Polymers in the form of either solutions or gels are being used to control water production, especially when oil bearing and water zones cannot be isolated. Results of field treatments have varied widely, but often no obvious reason can be given for the success or the failure of the treatment. The lack of understanding of the basic mechanisms by which polymers influence the flow of water and oil hinders a wider application of this process. The achievement of a disproportionate permeability reduction (DPR - reducing the water permeability while producing minimum reduction in the oil permeability) is crucial, and several mechanisms have been proposed to explain this effect. Among those mechanisms, polymer adsorption and lubrication effect are being thought as the main reason for DPR when polymers (without cross-linker) are used.

This paper presents a mechanistic study of the effect of polymer injection on single and two-phase flow. Experiments have been performed in glass micromodels, under both water-wet and oil-wet conditions. The oil and water end-point permeabilities have been obtained, before and after the injection of a cationic polyacrylamide, while observing the fluid distribution and flow patterns. During polymer injection, under water-wet conditions, polymer layers were seen to form and build-up on the crevices between the grains. This mechanism is known as adsorption-entanglement. Data on this phenomenon are scarce, and this paper presents the first visualization study of this phenomenon in porous media.

In water-wet models there was a significant permeability reduction to water, while there was no significant reduction in oil permeability. The entanglement of polymer reduces the area available for water to flow, whereas the oil, occupying the centre of the larger pores, was nearly unaffected by the polymer entanglement. In oil-wet models, where polymer layers were not observed, the oil and water effective permeability before and after polymer injection remained unchanged. Our results confirm that adsorption-entanglement for water-wet media, which produces the build-up of the polymer layers on the grain crevices, is a basic mechanism by which polymers can modify the flow characteristics and thus preferentially reduce the water relative permeability.

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