Low shear injection of different-nature polymers through various sand and sandstone cores has been performed to study the specific influence of adsorption on polymer propagation and flow behavior.
As predicted by the deletion layer theory, polymer propagates faster than brine through non adsorbing porous media giving a mobility ratio less than solution viscosity ratio. An increase in clay content both decreases permeability and increases polymer adsorption. Thus a polymer propagates through pores having a smaller cross-section at a lower rate. When pore sizes become close to macromolecule size, polymer adsorbs, forming bridges in pore throats, thus giving very poor and irregular propagation rates. A dynamic method for adsorption measurements on heterogeneous rocks is proposed. Our experimental results effectively fit a capillary flow model with a crossflow section reduced by an impenetrable adsorbed layer.
High-molecular-weight water soluble polymers are used in many oil field applications ranging from drilling to chemical flooding. Their high viscosifying power is primarily due to their large macromolecular size which in turn is often in the same range as pore size. As an example, an usual EOR grade xanthan gum molecule has a rod length of approximately 1 40m and currently is injected 2 into reservoir rocks with permeability k = 0.1 40m and porosity phi = 0.20. By applying the capillary bundle model the average pore throat diameter can be taker. as 2 (8 k/phi) 2.0 40m. If we consider that pore distribution can be pretty large in natural sansdstones, a significant number of pores will thus be smaller than the polymer molecules.
Yet, at high shear rates, polymer molecules in solution tend to orientate in the direction of flow and become deformed. This phenomenon, responsible for their shear thinning behavior (see, for example, the rheological curve in Fig.2), allows the macromolecules to pass through pores of smaller size than their own size. Nevertheless. as the shear rate decreases, hydrodynamic forces become weaker and the molecules less orientated in the direction of flow. When polymer molecules are subjected only to Brownian motion, the solutions viscosity finally reach a constant value (Newtonian regime). A xanthan gum molecule then behaves like a sphere 1 40m in diameter (its rod length) and obviously cannot easily pass through pores of smaller size.
When a polymer solution is injected into a formation a radial flow takes place. hear rate y is high (several hundred of s) just around the well bore but decreases proportionally to R (R being the invasion radius). At some distance from the well bore, hydrodynamic force become too weak to force the polymer molecules to pass through small pores. Propagation of the polymer thus becomes a problem, at least in low permeability reservoirs.
Propagation of polymers is also hindered by the tendency of polymer molecules to adsorb onto the rock. Adsorption of high molecular weight macromolecules is known to be largely irreversible. Significant desorption has been observed only at shear rates exceeding 2000s-1. At low shear rates, although permanent exchanges take place between adsorbed and free polymer molecules, the net quantity of adsorbed polymer does not vary over long periods.