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The present invention generally provides a method and apparatus for selectively sealing a bore. The tubular valve generally includes a closing member for seating in and closing the bore, and a pressure-actuated, retention member having first and second opposed piston surfaces opening and closing the

The present invention generally provides a method and apparatus for selectively sealing a bore. The tubular valve generally includes a closing member for seating in and closing the bore, and a pressure-actuated, retention member having first and second opposed piston surfaces opening and closing the valve. The tubular valve prevents sudden loss of pressure in the tubular and is controllable from the surface.

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The invention claimed is: 1. A method of operating a downhole valve comprising: providing the valve in a downhole tubular, the valve having: a closing member for sealing a bore; a retention member having a first and second piston surface substantially isolated from each other, the retention member

The invention claimed is: 1. A method of operating a downhole valve comprising: providing the valve in a downhole tubular, the valve having: a closing member for sealing a bore; a retention member having a first and second piston surface substantially isolated from each other, the retention member mechanically biased to interfere with the closing member to keep the valve open; a pressure chamber in communication with the second piston surface; and a control line in communication with the pressure chamber; applying a wellbore pressure to the first piston surface; increasing the pressure in the pressure chamber to a level sufficient to overcome the mechanical bias of the retention member, but insufficient to overcome both the pressure on the first piston surface and the mechanical bias, wherein the pressure in the pressure chamber is increased to the wellbore pressure through a metered flow orifice. 2. The method of claim 1, further comprising closing the valve upon a sudden loss of wellbore pressure. 3. The method of claim 2, further comprising equalizing the pressure in the pressure chamber with the wellbore pressure. 4. The method of claim 3, further comprising opening the valve. 5. The method of claim 1, wherein the pressure in the pressure chamber is increased by the control line. 6. The method of claim 1, further comprising decreasing the pressure in the pressure chamber through the metered flow orifice as the wellbore pressure gradually decreases. 7. The method of claim 6, further comprising closing the valve upon a sudden loss of wellbore pressure. 8. The method of claim 1, wherein the pressure in the pressure chamber is also increased to the wellbore pressure through a one way flow path. 9. A safety valve for use in a wellbore, comprising: a tubular housing having: a wall, a longitudinal bore defined by the interior of the wall, and a port formed through the wall for receiving a control line; a closing member operably coupled to the housing between: an open position where the closing member permits flow through the bore, and a closed position where the closing member seals the bore; and a piston: disposed in the housing, having a first portion in fluid communication with the port and in limited fluid communication with the bore, having a second portion substantially isolated from the first portion and in fluid communication with the bore, operable to allow closure of the closing member in response to fluid pressure exerted on the first portion exceeding pressure exerted on the second portion, and operable to open the closing member in response to pressure exerted on the second portion exceeding fluid pressure exerted on the first portion. 10. The safety valve of claim 9, further comprising a biasing member disposed in the housing and biasing the closing member toward the open position. 11. The safety valve of claim 9, wherein the limited fluid communication is unidirectional. 12. The safety valve of claim 9, further comprising a seal disposed between the portions. 13. The safety valve of claim 9, further comprising a bellows disposed between the portions. 14. The safety valve of claim 9, further comprising a unidirectional packing disposed between the portions. 15. The safety valve of claim 9, wherein the piston comprises a flow tube or the safety valve further comprises a flow tube attached to the piston. 16. The safety valve of claim 15, wherein: a recess is formed in the housing wall, and a seal is disposed between the housing and the flow tube. 17. The safety valve of claim 16, wherein: the flow tube has a shoulder extending into the recess, the piston portions are surfaces of the shoulder, and the shoulder has a seal disposed therein. 18. The safety valve of claim 17, wherein the flow tube has an orifice disposed through a wall thereof. 19. The safety valve of claim 17, further comprising a spring disposed in a pressure chamber between the flow tube and the housing, the spring biasing the closing member toward the open position. 20. The safety valve of claim 16, wherein the seal is a unidirectional packing. 21. The safety valve of claim 16, wherein the piston comprises a bellows attached to the flow tube and the housing. 22. The safety valve of claim 21, further comprising a cap disposed in the recess and attached to the housing, wherein: the bellows is attached to the cap, the seal is disposed between the cap and the housing, and an orifice is formed through the cap. 23. The safety valve of claim 15, wherein: the closing member is a flapper, the flow tube is operable to engage the flapper and push the flapper to the open position, the flow tube allows the flapper to move to the closed position by moving away from the flapper, the safety valve further comprises a biasing member biasing the flow tube toward the flapper, and the piston is operable to move the flow tube away from the flapper. 24. The safety valve of claim 9, wherein the piston comprises a rod and the safety valve further comprises a flow tube operably coupled to the piston. 25. The safety valve of claim 24, wherein: the housing has a pressure chamber formed therein in fluid communication with the port, the piston is disposed in the housing wall, and a seal is disposed between the piston and the housing. 26. The safety valve of claim 25, wherein the housing has an orifice formed therein. 27. The safety valve of claim 25, wherein the seal is a unidirectional packing. 28. The safety valve of claim 25, wherein: the closing member is a flapper, the flow tube is operable to engage the flapper and push the flapper to the open position, the flow tube allows the flapper to move to the closed position by moving away from the flapper, the safety valve further comprises a biasing member biasing the flow tube toward the flapper, and the piston is operable to move the flow tube away from the flapper. 29. The safety valve of claim 25, wherein: the housing comprises a first section, a second section, and a third section, the sections are connected by threaded connections, the piston is disposed in the second section, the pressure chamber is formed between the second and third sections, and the seal is disposed between the piston and the second section. 30. The safety valve of claim 9, wherein closing member is a flapper and the safety valve further comprises a spring biasing the flapper toward the closed position. 31. A safety valve for use in a wellbore, comprising: a tubular housing having: a wall, a longitudinal bore defined by the interior of the wall, and a port formed through the wall for receiving a control line; a closing member operably coupled to the housing between: an open position where the closing member permits flow through the bore, and a closed position where the closing member seals the bore; and a piston: disposed in the housing, having a first portion in fluid communication with the port and in limited fluid communication with the bore, having a second portion isolated from the first portion and in fluid communication with the bore, and operable to close the closing member in response to sufficient fluid pressure exerted on the first portion. 32. The safety valve of claim 31, wherein the limited fluid communication is unidirectional. 33. A safety valve for use in a wellbore, comprising: a tubular housing having: a wall, a longitudinal bore defined by the interior of the wall, and a port formed through the wall for receiving a control line; a closing member operably coupled to the housing between: an open position where the closing member permits flow through the bore, and a closed position where the closing member seals the bore; and a piston: disposed in the housing, having a first portion in fluid communication with the port, having a second portion isolated from the first portion and in fluid communication with the bore, and operable to close the closing member in response to sufficient fluid pressure exerted on the first portion, wherein: the piston comprises a flow tube or the safety valve further comprises a flow tube attached to the piston, a recess is formed in the housing wall, a seal is disposed between the housing and the flow tube and isolates a pressure chamber of the recess from the bore the flow tube has a shoulder extending into the recess, the piston portions are surfaces of the shoulder, the shoulder has a seal disposed therein, and the flow tube has an orifice disposed through a wall thereof providing limited fluid communication between the pressure chamber and the bore. 34. The safety valve of claim 33, further comprising a spring disposed in the pressure chamber between the flow tube and the housing, the spring biasing the closing member toward the open position. 35. A safety valve for use in a wellbore, comprising: a tubular housing having: a wall, a longitudinal bore defined by the interior of the wall, and a port formed through the wall for receiving a control line; a closing member operably coupled to the housing between: an open position where the closing member permits flow through the bore, and a closed position where the closing member seals the bore; and a piston: disposed in the housing, having a first portion in fluid communication with the port, having a second portion isolated from the first portion and in fluid communication with the bore, and operable to close the closing member in response to sufficient fluid pressure exerted on the first portion, wherein: the piston comprises a flow tube or the safety valve further comprises a flow tube attached to the piston a recess is formed in the housing wall, a seal is disposed between the housing and the flow tube and isolates a pressure chamber of the recess from the bore, the piston comprises a bellows attached to the flow tube and the housing and further isolating the pressure chamber from the bore, the safety valve further comprises a cap disposed in the recess and attached to the housing, the bellows is attached to the cap, the seal is disposed between the cap and the housing, and an orifice is formed through the cap and in communication with the pressure chamber and the bore. 36. A method for producing hydrocarbons from a wellbore, comprising: flowing hydrocarbons through a bore of a production tubing string disposed in the wellbore, the production tubing string comprising: a closing member, a chamber in fluid communication with a control line extending to the surface, and a piston: operably coupled to the closing member, having a first portion in fluid communication with the chamber, and having a second portion at least substantially isolated from the first portion and in fluid communication with the bore; and maintaining a pressure in the chamber substantially equal to a steady state pressure exerted on the second portion by the flowing hydrocarbons, thereby keeping the closing member in an open position and storing sufficient fluid energy in the chamber to automatically close the closing member in response to a surge in the flow of hydrocarbons through the bore. 37. The method of claim 36, wherein the production tubing string further comprises a biasing member biasing the closing member toward the open position. 38. The method of claim 36, wherein the chamber pressure is automatically maintained by limited fluid communication between the chamber and the bore. 39. The method of claim 36, wherein the chamber pressure is maintained by controlling pressure in the control line. 40. The method of claim 36, wherein the closing member is a flapper and the piston is a shoulder of a flow tube. 41. The method of claim 40, wherein the production tubing string further comprises a spring biasing the flow tube into engagement with the flapper. 42. The method of claim 36, wherein the piston comprises a bellows. 43. The method of claim 42, wherein: the closing member is a flapper, the production tubing string further comprises a flow tube coupled to the bellows. 44. The method of claim 43, wherein the production tubing string further comprises a spring biasing the flow tube into engagement with the flapper. 45. The method of claim 36, wherein: the closing member is a flapper, the production tubing string further comprises a flow tube, and the piston is a rod operably coupled to the flow tube. 46. The method of claim 45, wherein the production tubing string further comprises a spring biasing the flow tube into engagement with the flapper. 47. A safety valve for use in a wellbore, comprising: a tubular housing having: a wall, a longitudinal bore defined by the interior of the wall, and a port formed through the wall for receiving a control line; a closing member operably coupled to the housing between: an open position where the closing member permits flow through the bore, and a closed position where the closing member seals the bore; and a piston: disposed in the housing, having a first portion in fluid communication with the port, having a second portion in fluid communication with the bore, and operable to allow closure of the closing member in response to fluid pressure exerted on the first portion exceeding pressure exerted on the second portion, operable to open the closing member in response to pressure exerted on the second portion exceeding fluid pressure exerted on the first portion; and a unidirectional packing disposed between the portions. 48. A safety valve for use in a wellbore, comprising: a tubular housing having: a wall, a longitudinal bore defined by the interior of the wall, and a port formed through the wall for receiving a control line; a closing member operably coupled to the housing between: an open position where the closing member permits flow through the bore, and a closed position where the closing member seals the bore; a pressure chamber in fluid communication with the port and operable to store sufficient fluid energy to automatically close the closing member in response to a surge in flow through the bore; and a piston: disposed in the housing, operatively coupled to the closing member, and having a first portion in fluid communication with the pressure chamber and a second portion at least substantially isolated from the first portion and in communication with the bore, wherein the pressure chamber is in limited fluid communication with the bore. 49. The safety valve of claim 48, wherein the piston is operable to open the closing member in response to pressure exerted on the second portion exceeding fluid pressure exerted on the first portion. 50. The safety valve of claim 49, wherein the piston is further operable to allow closure of the closing member in response to fluid pressure exerted on the first portion exceeding pressure exerted on the second portion. 51. The safety valve of claim 48, further comprising a spring biasing the closing member toward the open position.