Dual-continuum multicomponent reactive transport with nth-order solute transfer terms for structured porous media

Abstract

The dual-continuum model (DCM) is a type of modeling approach often used to interpret anomalous and non-ergodic solute transport in which the breakthrough curve cannot be explained by the classical advection-dispersion equation. Experimental studies show that bentonites have a macro-porous domain containing free water and a microporous domain containing double-layer and interlayer water. Therefore, a DCM could be needed to describe water flow, solute transport, and chemical reactions through compacted bentonite. In most DCMs, the mass exchange between domains is based on a lumped first-order solute transfer term which is not always accurate. An nth-order solute transfer term for dual-continuum flow and reactive transport model for structured porous media is proposed here. The value of n is derived from the approximation of the analytical solution of diffusion through a thin slab. The parameters of DCMs are obtained by an inverse methodology. Solute transfer terms for compacted bentonite have been evaluated for 1-D and 2-D synthetic cases by solving the inverse problem for several values of n. The best results are obtained with an n of 0.72 and a scale term of 2.5. The reactive transport DCM has been tested with data from a permeability test conducted on a sample of full-scale engineered barrier experiment (FEBEX) bentonite. By accounting for the solute flux across the micromacro interface, the DCM overcomes the limitations of the single-continuum model (SCM) which fails to reproduce the long tails of the breakthrough curves of most chemical species of this test. The exponent n and the scaling factor were estimated from Cl − data. They are similar to those obtained from the 2-D synthetic case, thus indicating that the exponent and the scaling factor derived for the FEBEX bentonite could be used for other compacted bentonites.

A dual porosity model of high-pressure gas flow for geoenergy applications

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