membrane 66 forcing it into engagement with the inner wall of the casing string 64 in turn causing it to form a hermetic seal between the inner wall of the casing string 64 and the packer 68 fluid is continuously pumped downhole during this step in the process thereby increasing the pressure to the point that tension sleeve 70 connecting the setting tool 10 to the packer 68 shears in tension the pressure required to shear the tension sleeve 70 from the packer 68 is higher than that required to shear the pins 46 this position the u201c shear off u201d packer position is shown in fig2 a 2c once the packer 68 has been set the next step is to open the valve within the packer 68 so that the treatment fluids can be pumped down into the workstring into the formation this is accomplished by bleeding off the pressure of the fluid being pumped down through the workstring into the setting tool 10 by bleeding off the pressure the spring 16 which in the shear off packer position is in compression forces the inner mandrel 14 to move downward which in turn pushes against a sliding valve 72 formed within the packer 68 this action forces the sliding valve 72 downward which in turn aligns ports 76 within the sliding valve 72 with a port 78 formed within packer mandrel 74 which in turn communicates with a subterranean formation the setting tool 10 is locked into this position by activation of the locking device 50 it is this position that the locking keys 52 recess into the groove 56 retaining spring 54 forces the locking keys 52 into engagement with the groove 56 which axially fixes the inner mandrel 14 to the outer sleeve 12.