Method and apparatus for plugging a wellbore

A method of plugging a wellbore, comprising:

• running an apparatus into the wellbore on a tubular string, the apparatus including:
• a cement retainer disposed on the tubular string;
• and
• a radially expanded perforating assembly disposed below the cement retainer;
• setting the cement retainer to seal an annular area between the cement retainer and a casing wall therebetween;
• causing the perforating assembly to discharge and forming perforations in the casing wall;
• and
• injecting cement through the tubular string and into a formation adjacent the perforations.

An apparatus for use in plugging a wellbore, comprising:

• means for lowering a cement retainer and at least one radially expanded perforating assembly into the wellbore;
• means for setting the cement retainer against a casing;
• and
• means for discharging the perforating assembly.

A firing head for igniting a charge in a wellbore, comprising:

• a body having a central channel therethough;
• a two position firing pin disposed in the bore, the firing pin constructed and arranged to ignite the charge in a second position;
• and
• a frangible member disposed in the bore above the pin, the frangible member dividing the bore into an upper and lower portions, the upper portion having a higher pressure therein than the lower portion.

A firing head for discharging a perforating assembly, comprising:

• a body;
• a channel disposed in the body;
• a plug at least partially disposed in the channel;
• a rupture disc assembly disposed below the plug;
• a firing pin disposed below the rupture disc assembly;
• a roll pin at least partially disposed in the body and the plug;
• and
• a retention pin at least partially disposed in the firing pin.

A method for actuating a firing head for discharging a perforating assembly, comprising:

• supplying a force to break a membrane disposed above a firing pin;
• increasing a pressure above the firing pin;
• and
• causing the firing pin to strike a primer cap.