Perforating gun for underbalanced perforating

1. A perforating tool, comprising:

a perforating gun having at least one shaped charge, the at least one shaped charge generating a high-pressure gas when detonated;

a valve sub connected to the perforating gun, the valve sub having:

an enclosure having at least one port providing fluid communication between an exterior and an interior of the valve sub,

a mandrel disposed in the enclosure, the mandrel having a piston head and a fluid path extending at least partially through the mandrel,

a sleeve slidably mounted on the mandrel, the sleeve selectively blocking fluid flow through the at least one port, and

a pressure chamber defined by an inner surface of the sleeve and an outer surface of the piston head, the pressure chamber receiving the generated high-pressure gas via the fluid path, wherein the sleeve slides toward the perforating gun after a predetermined pressure is created by the generated high-pressure gas in the pressure chamber; and

a reservoir sub coupled to the valve sub, the reservoir sub having at least one chamber in fluid communication with the interior of the valve sub.

2. The perforating tool of claim 1 , wherein the at least one port is in fluid communication with the at least one chamber after the sleeve slides toward the perforating gun to generate an underbalanced condition.

3. The perforating tool of clam 1 , wherein the piston head is formed on an end of a shaft of the mandrel, wherein the sleeve includes a first bore in which the shaft is disposed and a second bore receiving the piston head, wherein a shoulder is formed at a juncture between the passage and the bore, the shoulder being positioned between the piston head and the perforating gun.

4. The perforating gun of claim 1 , wherein the enclosure has a first end proximate to the perforating end, a second end proximate to the reservoir sub, and a bore in which the sleeve translates between a first position and a second position, wherein the sleeve completely covers the at least one port in the first position and at least partially uncovers the at least one port in the second position.

5. The perforating gun of claim 1 , wherein the enclosure has a first end proximate to the perforating end and a second end proximate to the reservoir sub, and wherein the sleeve is proximate to the reservoir sub in the first position and proximate to the perforating gun in in the second position.

6. A dynamic underbalanced sub for use with a perforating gun having at least one shaped charge that generates a high-pressure gas when detonated, the dynamic underbalance sub comprising:

a valve sub connected to the perforating gun, the valve sub having:

an enclosure having a longitudinal cavity and at least one port providing fluid communication between an exterior and the cavity,

a mandrel disposed in the cavity and fixed to the enclosure, the mandrel having a piston head and a fluid path extending at least partially through the mandrel, the fluid path being in communication with an interior of the perforating gun,

a tubular sleeve slidably mounted on the mandrel, the sleeve shifting from a first position and a second position inside the cavity, wherein the sleeve blocks fluid flow through the at least one port in the first position, and

a pressure chamber defined by an inner surface of the sleeve and an outer surface of the piston head, the pressure chamber receiving the generated high-pressure gas via the fluid path, wherein the generated high-pressure gas in the pressure chamber displaces the sleeve to a second position wherein the at least one port is at least partially uncovered; and

a reservoir sub coupled to the valve sub, the reservoir sub having at least one chamber in fluid communication with the cavity, and wherein the sleeve slides away from the reservoir sub when shifting from the first position to the second position.

7. The dynamic underbalance sub of claim 6 , wherein the piston head is formed on an end of a shaft of the mandrel, and wherein the sleeve has a first bore formed complementary to the shaft and a larger second bore in which the piston head is disposed.

8. The dynamic underbalance sub of claim 6 , further comprising a first set of seals between the enclosure and the sleeve, and a second set of seals between the sleeve and the mandrel, the first and the second set of seals isolating the perforating gun from a fluid flowing through the at least one port.

9. The dynamic underbalance sub of claim 6 , further comprising a retaining member selectively locking the sleeve to the mandrel.