Abstract
There are abundant examples of natural, engineering and industrial applications, in which “solute transport” and “mixing” in porous media occur under multiphase flow conditions. Current state-of-the-art understanding and modelling of such processes are established based on flawed and non-representative models. Moreover, there is no direct experimental result to show the true hydrodynamics of transport and mixing under multiphase flow conditions while the saturation topology is being kept constant for a number of flow rates. With the use of a custom-made microscope, and under well-controlled flow boundary conditions, we visualized directly the transport of a tracer in a Reservoir-on-Chip (RoC) micromodel filled with two immiscible fluids. This study provides novel insights into the saturation-dependency of transport and mixing in porous media. To our knowledge, this is the first reported pore-scale experiment in which the saturation topology, relative permeability, and tortuosity were kept constant and transport was studied under different dynamic conditions in a wide range of saturation. The critical role of two-phase hydrodynamic properties on non-Fickian transport and saturation-dependency of dispersion are discussed, which highlight the major flaws in parametrization of existing models.
Highlights
Transport in porous materials is important for many industrial applications and natural processes such as transport of chemicals in enhanced oil recovery, fertilizers in agricultural soils, salt water intrusion into fresh water coastal aquifers, groundwater contaminations, and deliverance of pharmaceutical products to live tissues[1,2,3,4,5,6,7]
The dispersion coefficient, pore scale velocity, and maximum concentration were estimated by fitting equation (1) to the experimental data
The accuracy of the fitting results was estimated based on the normalized mean square error (NMSE)
Summary
Transport in porous materials is important for many industrial applications and natural processes such as transport of chemicals in enhanced oil recovery, fertilizers in agricultural soils, salt water intrusion into fresh water coastal aquifers, groundwater contaminations, and deliverance of pharmaceutical products to live tissues[1,2,3,4,5,6,7]. Under single-f-luid phase conditions, hydro-dynamic dispersion (D) is empirically proposed to be a power law function of average pore-scale velocity (u), defined as D = Dm + un 8 In this relation, Dm is the molecular diffusion coefficient, and α is the diffusivity coefficient. From the hydro-dynamic point of view, the existence of two fluid phases in a porous medium will create some hydrodynamically stagnant zones in each fluid with very weak (close to zero) velocity fields, which will make the local transport regime to be diffusive. This is in contrast to the remaining area occupied by the same fluid where transport is advective-dispersive. This study is a follow-up study of Karadimitriou et al.[25] in which for the first time using direct image analysis, the impact of flow rate on transport properties under the same saturation topology is studied
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