Low polymer retention in the reservoir is essential to the success of a polymer EOR operation. It is usually estimated from laboratory core flood experiments conducted under conditions as close as possible to those prevailing in the field. This paper addresses the effects of core wettability and oil saturation on polymer adsorption/retention.
Adsorption of EOR water-soluble polymers onto oil or, more generally, hydrophobic substrates was estimated from a series of core flood experiments on alkane-coated (silane-treated) substrates and compared to adsorption on the bare siliceous substrates. A significant adsorption onto hydrophobic surfaces was found, the effect being stronger for nonionic Polyacrylamide and polysaccharides (scleroglucan and xanthan) than for anionic (hydrolyzed) Polyacrylamides.
In a second series of core flood experiments, Polyacrylamide adsorption/retention in cores under residual oil saturation was measured and compared to the adsorption/retention in similar oil-free, 100% brine saturated, cores. In oil-wet cores, retention decreased considerably in the presence of oil, while it increased slightly in water-wet systems.
These effects may be explained in terms of water/rock and water/oil interfacial area distribution within the pore space, which is governed by wettability. In water-wet cores, droplets of residual oil offer an additional adsorbing surface to the polymer, whereas in oil-wet cores the oil wetting film offers the polymer an adsorbing surface much smaller than the (no longer accessible) rock surface.