the type known in the art as a mechanically actuated retrievable packer in other words through mechanical manipulation of the work string 2 the lower packer 6 may be expanded into sealing engagement with the interior bore 1a of casing 1 at any selected point further manipulation of the work string 2 will result in the collapsing of the lower packer 6 to permit it to be moved to another position the packer 6 may for example comprise the baker model r 3 single grip retrievable casing packer thus the first step involved in the process after the run in of the combined perforating and gravel setting apparatus 10 into the well casing is to effect the setting of the lower packer 6 by manipulation of the work string 2 this results in the expansion of gripping teeth 6a and annular seals 6b conventionally provided on the packer into engagement with the casing bore 1a fig2 as previously mentioned the top end of the lower packer 6 is conventionally secured as by threads to the bottom end of an elongated liner assembly 20 the liner assembly 20 is constructed in the same general manner as the liner assembly employed in the gravel packing apparatus described in the aforementioned u s pat no 3 987 854 the construction of the liner assembly 20 will not therefore be described in great detail but the principal elements thereof starting at the bottom of the liner assembly fig1 b and moving upwardly include the following items first is an o ring seal sub 21 providing a mounting for an o ring seal 21a which cooperates in sealing relationship with the lower tubular portion 30b of the crossover mandrel 30 next the top end of the o ring seal sub.