FLOW CHANNELING IN STRONGLY HETEROGENEOUS POROUS-MEDIA - A NUMERICAL STUDY

Flow and solute transport through porous media having strongly variabl e permeability were studied for parallel and convergent/divergent flow s. The variation in hydraulic conductivity K causes (1) the fluid to f low through a porous medium along least resistive pathways and (2) sol ute dissolved in the fluid to be transported with widely differing vel ocities. Numerical simulations were performed to study flow and solute transport in a three-dimensional heterogeneous porous block. It is fo und that for a strongly heterogeneous medium the particles (or solutes ) travel through the medium along preferred flow paths, which we call channels. These channels possess hydraulic properties that are differe nt from those of the global porous medium and which are invariant rega rdless of the direction from which the hydraulic gradient is applied t o the porous block. The log-hydraulic conductivities along these chann els have a greater mean value and a smaller standard deviation than fo r the global porous medium. These differences or ''shifts'' were calcu lated as a function of the hydraulic conductivity variance of the glob al porous medium. Tracer breakthrough curves for a pulse injection wer e also calculated. For small standard deviations of the global hydraul ic conductivity distribution, a peak in the breakthrough curve is foun d which spreads out around its peak value as the standard deviation is increased. However, as the standard deviation is increased further, a new peak emerges at a much earlier time. This may be the result of in creasing channeling effects at large standard deviations. For the cage of a spherical pressure boundary around point tracer injection, the f low follows the usual divergent pattern only for small variations in h ydraulic conductivity. When the standard deviation in log K is large, a significant portion of the flow becomes channelized; i.e., it tends toward a linear flow pattern.