EFFECTS OF PORE-LEVEL REACTION ON DISPERSION IN POROUS-MEDIA

The Darcy-level consequences of the transport of reactive tracers is a nalyzed by detailed pore-level modeling, based on a network model. Mom ents of the residence-time distribution of the conservative process (r eversible reactions) are useful for investigation of the spreading of tracers, even when complete evaluation of the residence-time distribut ion is not available. We carry out simulations to show how reaction te rms have to be included in the convection-dispersion equation to corre ctly predict the Darcy-level effects of reversible reactions at the po re-level. In the case of spatially homogeneous rate constants, the val ue of the dispersion coefficient corresponds to that of a nonreactive tracer. Spatial heterogeneities of the rate constants give rise to a d ispersion 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 i n terms of two dimensionless groups, the Damkohler number Da and the v ariance 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 distributio ns of rate constants. The limit Da --> 0 represents a breakdown of the macroscopic equation, though the relative error in the low-order mome nts of the residence-time distribution is less than 29% for 0.1 < Da < 1. A binary distribution of the rate constant at its percolation thre shold 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 Publishe d by Elsevier Science Ltd.