Apparatus and methods are described that are particularly suited for creating a seal in a borehole annulus. In one embodiment, an outer surface 10s of an expandable conduit 10 is provided with a formation 20 that includes an elastomeric material (e.g. a rubber) that can expand and/or swell when the material comes into contact with an actuating agent (e.g. water, brine, drilling fluid etc). The expandable conduit 10 is located inside a second conduit (e.g. a pre-installed casing, liner or open borehole) and radially expanded. The actuating agent can be naturally occurring in the borehole or can be injected or pumped therein to expand or swell the elastomeric material to create the seal.