Experimental and numerical investigations on the effect of fracture aperture distribution and geometry on fluid flow and solute transport in natural fractures

Madeleine Stoll1, Florian Huber1, Michael Trumm1, Frieder Enzmann2, Achim Wenka3, Eva Schill4, Thorsten Schäfer5
1 Institut für nukleare Entsorgung, Karlsruher Institut für Technologie
2 Institut für Geowissenschaften, Johannes Gutenberg-Universität Mainz
3 Institut für Mikroverfahrenstechnik, Karlsruher Institut für Technologie
4 Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt; und Institut für nukleare Entsorgung, Karlsruher Institut für Technologie
5 Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena; und Institut für nukleare Entsorgung, Karlsruher Institut für Technologie

O 12.3 in Reactive transport in heterogeneous aquifers

24.03.2018, 09:30-09:45, 3

The impact of flow channel geometry on solute transport was studied experimentally and numerically on two differently altered fractured granites from Soultz-sous-Forêts (France). With the help of three injection and three extraction locations at the top and bottom of the fractured cores different dipole flow fields were induced. The breakthrough curves (BTCs) of the injected conservative tracer (Amino-G) were measured using fluorescence spectroscopy. Based on tomographic data obtained by µCT (resolution 60 µm) semi 3D numerical models were generated for both fractures by implementing the measured 3D aperture distribution to the 2D fracture geometry. Fluid flow and tracer transport are simulated using COMSOL Multiphysics®. The Navier-Stokes equations were solved using the “Laminar Flow” option. In order to study purely the physical transport process neither chemical interactions with the fracture surfaces (e.g. sorption) are incorporated nor any potential matrix diffusion.
In accordance to their geological history, both fractures are significantly different in terms of spatial heterogeneities. The fluid flow and the solute transport behavior clearly reflect these geometrical differences. The altered granite fracture is highly complex in geometry with a smaller mean aperture (0.162±0.132 mm) compared to the unaltered fractured core (0.357±0.112 mm). In contrast, the unaltered fracture has a geometry with a minor number of asperities and aperture distribution shifted to higher aperture values.
Regarding the experimental findings, the BTCs of the altered fracture are less homogenous compared to the ones through the unaltered fracture which confirms the tomographic imaging results. In case of the altered fracture there is a good agreement between modelled and experimental BTCs. We conclude that fluid flow through this fracture is overall dominated by the complex fracture geometry and the high number of asperities whereas the aperture distribution plays only a negligible role.
The experimental BTCs of the different flow configurations of the unaltered fracture show less deviation between each other. The wide aperture and the aperture distribution might decrease the influence of fracture walls and the fully open fracture (no asperities under the given µCT resolution) might be the reason for the similar BTCs. However, the modelled BTCs are more irregular compared to the experimental ones with an overestimation in the tailing region giving an overall lesser agreement between the experimental and modelled BTCs.
In this work we show the major advantage in knowing the internal structure and the spatial heterogeneities of single fractures in order to interpret and understand solute transport processes in complex natural domains. Despite the simplification from a fully 3D to a semi 3D model, the presented results provide a step forward in understanding fundamental solute transport processes through natural single fractures with natural surface roughness/geometry.

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