Direct simulation of heterogeneous diffusion and inversion procedure applied to an out-diffusion experiment. Test case of Palmottu granite.
An out-diffusion laboratory experiment using a non-reactive tracer was fitted using the Time Domain Diffusion (TDD) method. This rapid particle tracking method allows simulation of the heterogeneous diffusion based on pore-scale images and local values of diffusivities. The superimposed porosity and mineral 2D maps act as computation grids to condition diffusion pathways. We focused on a Palmottu granite sample, in which the connected pore space has a composite microstructure with cracks linking microporous minerals and is above the percolation threshold. Three main results were achieved: (i) When compared to the fitting obtained with one coefficient (best mean square residual R = 1.6 x 10(-2)), diffusion is shown to be suitably characterised with two coefficients related to cracks and microporous minerals (best R = 6.5 x 10(-4)), (ii) rather than imposing a local apparent diffusion coefficient D(a) independent of the local porosity Phi, a best fit is obtained by applying Archie's relationship D(a) = D(0) x G with G = Phi(m) to each pixel of the calculation grids (G is the geometry factor, D(0) is the diffusion coefficient in free fluid, and m is Archie's exponent), and (iii) the order of magnitude of the fitted diffusion coefficient or Archie's exponents (m=0 for microcracks and m=1.82 for microporous minerals) is physically realistic.
Sardini
P
P
HYDRASA Laboratory (Hydrogeology, Clays, Soils and Alterations), UMR 6532 CNRS, University of Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France. paul.sardini@univ-poitiers.fr
Robinet
J C
JC
Siitari-Kauppi
M
M
Delay
F
F
Hellmuth
K H
KH
D003198:Computer Simulation; D004058:Diffusion; D005387:Finland; D007091:Image Processing, Computer-Assisted; D015233:Models, Statistical; D008962:Models, Theoretical; D016062:Porosity; D011848:Radioactive Pollutants; D011850:Radioactive Waste; D012822:Silicon Dioxide; D013997:Time Factors; D014872:Water Movements; D014873:Water Pollutants; D014878:Water Pollution, Radioactive / Q000032:analysis*; D018508:Water Purification / Q000379:methods*
D011848:Radioactive Pollutants; D011850:Radioactive Waste; D014873:Water Pollutants; C007886:granite; D012822:Silicon Dioxide