An apparatus and methods for preventing the accumulation of unwanted materials in an enlarged inner diameter portion of a casing or housing. In one aspect of the invention, a sleeve is disposed in the housing to isolate an annular area defined by the outer surface sleeve and the wall of the enlarged inner diameter portion. The sleeve prevents unwanted materials from being disposed in the annular area. The sleeve can later be expanded into the enlarged inner diameter portion, removed from the wellbore or destroyed. In another aspect of the invention, the sleeve is provided and disposed to cover the enlarged inner diameter portion. By covering the enlarged inner diameter portion, unwanted material is prevented from accumulating at said portion and from interfering with the expansion of the next casing into said portion to form a monobore. The sleeve can be made from materials that are dissolvable, elastically deformable, or retrievable.