Method and apparatus for actuating a downhole tool

A downhole apparatus, comprising:

• a body defining a bore and a chamber,
• a first piston;
• and
• a second piston,
• wherein the first piston is movable from an open position to a closed position, the first piston in the open position permitting fluid communication between the bore and the chamber, the first piston in the closed position preventing fluid communication past the first piston, between the bore and the chamber, and
• wherein the second piston is movable at least partially in response to a pressure differential between the bore and the chamber.

A method of actuating a downhole apparatus in a wellbore, comprising:

• increasing a pressure of at least a portion of the wellbore to a first fluid pressure, wherein the first fluid pressure is communicated into a chamber of the downhole apparatus;
• reducing the pressure in the at least a portion of the wellbore from the first fluid pressure to a second fluid pressure, wherein the chamber is held substantially at the first fluid pressure, causing a pressure differential to be applied to a piston of the downhole apparatus that is in communication with the chamber, wherein the piston moves in response to the pressure differential, opening a fluid passageway;
• and
• actuating the downhole apparatus by pumping fluid through the fluid passageway after opening the fluid passageway.

A downhole apparatus, comprising:

• a body defining a bore, a first port extending radially therein, and a chamber;
• a first piston positioned in the chamber, wherein the first piston is movable from an open position to a closed position, wherein the first piston in the open position permits fluid pressure communication from the bore to the chamber via the first port, and wherein the first piston in the closed position prevents fluid pressure communication between the bore and the chamber via the first port;
• and
• a second piston positioned in the chamber, wherein the second piston is movable from a first position to a second position at least partially in response to a pressure differential between the chamber and the bore.