Predictions of non-Fickian solute transport in different classes of porous media using direct simulation on pore-scale images

摘要

We present predictions of transport through micro-CT images of porous media that include the analysis ofncorrelation structure, velocity, and the dynamics of the evolving plume. We simulate solute transport throughnmillimeter-sized three-dimensional images of a beadpack, a sandstone, and a carbonate, representing porousnmedia with an increasing degree of pore-scale complexity. The Navier-Stokes equations are solved to computenthe flow field and a streamline simulation approach is used to move particles by advection, while the random walknmethod is employed to represent diffusion.We show how the computed propagators (concentration as a functionnof displacement) for the beadpack, sandstone, and carbonate depend on the width of the velocity distribution.nA narrow velocity distribution in the beadpack leads to the least anomalous behavior, where the propagatorsnrapidly become Gaussian in shape; the wider velocity distribution in the sandstone gives rise to a small immobilenconcentration peak, and a large secondary mobile peak moving at approximately the average flow speed; innthe carbonate with the widest velocity distribution, the stagnant concentration peak is persistent, with a slowernemergence of a smaller secondary mobile peak, characteristic of highly anomalous behavior. This defines differentntypes of transport in the three media and quantifies the effect of pore structure on transport. The propagatorsnobtained by the model are in excellent agreement with those measured on similar cores in nuclear magneticnresonance experiments by Scheven, Verganelakis, Harris, Johns, and Gladden, Phys. Fluids 17, 117107 (2005).