Experimental investigation of the link between pore scale velocities, transport and reactivity in porous media

Abstract:

Pore scale characterization of flow velocities and concentration spatial distributions is a key to understanding non-Fickian transport and mixing in porous media. We present a millifluidic setup aimed at investigating those processes in transparent porous media, at the pore scale. The porous media are quasi-2D, consisting of a Hele-Shaw cell containing cylindrical grains. They are made by soft lithography from a numerical model and provide full control on the geometry (medium porosity, permeability and heterogeneity). The setup allows for the study of primary drainage/imbibition, or the joint continuous injection of two fluids (e. g. water and air). A camera records the distributions of fluid phases, the position of solid tracers, and spatially-resolved images of light emissions inside the flow cell. The pore scale velocity field is thus measured from particle tracking, while pore scale concentration fields are measured accurately in passive transport experiments, using fluorescein; both continuous injection and finite volume solute injections can be achieved. Using two chemo-luminescent liquids, the reaction of which produces photons in addition to the reaction product, we are also able to study the local production rate of the reaction product as the reactive liquids flow through the system [1]. Pressure drops across the medium are also measured. This complete characterization (phase distributions, velocity and concentration fields, pressure drops) of the system allows to explain non-Fickian behaviors and test models that upscale transport and mixing properties from pore scale data. As examples, we shall discuss the upscaling of transport from the knowledge of Lagrangian velocities and the relationships between conservative and reactive transport under mixing-limited conditions (very large Damkhöler number). Other applications include the prediction of the mixing rate from the sole knowledge of the flow stretching [2], and the characterization of mixing by unsaturated flows [3].

[1] P. de Anna et al. (2014), Environ. Sci. Technol. 48, 508-516. [2] R. Turuban et al. (2014), Mixing and dispersion upscaling from a 2D pore scale characterization of Lagrangian velocities, submitted. [3] J. Jiménez-Martínez et al. (2014), Persistence of incomplete mixing in unsaturated porous media, submitted.