Releasing tool for pressure activated packer

A releasing tool for use in releasing a packer set in a well casing and having a normally pressure balanced annular releasing piston disposed in an annular chamber surrounding an inner mandrel body, and a pair of ports respectively connecting between the bore of the packer and said annular chamber at opposite ends of said piston, said releasing tool comprising:

• a tubular housing insertable within the bore of a packer;
• an axially shiftable tubular mandrel disposed within said tubular housing and defining in one axial position a first chamber at atmospheric pressure and a second chamber exposed by an external port to existing tubing pressure;
• shearable means for retaining said mandrel in said one position;
• spaced sealing means on said housing isolating one of said packer ports and connecting said isolated ports to said second chamber port;
• means in said axially shiftable mandrel for blocking fluid passage therethrough, whereby tubing pressure may be increased to exert sufficient force on said axially shiftable mandrel to shear said shearable means and move said mandrel to a second position wherein said first and second chambers are interconnected, thereby producing a pressure imbalance on said packer releasing piston to effect said shifting of the releasing piston to release the packer.