Compressible liquid mechanism for downhole tool

An annulus pressure responsive downhole tool apparatus, comprising:

• a housing:
• an operating assembly including an operating element disposed in said housing, and an operating mandrel slidably disposed in said housing and operably associated with said operating element;
• a first power piston slidably disposed in said housing, and operably associated with said operating mandrel and said operating element for moving said operating element relative to said housing by moving said operating mandrel in said first direction relative to said housing, said first power piston having first and second sides;
• first and second pressure conducting passage means defined within said housing for communicating a well annulus exterior of said housing with said first and second sides, respectively, of said first power piston;
• retarding means, disposed in said second pressure conducting passage means, for delaying communication of a sufficient portion of a change in well annulus pressure to said second side of said first power piston for a sufficient time to allow a pressure differential across said first power piston to move said first power piston in said first direction between a first and a second position relative to said housing and thereby to move said operating mandrel in said first direction;
• a second power piston connected to said operating mandrel for movement therewith in said first direction, said second power piston including a high pressure side and a lower pressure side;
• port means for communicating said high pressure side of said second power piston with said well annulus to create a pressure differential across said second power piston in said first direction;
• and
• isolation seal means for initially isolating said high pressure side of said second power piston from said port means when said first power piston is in its said first position.

A method of operating an annulus pressure responsive downhole tool, comprising:

• (a) increasing well annulus pressure;
• (b) communicating said increased well annulus pressure to a first side of a power piston of said tool while delaying communication of said increase through a compressible liquid to a second side of said power piston for a sufficient time to allow a pressure differential across said power piston to move said power piston between a first position and a second position within said tool and to thereby move an operating element from an initial position to an intermediate position within said tool;
• (c) compressing said compressible liquid by a volume substantially equal to a displacement of said power piston as said power piston moves from its first to its second position and as said operating element correspondingly moves from its initial position to its intermediate position;
• and
• (d) moving said operating element from its intermediate position to a final position by means other than said power piston without further compressing said compressible liquid as a result of said moving said operating element from its said intermediate position to its said final position.