An expandable packer tool comprising a mandrel tool body; a sleeve-like packer element mounted on the tool body; and a setting mechanism that is operable to axially compress the packer element such that the diameter of the packer element in its compressed state is increased compared to it uncompressed state. An extrusion barrier can comprise a ring of seal elements disposed around the tool body on either side of the packer element, and arranged to engage the packer element so that expansion of the packer element causes the ring of seal elements to expand in a corresponding manner so as to form an extrusion barrier; wherein the ring of seal elements comprises wedge-shaped seal members comprising first seal members having a inner base wider than an outer surface, and second seal members having an inner base narrower than an outer surface, the ring comprising alternating first and second members, the inclined faces of each wedge-shaped member engaging the corresponding faces of adjacent members. A retraction mechanism for the packer element comprises a cut-out in its inner surface near at least one end; and the setting mechanism comprises an inner mandrel that has an outward projection that engages in the cut-out so that the inner mandrel can exert a pulling force on the packer element.