Bidirectional downhole isolation valve

1. An isolation valve for use in a wellbore, comprising:

a housing;

a piston longitudinally movable relative to the housing;

a flapper movable between an open position and a closed position, the flapper operable to isolate an upper portion of a bore of the valve from a lower portion of the bore in the closed position; and

a pressure bypass operable to bypass the closed flapper when a predetermined pressure is reached, the pressure bypass comprising a port formed through a wall of the piston.

2. The isolation valve of claim 1 , wherein the pressure bypass provides a flowpath around the closed flapper.

3. The isolation valve of claim 1 , wherein the predetermined pressure is a design pressure of the flapper.

4. The isolation valve of claim 3 , wherein the design pressure of the flapper is selected from one of a yield strength and a fracture strength of the flapper.

5. The isolation valve of claim 1 , wherein the port provides fluid communication between the upper portion of the bore of the valve and the lower portion of the bore in an open position.

6. The isolation valve of claim 1 , the pressure bypass further comprising a rupture disk disposed in the port and operable to rupture in response to reaching the predetermined pressure.

7. The isolation valve of claim 6 , wherein the rupture disk is one of a metal, polymer, or composite material.

8. The isolation valve of claim 1 , further comprising an abutment configured to receive the flapper in the closed position, thereby retaining the flapper in the closed position.

9. The isolation valve of claim 8 , further comprising a relief notch formed in the abutment.

10. The isolation valve of claim 8 , wherein the abutment is a shoulder of a lock sleeve operable to engage the housing.

11. The isolation valve of claim 8 , wherein the abutment has a port formed therethrough to prevent sealing between the flapper and the abutment.

12. The isolation valve of claim 1 , wherein the pressure bypass is formed through a wall of the piston above the flapper.

13. The isolation valve of claim 1 , wherein the pressure bypass is operable to relieve a pressure differential.

14. A method of relieving pressure in an isolation valve, comprising:

deploying the isolation valve in a string in a wellbore;

closing a flapper of the isolation valve to isolate a bore of the valve; and

opening a pressure bypass to bypass the closed flapper in response to reaching a predetermined pressure, wherein the pressure bypass comprises a port formed through a wall of a piston.

15. The method of claim 14 , wherein opening the pressure bypass comprises providing a flow path around the closed flapper.

16. The method of claim 14 , wherein the predetermined pressure is a design pressure of the flapper.

17. The method of claim 14 , further comprising moving the piston towards an abutment of the isolation valve to close the flapper.

18. The method of claim 14 , wherein opening the pressure bypass comprises rupturing a rupture disk disposed in the port of the pressure bypass.

19. An isolation valve for use in a wellbore, comprising:

a housing;

a flapper movable between an open position and a closed position, the flapper operable to isolate an upper portion of a bore of the valve from a lower portion of the bore in the closed position;

a pressure bypass operable to bypass the closed flapper when a predetermined pressure is reached; and

an abutment configured to receive the flapper in the closed position, thereby retaining the flapper in the closed position, wherein the abutment is a shoulder of a lock sleeve operable to engage the housing.

20. The isolation valve of claim 19 , wherein the pressure bypass comprises a relief notch formed in the abutment.

21. The isolation valve of claim 19 , wherein the abutment has a port formed therethrough to prevent sealing between the flapper and the abutment.