DIFF measurements in different pressure conditions. 

In this report, the data obtained with PFL measurements were also analysed against the other existing data. This consisted of the comparison of PFL DIFF data and injection test data in ONKALO and statistical analysis of PFL transmissivities from surface based drillholes and ONKALO pilot holes within certain rock domain. Additionally, the analyses also included comparisons between the sum of flows detected with the PFL DIFF tool in the ONKALO pilot holes and outflows from probe holes located at the same tunnel section.  The PFL flow anomalies that were not possible to correlate with the fractures mapped from the drillcore or drillhole wall images were also studied in detail.

This study shows that the PFL DIFF tool is working well in the subsurface ONKALO-holes. There are no clear signs of turbulent flow under a large drawdown. Large gradient of the hydraulic head does not cause any additional technical problems for the PFL DIFF tool. The reported PFL DIFF flow logging results represent flows from single fractures or a network of fractures intersecting the subsurface drillholes during steady flow conditions.


Hydrogeology, ONKALO, subsurface drillholes, PFL, transmissive fractures, hydraulically conductive fractures, groundwater flow


POSIVA 2017-04_web (pdf) (35 MB)
Appendix 1_correlated_fractures_web (xlsx) (82.7 KB)
Appendix 1_PFL data in ONKALO-holes_web (xlsx) (190.4 KB)


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