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A downhole valve is disclosed for use in a tubing-conveyed string of tools in a well bore, constructed to be actuated to open under pressure forces by dropping a drop bar from the well surface through the tubing. The valve, deployed in the well in a closed condition, has a frangible member arranged

A downhole valve is disclosed for use in a tubing-conveyed string of tools in a well bore, constructed to be actuated to open under pressure forces by dropping a drop bar from the well surface through the tubing. The valve, deployed in the well in a closed condition, has a frangible member arranged to be broken by the drop bar to enable the valve to open. The valve opens under well pressure forces when well pressure exceeds tubing pressure, and opens under tubing pressure forces when tubing pressure exceeds or substantially equals well pressure. The valve is particularly useful when opened in conjunction with the detonation of a perforating gun for overbalanced or underbalanced well perforation. Methods of operating a string of tools including the valve are also disclosed.

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[ What is claimed is:] [1.] A downhole valve for use in a string of tools conveyed on tubing under internal tubing pressure in a well bore under well pressure, the valve comprisinga housing defining a port for fluid communication between said tubing and the well bore,a movable member arranged to blo

[ What is claimed is:] [1.] A downhole valve for use in a string of tools conveyed on tubing under internal tubing pressure in a well bore under well pressure, the valve comprisinga housing defining a port for fluid communication between said tubing and the well bore,a movable member arranged to block said port in a first, closed position and to move, upon actuation of the valve, to a second, open position to expose said port, anda floating piston arranged to move within an annular piston chamber defined about said movable member between said housing and said movable member, and to bear against said movable member while well pressure is greater than tubing pressure,said movable member constructed to be moved to said second position by tubing pressure, in response to the actuation of the valve, while tubing pressure is greater than well pressure and while tubing pressure is equal to well pressure,said movable member further constructed to be moved to said second position, while tubing pressure is less than well pressure and in response to the actuation of the valve, by force exerted by said floating piston.