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Recent publications

Workreport 2017-22



Matrix Pore Water Study on REPRO Samples


Voutilainen, M., Ikonen, J., Kekäläinen, P., Sammaljärvi, J., Siitari-Kauppi, M., Lindberg, A., Kuva, J., Timonen, J.



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The chloride concentration of matrix pore water was studied using six drill core samples from REPRO niche. The samples were collected so that they were preserved in the naturally saturated state and chloride was then leached out from the matrix pore water using an out-diffusion experiment. The chloride out-diffusion curves were modelled using a novel mathematical tool to get information on total amount of chloride and its pore diffusion coefficient. The developed analysis tool takes into account some heterogeneities of the sample mineralogy and structure. According to these analyses, the matrix pore water of investigated samples contains chloride from (0.8 ± 0.2) g/l to (2.3 ± 0.7) g/l whereas the previously measured chloride concentrations from water conducting fractures near by REPRO niche show significantly higher values (5.2−5.9 g/l). According to these results, the chloride concentration of matrix pore water and fracture water are not in balance. This was interpreted as being a consequence of anion exclusion that reduces available pore space for anions and thus hinders their diffusion in respect to cations and neutral molecules. Effect of anion exclusion is typically observed in tight clay rocks but also crystalline rocks with the high biotite content and relatively high quantity of the nanometer scale pore appertures.

The pore diffusion coefficients of chloride determined from the out-diffusion curves were from (0.9 ± 0.2) × 10−11 m2/s to (9 ± 2) × 10−11 m2/s from the transport dominating early part of the curves and from (0.4 ± 0.1) × 10−11 m2/s to (0.9 ± 0.2) × 10−11 m2/s from their late part. The transport properties of samples were determined using He-gas methods. The results for permeability varied from (6 ± 1) × 1019 m2 to (64 ± 1) × 1019 m2. The results from the He-gas through-diffusion measurements were converted to water phase values and then the pore diffusion coefficients varied from (0.50 ± 0.05) × 10−11 m2/s to (8.3 ± 0.7) × 10−11 m2/s. The results were in the same order of magnitude as from the chloride out-diffusion measurements.

The porosities were measured using three different techniques (water gravimetry, 14C-PMMA autoradiography and argon pycnometry) and average porosities of the samples varied from (0.6 ± 0.2) % to (0.9 ± 0.2) % (error weighted results over all three measurements). According to 14C-PMMA results, the pore structure of the samples consists of intra- and intergranular fissures (mainly quartz and feldspar grains), biotite rich areas with slightly higher porosity than average porosity (about 1 %), and highly porous (over 5 %) roundish patches (cordierite and pinite) as well as homogeneously porous mineral grains (even at the investigated scale).


Crystalline rock, matrix pore water, porosity, mineralogy, pore diffusion coefficient, C-14-PMMA autoradiography, fracture water.


WR 2017-22_web (pdf) (4.4 MB)


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