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An apparatus for selectively isolating a firing head associated with a perforating gun may include an igniter coupled to a firing head, a time delay module coupled to the igniter and generating a pressure pulse after being activated by the igniter, a metering module, and a second firing head. The me

An apparatus for selectively isolating a firing head associated with a perforating gun may include an igniter coupled to a firing head, a time delay module coupled to the igniter and generating a pressure pulse after being activated by the igniter, a metering module, and a second firing head. The metering module may be coupled to the time delay module and including a housing having a bore and at least one opening exposed to a wellbore annulus. A piston disposed in the housing bore may have at least one passage. The piston is axially displaced from a first position to a second position by the generated pressure pulse. The second firing head is coupled to the metering module and is in fluid communication with the housing bore. The piston blocks fluid communication from the at least one opening of the housing and the second firing head in a first position and allows fluid communication from the at least one opening of the housing to the second firing head in the second position.

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1. An apparatus for selectively isolating a firing head associated with a perforating gun, comprising: a first firing head;an igniter coupled to the first firing head;a time delay module coupled to the igniter, the time delay module generating a pressure pulse after being activated by the igniter;a

1. An apparatus for selectively isolating a firing head associated with a perforating gun, comprising: a first firing head;an igniter coupled to the first firing head;a time delay module coupled to the igniter, the time delay module generating a pressure pulse after being activated by the igniter;a metering module coupled to the time delay module, the metering module including: a housing having a bore and at least one opening exposed to a wellbore annulus, anda piston disposed in the housing bore, the piston having at least one passage, the piston being axially displaced from a first position to a second position by the generated pressure pulse, the piston sealing the at least one opening of the housing in the first position and allowing fluid communication through the at least one opening of the housing to the bore in the second position; anda second firing head coupled to the metering module, the second firing head being in fluid communication with the housing bore and in pressure communication with the wellbore annulus when the piston is in the second position. 2. The apparatus of claim 1, wherein the time delay module has a deflagration component and a high-order detonation component, the time delay module generating the pressure pulse using at least the high-order detonation component. 3. The apparatus of claim 1, wherein the first firing head includes a first pin piston and the second firing head includes a second pin piston, and wherein the first pin piston and the second pin piston are propelled using fluid pressure. 4. The apparatus of claim 3, wherein the first pin piston is in fluid communication with a fluid in the wellbore annulus while the second pin piston is isolated from a pressure in the wellbore annulus. 5. The apparatus of claim 1, wherein the piston includes at least one frangible element connecting the piston to the housing and at least one seal forming a fluid barrier between the piston and an inner surface of the housing. 6. The apparatus of claim 1, wherein the housing bore is at substantially atmospheric pressure when the piston is in the first position. 7. The apparatus of claim 1, wherein the first firing head and the second firing head are pressure activated. 8. The apparatus of claim 7, wherein the first firing head and the second firing head are activated by substantially the same pressure. 9. The apparatus of claim 7, wherein the first firing head and the second firing head are activated by substantially different pressures. 10. The apparatus of claim 1, wherein the housing includes a barrel section, and wherein the barrel section has an internal volume selected to prevent fluid pressure in the housing bore to decrease less than ten percent when the second firing head is activated. 11. The apparatus of claim 1, wherein the housing includes a vent portion, the vent portion includes at least one vent exposed to the wellbore annulus and a vent piston, the vent piston sealing the at least one vent in a first position and unsealing the at least one vent in a second position, wherein a fluid pressure in the housing bore displaces the vent piston from the first position to the second position. 12. An apparatus for selectively isolating a firing head associated with a perforating gun, comprising: a first pressure activated firing head;an igniter coupled to the first firing head, the igniter generating a shock wave when detonated by the first pressure activated firing head;a time delay module coupled to the igniter, the time delay module having a deflagration component and a high-order detonation component, the time delay module generating a pressure pulse using at least the high-order detonation component after being activated by the igniter;a metering module coupled to the time delay module, the metering module including: a housing having a bore, at least one opening exposed to a wellbore annulus,a piston disposed in the housing bore, the piston having at least one passage, the piston being axially displaced from a first position to a second position by the generated pressure pulse, the piston sealing the at least one opening of the housing in the first position and allowing fluid communication through the at least one opening of the housing in the second position; anda second pressure-activated firing head coupled to the metering module, the second firing head being in fluid communication with the housing bore and in pressure communication with the wellbore annulus when the piston is in the second position, the second pressure-activated firing head being activated by a pressure increase in the bore. 13. The apparatus of claim 12, wherein the first pressure-activated firing head, the time delay module, the metering module, and the second pressure-activated firing head are each formed as cylindrical bodies, wherein the first pressure-activated firing head is connected to one end of the time delay module and the metering module is connected to an opposing end of the time delay module, and second pressure activated firing head is connected to the metering module. 14. A method for selectively isolating a firing head associated with a perforating gun, comprising: forming a perforating tool by: coupling an igniter to a first firing head;coupling a time delay module to the igniter;coupling a metering module to the time delay module, the metering module including: a housing having a bore and at least one opening, anda piston disposed in the housing bore, the piston having at least one passage and;coupling a second firing head to the metering module, the second firing head being in fluid communication with the housing bore and only in pressure communication with a wellbore annulus when the piston is in the second position;conveying the perforating tool into a wellbore;activating the igniter using the first firing head;activating the time delay module using a shock wave generated by the activated igniter;generating a pressure pulse using the activated time delay module;using the generated pressure pulse to axially displace the piston from a first position to a second position by the generated pressure pulse, the piston sealing the at least one opening of the housing in the first position and allowing fluid communication through the at least one opening of the housing to the bore in the second position; andincreasing a pressure in a wellbore annulus after the bore of the metering module is filled with a fluid. 15. The method of claim 14, wherein the first firing head is in pressure communication with the wellbore annulus while the perforating tool is being conveyed in the wellbore and the second firing head is hydraulically isolated from the wellbore annulus while the perforating tool is being conveyed in the wellbore.