NOVELTY - The packer has two annular segments coaxially, axially and displaceably arranged on a hollow mandrel. An inner end of each segment is provided with an inner seal against the mandrel. An outer end expands radially and slides axially over an inner end of a neighboring segment when the segments are pressed axially against each other. An axially displaceable actuating piston exerts an axial force of sufficient magnitude to cause the outer end of the segment to slide over the inner end of the neighboring segment and expand radially into sealing contact with a wellbore wall.    USE - Packer for providing seal between an inner cylindrical mandrel and a wellbore wall of a hydrocarbon well e.g. oil well and gas well, in a geothermal plant.    ADVANTAGE - The packer includes a packer element with a series of independent segments for providing seal better against rough surfaces than a continuous sleeve, so that the material of an elastic sleeve on sides of the depression pulls the sleeve out from the depression when the elastic sleeve is positioned above the depression, thus preventing the elastic material from entering into the depression to seal efficiently. The packer allows a clamping ring to prevent the inner end of the segment from moving radially and squeeze the seal against the mandrel. The segments can be radially expanded against a wellbore wall by force of sufficient magnitude to prevent fluid from passing through the annulus between the mandrel and the wellbore wall. The mandrel is compact, and allows a pipe allowing fluid to flow through an inner longitudinal passage in the mandrel. The packer utilizes stiffer segments at distance from a center part to prevent extrusion of the softer segments along the longitudinal axis of a plug, so that the relatively soft segments easily conform to depressions and elevations in a wellbore to resist higher pressures and temperatures in the packer element.    DETAILED DESCRIPTION - The segments are made of a deformable material. The actuating piston is a hydraulic piston.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of an annular segment.    Clamping ring (121)    Inner frusto-conical sliding surface (124)