Physical adsorption and mechanical entrapment are two major causes of polymer retention in porous media. Physical adsorption is believed to be an equilibrium process and is often modelled by assuming a Langmuir isotherm. The outcome is steady-state pressure response because the permeability reduction is also accounted for by adsorption. However, some experimental data show gradual increase of pressure with time implying that polymer retention is a kinetically-controlled process.
In this paper, we discuss simultaneous effect of sorption and mechanical retention on the polymer retention in porous media. An exact solution for 1-D flow problem for the case of constant filtration coefficient and Langmuir sorption isotherm, including explicit formulae for breakthrough concentration and pressure drop across the core is derived. The general model with varying filtration coefficient was successfully matched with coreflood data confirming the occurrence of simultaneous sorption with deep bed filtration.
In the absence of mechanical entrapment, the physical adsorption causes delay in the polymer front and does not affect the polymer concentration behind the front. Addition of mechanical entrapment results in very slow recovery of the injected concentration at the outlet (for varying filtration coefficient) or reaching to a steady-state concentration, which is only a fraction of the injected concentration (for a constant filtration coefficient). The results indicate that the accurate assessment of polymer retention requires both pressure and effluent concentration data at the outlet of the porous medium. In many cases, the kinetics involved in the mechanical entrapment could be very slow. In such cases, the experiments with few pore-volumes of injected polymer can be misleading and should not be directly used for field simulations.