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A method and apparatus for effecting the perforation of a well casing and the adjoining production formation comprises a tubular housing communicating between a perforating gun housing and the lower end of a tubular string. The perforating gun housing which is adapted to be carriable by the tubular

A method and apparatus for effecting the perforation of a well casing and the adjoining production formation comprises a tubular housing communicating between a perforating gun housing and the lower end of a tubular string. The perforating gun housing which is adapted to be carriable by the tubular string contains an impact actuated firing head and the tubular housing contains a port normally closed by sleeve valve. The valve is biased towards an open position but is selectively secured in its closed position by a frangible restraining mechanism which extends into the free fall path of a detonating bar dropped through the tubular tool string and is removed by the detonating bar to initiate the opening of the port sleeve valve prior to impacting the firing head. In a modification, the sleeve valve is shifted to its open position by fluid pressure produced in the casing annulus subsequent to the firing of the perforating gun.

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1. Well perforating apparatus comprising a tubular housing having means on its upper end for connecting to a tubular tool string; means on the lower end of said tubular housing for communicating to a perforating gun housing containing an impact actuated firing head; port means in said tubular housin

1. Well perforating apparatus comprising a tubular housing having means on its upper end for connecting to a tubular tool string; means on the lower end of said tubular housing for communicating to a perforating gun housing containing an impact actuated firing head; port means in said tubular housing; shiftable valve means for opening and closing said port means; means for biasing said shiftable valve means to a port opening position; means for securing said shiftable valve means in a port closing position; and restraining means resisting movement of said securing means from said port closing position; said restraining means being disposed in the free fall path of a detonating means transmitted through the tubular tool string and removed thereby, whereby the detonating bar initiates the opening of said radial port means prior to impacting said firing head. 2. The apparatus defined in claim 1 wherein said shiftable valve means comprises a sleeve axially shiftable on said tubular housing and said means for biasing said shiftable valve means comprises a spring element surrounding said tubular housing and abutting said sleeve. 3. The apparatus of claim 1 wherein said securing means comprises a collet coaxially disposed relative to said tubular housing and having spring arms resiliently expandable into latching engagement with the lower end of said tubular housing; and said restraining means comprises a frangible barrier disposed in the path of the detonating bar and holding said collet arms in a radially expanded position of engagement with said lower end of said tubular housing to comprise a lower portion of said shiftable valve means. 4. The apparatus defined in claim 3 wherein said shiftable valve means comprises a sleeve axially shiftable on said tubular housing and said means for biasing said shiftable valve means comprises a spring surrounding said tubular housing and abutting said sleeve. 5. The apparatus of claim 3 wherein said frangible barrier comprises a disc element comprising tempered glass. 6. Well perforating apparatus comprising a tubular housing having means on its upper end for communicable extension to a tool string and means on its lower end for communicable extension to a perforating gun having an impact actuated firing head; port means in said tubular housing; first means for opening and closing said port means; securing means for securing said first means in a port closing position; restraining means for resisting movement of said securing means from said port closing position; said restraining means projecting into the free fall path of movement of a detonating means transmitted through the tubular tool string and being shiftable thereby to release said securing means; and means other than the detonating means for axially shifting said first means to a port opening position subsequent to passage thereacross of the detonating means. 7. The apparatus of claim 6 wherein said securing means comprises a collet having peripherally spaced latching arms held by said restraining means in engagement with said valve sleeve to prevent movement thereof to said port opening position. 8. Well perforating apparatus comprising a tubular housing having means on its upper end for communicable extension to a tool string, and means on its lower end for communicable extension to a perforating gun having an impact actuated firing head; port means in said tubular housing; a sleeve valve for opening and closing said port means; securing means for securing said valve sleeve in a port closing position; restraining means for resisting movement of said securing means from said port closing position; said restraining means projecting into the free fall path of movement of detonating means transmitted through the tubular tool string and being shiftable thereby to release said securing means; and means responsive to the fluid pressure produced in the casing annulus adjacent said housing for shifting said sleeve valve to its said open position relative to said port means subsequent to the firing of the perforating gun. 9. The apparatus of claim 8 wherein said means responsive to the fluid pressure produced in the casing annulus comprises a first piston surface on said valve sleeve exposed to said annulus fluid pressure and an opposed piston surface exposed to a lower fluid pressure within said housing. 10. The apparatus of claim 8 wherein said restraining means comprises the ring portion of a collet; shearable means securing said ring portion in said position preventing movement of said securing means from said port closing position; said collet having a plurality of peripherally spaced arms, each arm terminating in a head portion projecting partially into said free fall path of movement of the detonating means, thereby shearing said shearable means and shifting said collet downwardly to free said securing means. 11. The apparatus of claim 7 wherein said securing means comprises a collet having peripherally spaced latching arms held by said restraining means in engagement with said valve sleeve to prevent movement thereof to said port opening position. 12. The apparatus of claim 11 wherein said means responsive to the fluid pressure produced in the casing annulus comprises a first piston surface on said valve sleeve exposed to said annulus fluid pressure and an opposed piston surface exposed to a lower fluid pressure within said housing. 13. The apparatus of claim 11 wherein said restraining means comprises the ring portion of a collet; shearable means securing said ring portion in said position preventing movement of said latching means from said port closing position; said collet having a plurality of peripherally spaced arms, each arm terminating in a head portion projecting partially into said free fall path of movement of the detonating means, thereby shearing said shearable means and shifting said collet downwardly to free said latching means. 14. The method of underbalance perforating a well casing and adjacent formation by an impact fired perforating gun which is adapted to be carriable by a tubing string, impact fired, perforating gun, said tubing string having an axially shiftable valve adapted to open and close port means in said tubing string immediately above said perforating gun; comprising the steps of: shifting said valve to said port closing position against a bias and securing said valve in said closed position by securing means latch having a movement preventing portion projecting into the tubing bore; positioning the perforating gun at the desired location in the casing; maintaining the fluid pressure in the tubing string adjacent the port means at a level below the anticipated formation fluid pressure; and positioning detonating means through the tubing to trip the securing means and fire the perforating gun, thereby opening said port means for immediate flow of formation fluids up said tubing string. 15. The method of claim 14 wherein a disc of glass constitutes the movement preventing portion of the securing means, said glass disc being shattered by the detonating means without substantially reducing the kinetic energy of the detonating bar. 16. The method of underbalance perforating a well casing and adjacent formation by dropping a detonating bar on a tubing string communicable perforating gun, comprising the steps of: providing port means in the tubing string above the perforating gun and a shiftable sleeve valve biased to an open position relative to the port means; latching the sleeve valve in a closed position by a latching mechanism including a latch release operable by the passage of a detonating bar through the tubing string; positioning the perforating gun at the desired location in the casing; maintaining the fluid pressure in the tubing string adjacent the port means at a level below the anticipated formation fluid pressure; and dropping a detonating bar through the tubing to trip the latch and fire the perforating gun, thereby opening said port means for immediate flow of formation fluids up said tubing. 17. The method of claim 16 wherein said latch release constitutes a tempered glass mass transversely disposed relative to the path of the detonating bar; said glass mass being breakable by a small portion of the kinetic energy of said detonating bar. 18. The method of underbalance perforating a well casing and adjacent formation by a tubing string communicable, impact fired, perforating gun, said tubing string having an axially shiftable valve adapted to open and close radial ports in said tubing immediately above said perforating gun comprising the steps of: securing said axially shiftable valve in said port closing position by a shiftable latch mechanism; maintaining the fluid pressure in the tubing string adjacent the radial ports at a level below the anticipated formation fluid pressure; dropping a detonating bar through the tubing to shift said latch mechanism out of engagement with said axially shiftable valve; and moving said axially shiftable valve to its port opening position by energy supplied from a source other than the detonating bar. 19. The method of claim 18 wherein the valve shifting energy is derived from compressing a spring when the valve is secured in its port closing position. 20. The method of claim 18 wherein the valve shifting energy is derived from fluid pressure produced in the casing annulus adjacent the shiftable valve subsequent to firing the perforating gun.