string a short distance above the packer at the time that the packer is set the tubing and casing both will be full of weighted fluid such as mud in order to maintain the producing formation under control after the packer is set the pressure of the mud in the tubing casing annulus is increased to test the packer then the annulus pressure is further increased to open the sliding sleeve device by applying a downward pressure to the port closure sleeve 63 to shear the screws 70 so that the port closure sleeve 63 can be moved to its fully open position by the spring 78 this opens the lateral ports 22 of the well tool so that a lighter medium such as water or oil may be used to displace the mud from the tubing by pumping it down the tubing and forcing the mud outwardly through the lateral ports 22 into the annulus where it rises to the surface in many cases it is desirable to close the sliding sleeve valve 24 as soon as the mud is displaced from the tubing so that the mud will remain in the tubing casing annulus and the water will remain in the tubing string now if the producing formation is of abnormal bottom hole pressure it is only then necessary to.