The invention provides a method and apparatus for use in a wellbore gravel pack operation. The method comprises providing an apparatus in a downhole annulus. The apparatus comprises a mandrel and a swellable element formed from a material selected to increase in volume when exposed to a downhole stimulus. The method comprises placing a gravel pack below the apparatus via the downhole annulus in which the apparatus is located, and placing a gravel pack above the apparatus. Subsequent to placing the gravel packs, the swellable element is increased in volume to create an annular barrier in the wellbore. The invention allows isolation of multiple intervals of a well in a single gravel pack operation using swellable elastomers, and does not rely on the use of shunt tube alternate path systems.