Hydrodynamics and Long-term Permeability Evolution in Clogging Porous Media

Permeability reduction caused by colloid deposition in porous media, or clogging, is important in water treatment, aquifer hydraulics, and subsurface remediation. Analysis of six published data sets, representing a variety of particles, porous media and fluids, indicates greater clogging at lower fluid velocity. There is a unique relationship between a clogging parameter in a modified O'Melia and Ali model and the depositing particle's Peclet number. The adopted Peclet number is the ratio of advective to Brownian particle transport within a porous medium and includes hydrodynamics, particle size, and the grain size of the porous medium. Although these data quantify the dependence of clogging on Peclet number, they do not describe steady-state clogging, achieved under conditions of constant flow, with a constant permeability and a constant mass of deposited particles. Data and models for steady-state clogging are lacking because classical filtration research focused on water treatment filters, which are backwashed periodically, and so are not allowed to reach steady state. Steady-state clogging is relevant to flow in natural subsurface environments as an important limiting case in the feedback process between particle deposition and permeability evolution, and as an initial condition for models that describe permeability alteration from mechanical or seismic forces. A model for steady-state clogging assumes deposit permeability to be negligible, then calculates Poiseuille flow in open tubes through the clogged media. The radius of these tubes is a characteristic pore size; the number of these tubes is determined by the applied flow rate and the deposit shear strength, using published estimates. With these assumptions, the model predicts that the pressure drop across the clogged sample is independent of the imposed flow rate, so steady-state permeability will be proportional to flow rate. Deposition experiments (destabilized montmorillonite on quartz sand) to test the power-law relationship between hydrodynamics and permeability and the steady-state clogging model are in progress.