Effects of pore-level reaction on dispersion In porous media

The Darcy-level consequences of the transport of reactive tracers is analyzed by detailed pore-level modeling, based on a network model. Moments of the residence-time distribution of the conservative process (reversible reactions) are useful for investigation of the spreading of tracers, even when complete evaluation of the residence-time distribution is not available. We carry out simulations to show how reaction terms have to be included in the convection-dispersion equation to correctly predict the Darcy-level effects of reversible reactions at the pore-level. In the case of spatially homogeneous rate constants, the value of the dispersion coefficient corresponds to that of a nonreactive tracer. Spatial heterogeneities of the rate constants give rise to a dispersion coefficient that depends on the strength of the disorder in the reaction rates and the dispersion coefficient depends nonlinearly on the mean flow velocity. The effects of reaction can be summarized in terms of two dimensionless groups, the Damkohler number Da and the variance of the rate constant distribution. For Da much greater than 1, a macroscopic convection-dispersion-reaction equation offers a valid description of transport, even for spatially heterogeneous distributions of rate constants. The limit Da --> 0 represents a breakdown of the macroscopic equation, though the relative error in the low-order moments of the residence-time distribution is less than 29% for 0.1 < Da < 1. A binary distribution of the rate constant at its percolation threshold yields the maximum value of the dispersion coefficient. Plots of the Darcy-level Peclet number, UL/D-parallel to, with respect to the length of the system, L, reaches an asymptotic value at a length much larger than the typical pore length. This indicates the presence of a correlation length much larger than the pore length. (C) 1997 Published by Elsevier Science Ltd.