Patent GB-677824-A (GB)
Title: Improvements in devices containing hollow explosive charges for perforating or cutting bore-hole linings or casings
IPC: F42B3/08
Filed: 1950-01-23
Abstract: 677,824. Blasting devices. SOC. DE PROSPECTION ELECTRIQUE PROCEDES SCHLUMBERGER. Jan. 23, 1950. [Jan. 22, 1949; Jan. 22, 1949; April 5, 1949; April 5, 1949; April 5, 1949; Nov. 8, 1949. Nov. 8, 1949] No. 1743/50. Class 9 (i). [Also in Group XXXVII] An explosive device, Fig. 1, for perforating well casings and like operations comprises a steel block 1 drilled radially to form cylindrical cavities 2, 2, 2<SP>11</SP> in each of which is placed a hollow charge. Axial channels 3, 3<SP>1</SP>, 3<SP>11</SP> between the cavities accommodate a detonating fuze 15. Each charge 5 has a jacket 4 of elastic material, preferably rubber soluble in hydrocarbons. Into each charge 5 is fitted a conical liner 6 having a cylindrical extension 7, the liner preferably being made of a substance with a low vaporisation temperature, e.g., zinc, zinc alloy, or magnesium. The charges are centred in the cavities 2 by a forward washer 9 and a rear washer 8 provided with notches to pass the fuze cord 15. Each cavity 2 is closed by a cap 11 held in position by a spring blade 13 fitting within slots 14 in the block. The cap 11 is preferably of friable, relatively dense material, e.g. cast iron, and its thickness is decreased towards the centre so that the cap will withstand the hydrostatic pressure in the borehole without offering excessive resistance to the blast from the charge. The fuze 15, which is pressed into a slot of part circular-section in each jacket 4 is ignited by a detonator initiated electrically, the conductor wires (not shown) being placed alongisde the fuze. In a modification the cap 11 is screw-threaded and bears against the forward end of the charge. The device shown in Fig. 6 has at the forward end a chamber 16<SP>1</SP> containing a detonator 17 attached to the fuze 15. The chamber is closed by a plug 18. Leads 19, 19<SP>1</SP> from the detonator are connected to metal contacts 21, 21<SP>1</SP> supported by an insulating sleeve 22 in a chamber 20. The leads after leaving the contacts 21, 21<SP>1</SP> are taken along the axial channels 3<SP>1</SP>, 3<SP>11</SP>, etc., to the cable supporting the device and connected to a source of current at ground level. The chamber 20 which is closed by a plug 23 contains a metal float 24 which, as water enters the chamber, rises to short-circuit the detonator 17 through the contacts 21, 21<SP>1</SP> and hence prevent ignition. In a modification the lead wires are covered with a water-pervious textile insulation and wound on a metal mandrel within the chamber 20. Water entering the chamber penetrates the insulation and the wires are shot-circuited through the mandrel. In an alternative arrangement one lead from the detonator is connected to the block 1 which forms the earth return and the second lead is connected through a resistance to a metal tube insulated in the chamber 20. Water entering the chamber 20 destroys the insulation and the igniter 17 is again short-circuited. In another arrangement, Fig. 9, one lead 19<SP>1</SP> is earthed to the block 1 and the other lead is connected through contacts 30, 30<SP>1</SP> to a conductor 27. The contacts 30, 30<SP>1</SP> are held together by a safety-pin type spring 28 held closed by a gummed strip 29. Water entering the chamber 20 detaches the strip 29 and the contacts separate to break the circuit to the igniter 17. Alternatively firing may be prevented by wetting of the ignition compound. The detonating fuze 15 terminates in a sleeve closed by a plug through which the leads 19, 191 pass to a container filled with an ignition compound in which is embedded a bridge wire. The container is of hygroscopic material, e.g., unvarnished cardboard and the plug is water permeable. The sleeve is situated in the chamber 20 so that the water entering the device filters into the sleeve and prevents ignition. The plug may be impermeable while openings in the sleeve allow water to enter. These openings are covered with cellophane until the fuze is placed in position. A warning device connected to a neon lamp may be used to indicate the presence of water in the apparatus. The hollow charge may be frusto-conical, its outer surface converging towards a semi-angle of 30 degrees while the cavity semi-angle is 55 degrees. Each jacket 4 may have a forward extension lodging in the aperture of the block and a rear extension fitting within a recess in the device, a plug holding the jacket 4 in position, while the fuze 15 is separated from the charge by a thin partition. The fuze is then initiated by an electric detonator. Figure 15 shows a hollow charge 5 in a case 6 suitable for insertion in the device of Fig. 1. At the rear of the explosive charge is a priming charge 74 adapted to be ignited by a Buckford fuze 15. The two charges are separated by a conical chamber 76 with vertex angle of the order of 120 degrees. The chamber 76 may be filled with inert material. The charge 74 may be fltted with a liner 77 and a wall 78 may confine the charge 5. The liner 77 may be cup-shaped.