HR: 14:10h
AN: H13K-02    [Abstracts]
TI: An Example of Non-Fickian Dispersion in Porous Media Explained by Heterogeneous Microscale Matrix Diffusion
AU: * Gouze, p
EM: philippe@msem.univ-montp2.fr
AF: Laboratoire de Geosciences, Universite de Montpellier, Montpellier, 34095 cdx5, France
AU: Le Borgne, T
EM: Leborgne@msem.univ-montp2.fr
AF: Laboratoire de Geosciences, Universite de Montpellier, Montpellier, 34095 cdx5, France
AU: Melean, Y
EM: melean@msem.univ-montp2.fr
AF: Laboratoire de Geosciences, Universite de Montpellier, Montpellier, 34095 cdx5, France
AU: Dentz, M
EM: marco.dentz@gmail.com
AF: Department of Geotechnical Engineering and Geosciences, Technical University of Catalonia, Barcelona, 08034, Spain
AU: Leprovost, R
EM: leprovost@msem.univ-montp2.fr
AF: Laboratoire de Geosciences, Universite de Montpellier, Montpellier, 34095 cdx5, France
AU: Carrera, J
EM: jcarrera@ija.csic.es
AF: Institute of Earth Sciences Jaume Almera, CSIC, Solé i Sabarís, s/n, Barfcelona, 08028, Spain
AB: We report a set of single well injection withdrawal tracer tests (SWIWTT) and transmission tracer tests (TTT) at laboratory scale in high porosity limestone using newly developed equipment. Tracer concentration breakthrough curves (BTC) are measured over 4 to 5 orders of magnitude allowing us to precisely characterize the non-Fickean dispersion behavior. Using the SWIWTT for distinctly different exploration volume and the TTT performed through the core corresponding to the same level in the borehole, we interpret the non-Fickean dispersion behavior in term of mobile-immobile mass transfers due to diffusion in the microporosity. All the BTC are similar and do not display the usual single slope power law concentration decrease C(t) ~ t^-γ corresponding to the conventional MIM mass transfer model. At intermediate time t₁<t<t₂ , when t₁ is larger than the advection characteristic time, the concentration decreases roughly as C(t) ~ t^-2. Then, at large times (t> t₂) the asymptotic decrease is C(t) ~ t^-1.5. Analyzing the BTC set using both CTRW approach and memory function approach, we explain the measured BTC by a dual slope transition time distribution that denotes, in term of MIM mass transfer model, a dual control of the retention time probability of the tracer in the immobile domain. The origin of these distinctly different characteristic times of retention, which appears to be independent of the scale, is difficult to solve by conventional analysis of the medium. To test the assumption that the heterogeneity of the microporosity controls this unusual non-Fickean behavior, we performed numerical diffusion experiments using X-ray microtomography to quantify the mobile-immobile interface geometry and the immobile microposisity distribution. Result shows that, a memory function similar to the one required for fitting the BTC set is obtained by the numerical diffusion experiment as soon as the heterogeneity of the immobile domain is taken into account. We conclude that, for limestone reservoirs, the strong non-Fickean dispersion behavior is explained by matrix diffusion only, and the unusual dual slope pretension time is controlled by the microscale heterogeneity of the diffusivity in the rock matrix.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1857 Reservoirs (surface)
DE: 1858 Rocks: chemical properties
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2007 Fall Meeting