Polymer Plug is a volume of polymer slurry placed in a wellbore, which, in time and under the correct temperature conditions, will develop to provide a high-viscosity platform on which a cement plug can be placed. Polymer plugs are typically used when a cement plug must be set accurately within the wellbore, The viscous material prevents the dense cement slurry from fingering through the lighter wellbore fluid during placement, helping to ensure that cement is placed over the desired interval. (Source: Schlumberger Oilfield Glossary)
