a plurality of packing elements are mounted in vertically spaced relationship on a tubing string with the spacing of the elements corresponding generally to the spacing of a plurality of sets of perforations in a well conduit . the lowermost packing unit is provided with radially expanding locking elements which engage a locking groove provided in the well conduit . all packing units incorporate expandable elastomeric sealing members and are set by the application of tension to the tubing string and are unset by the subsequent application of a higher degree of tension to the tubing string .