SEAL UNITS prevent fluid movement at the interfacebetween the outer surface of the casing and the inside ofthe cement sheath. In spite of the improved practice ofreleasing pressure on the casing after the top plug bumps,investigations by different research and developmentgroups confirm that leakage at this interface is not onlypossible, but probable, when subjected differentialpressures often encountered in completing or producinga well. Inter-zonal communication at this point, like“channeling” in the remainder of the annulus, will preventefficient depletion of the producing zone, or causestimulation work (fracturing, acdizing, etc) to beineffective due to uncontrolled fluid migration in theannulus.