Perforating gun using a bubble activated detonator

Abstract: An exploding foil bubble activated detonator, adapted for use in a perforating gun, includes an exploding foil (20b), the exploding foil (20b) including a neck section (20b2) which vaporizes when a current of sufficient magnitude and duration flows therethrough, a layer (22) of polyimide material deposited over said foil (20b), and a spacer layer (24) deposited over said layer (22) of polyimide material, the spacer layer (24) including a guiding hole (24a) disposed directly over the neck section (20b2) of the foil (20b). When the current flows through the neck section (20b2) of the foil (20b), a turbulence is caused to occur directly above the neck section (20b2). This turbulence causes a portion of the polyimide material to expand to form a bubble (22a). The bubble (22a) is shaped and sized by the guiding hole (24a) in the spacer layer (24) which is disposed directly over the neck section (20b2) of the foil (20b). The shaped and sized bubble (22a) in the polyimide material impacts an explosive (26b) thereby detonating the explosive (26b). A selective gun firing system may include the bubble activated detonator and functions to allow an operator to selectively fire, in a predetermined sequence, a plurality of perforators in the perforating gun, such as from bottom gun to top gun. A hall effect sensor disposed in a well truck is connected to a wireline to which the perforating gun is connected and senses a current in the wireline. A novel safety barrier is inserted into a hole in a barrel of a prior art exploding foil initiator (EFI) detonator for blocking a flying plate flying in the hole and thereby preventing accidental detonations of the EFI detonator.

Description: BACKGROUND OF THE INVENTION The present invention relates to detonating methods and apparatus. In the past, defence laboratories and related industries approached oil service companies to introduce a new detonator concept for use in perforating guns disposed in subsurface wells, an exploding foil initiator (EFI) detonator. The EFI detonator avoids the dangers associated with induced currents in primary highly sensitive explosives, the induced currents being produced when lead wires associated with arming apparatus in Electro Explosive Devices (EED) are exposed to electromagnetic fields originating from RF, radar, TV, and other electric transmissions. The EFI detonator is also termed the "flying plate" detonator, and a typical example of such a detonator is illustrated in figures 1, 2a, and 2b of the drawings and described in US Patent 4,788,913 to Stroud et al, entitled "Flying-Plate Detonator using a High Density High Explosive". The flying plate detonator includes a foil connected to a source of current, a small neck section of the foil exploding or vaporizing when a high current flows through the neck section of the foil. A disc is disposed in contact with the foil, the exploding neck section of the foil shearing a small flyer from the disc, the flyer travelling or flying through a barrel when sheared from the disc. The flyer impacts a secondary explosive and initiates a detonation of the secondary explosive. However, if the barrel is not centred correctly, an obstruction will appear between the disc and the secondary explosive. This obstruction will often prevent the flyer from impacting the secondary explosive. Therefore, when the flying plate EFI detonator is disposed in a perforating gun, this obstruction may prevent the perforating gun from detonating. A pyrotechnic device incorporating a flying plate detonator is disclosed in GB-A-2 100 395, where the flying plate is referred to as a "minislapper". However, according to the teachings of GB-A-2 100 395, the detonation of the pyrotechnic composition of the device is not believed to be due to the impact of the flying plate (or "minislapper"), but rather due to the hot plasma from the exploding (or vaporizing) foil. SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide an alternative detonator apparatus for use in a perforating gun, which alternative detonator apparatus inherently includes all the advantages associated with the EFI flying plate detonator, but which does not include the disadvantages of the EFI flying plate detonator. According to a first aspect of the present invention, there is provided a detonator for detonating an explosive, the detonator comprising: a conductive foil having a bridge portion which vaporizes when an electrical current is passed therethrough; means for passing an electrical current through the bridge portion of the conductive foil; a layer of plastics material disposed on the conductive foil; and a spacer layer disposed on the layer of plastics material, the spacer layer including a hole which is disposed directly over the bridge portion of the conductive foil and which leads to the explosive; characterised in that the respective thicknesses of said layer of plastics material and of the spacer layer are selected such that vaporization of the bridge portion of the conductive foil forces a bubble of the plastics material into the hole to impact and detonate the explosive. According to a second aspect of the present invention, there is provided a method of detonating an explosive, the method comprising the steps of: disposing a layer of a plastics material between a conductive foil having a bridge portion which vaporizes when an electrical current is passed therethrough and a spacer layer having a hole which is disposed directly over said bridge portion and which leads to the explosive; and passing an electrical current through the bridge portion of the conductive foil to vaporize said bridge portion; characterised in that the respective thicknesses of said layer of plastics material and of the spacer layer are selected such that vaporization of the bridge portion of the conductive foil forces a bubble of the plastics material into the hole to impact and detonate the explosive. Further scope of applicability of the present invention will become apparent from the detailed description presented hereinafter. It should be understood, however, that the detailed description and the specific examples, while representing a preferred embodiment of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become obvious to one skilled in the art from a reading of the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS A full understanding of the present invention will be obtained from the detailed description of the preferred embodiment presented hereinafter, and the accompanying drawings, which are given by way of illustration only and are not intended to be limitative of the present invention, and wherein: figure 1 is a sketch of the prior art apparatus and method of detonation using an exploding foil initiator (EFI) including a flying plate and a barrel for guiding the flying plate; figures 2a and 2b are further sketches of the prior art method of detonation using EFI and the flying plate; figure 3 is a sketch of the EFB apparatus in accordance with the present invention using an EFI without the flying plate or barrel; figure 4 illustrates the layers of material shown in figure 3; figures 5a-5b illustrate a cross sectional view of the EFB apparatus of figure 3 taken along section lines 5-5 of figure 3 and further illustrating a front view of this sectional view; figures 6a-6b illustrate a cross sectional view of the EFB apparatus of figure 3 taken along section lines 6-6 of figure 3 and further illustrating a front view of this sectional view; figure 7a illustrates a front longitudinal sectional view of the EFB apparatus of figure 3 before vaporization or explosion of the neck section of the foil bridge; figure 7b illustrates a front longitudinal sectional view of the EFB apparatus of figure 3 after vaporization or explosion of the neck section of the foil bridge, showing the bubble formed by the polyimide layer for initiating the detonation of the secondary explosive; figure 8 illustrates a perforating gun including a plurality of perforators, the perforating gun including a selective gun firing system, the selective gun firing system including the EFB apparatus of figures 3-7B; figures 9a-9b illustrate a hall effect current sensor for use in a well logging truck which senses the current in the wireline connected to the perforating gun and to the EFB detonator apparatus in the selective gun firing system; figure 10 illustrates a firing head assembly adapted to be connected to a perforating gun, the firing head assembly including the conventional EFI detonator, the EFI detonator including a port plug which, when removed, enables a steel rod to be inserted in the barrel of the EFI detonator thereby providing a safety barrier and a safe-arm feature for the detonator; and figure 11 illustrates a safety barrier which is inserted into a port through a wall of a firing head assembly, the safety barrier being inserted after a port plug is removed, the safety barrier including a steel rod for blocking the flying plate in the conventional EFI detonator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A discussion of the exploding foil bubble activated detonator (EFB detonator), for use in a perforating gun, in accordance with one aspect of the present invention, is set forth in the following paragraphs with reference to figures 1-7b of the drawings. Referring to figure 1, a prior art apparatus is illustrated for detonating a secondary explosive using an Exploding Foil Initiator (EFI), a flying plate and a barrel for guiding the flying plate. In figure 1, a pair of electrodes 10 energize a foil 12 by flowing a current through the foil. A disc 14, such as a plastic disc, is disposed adjacent the foil 12. A barrel 16 is disposed adjacent the disc 14, the barrel 16 having a hole 16a therein for guiding a flying plate which is sheared from the disc 14 when the foil 12 vaporizes in response to a current flowing therethrough. A secondary explosive 18, such as HNS or PETN, is disposed adjacent the barrel 16 for receiving the flying plate and detonating when the flying plate impacts the explosive. In operation, when current of sufficient magnitude flows through foil 12, a neck portion of foil 12 vaporizes thereby causing a flying plate to be sheared from the disc 14. The flying plate is guided by the hole 16a in barrel 16, the flying plate impacting the secondary explosive 18, detonating the explosive. Figures 2a and 2b illustrate the concept, set forth above with reference to figure 1, in more detail. Figures 2a and 2b are discussed in detail in U.S. Patent 3,978,791 to Lemley et. al. Flying plate detonators, of the type shown in figures 1, 2a, and 2b, are also discussed in detail in U.S. Patent 4,788,913 to Stroud et. al. In figure 2a, the disc 14 is disposed adjacent the foil 12, and a current flows through the foil 12. A flying plate has not yet been formed. In figure 2b, a flying plate 14a is sheared from disc 14 when a current of sufficient magnitude and duration flows through the foil 12. The flying plate 14a is guided by hole 16a of barrel 16 during its flight through the barrel. The impact of the flying plate 14a on the secondary explosive 18 detonates the explosive 18. In the figure 1, 2a, and 2b prior art embodiments, the barrel 16 must be centered directly above the disc 14 and the foil 12. If the barrel is not centered directly above the disc 14 and the foil 12, the barrel itself will act as an obstruction to the flight of the flying plate 14a within the barrel. Referring to figure 3, an exploding foil bubble activated (EFB) detonator apparatus 20 for use in a perforating gun, in accordance with the present invention, is illustrated. In figure 3, the EFB detonator 20 includes a base part 20a in which a current "I" flows. A ceramic disc 20g is first deposite or disposed over the top of base part 20a. A foil bridge 20b is disposed over the ceramic disc 20g, the foil 20b including an enlarged part 20b1 on both ends and a small neck section 20b2 interconnected between the two enlarged parts 20b1. A first conductor 20c is disposed through ceramic disc 20g and a second conductor 20d is also disposed through ceramic disc 20g, the first conductor 20c conducting the current "I" toward one enlarged part 20b1 of foil 20b, and the second conductor 20d is adapted for conducting a return current away from the other enlarged part 20b1 of foil 20b to ground potential. When a current "I" of sufficient magnitude and duration flows through the small neck seation 20b2, the neck section 20b2 explodes or vaporizes, causing a turbulence to occur in the vicinity of the neck section. This turbulence is ultimately responsible for implementing the positive benefits and advantages in accordance with the present invention, which benefits and advantages will be explained in more detail in the following paragraphs. A first dot 20e appears on the top of base part 20a and a second dot 20f appears on the top of base part 20a, opposite the first dot 20e. The dots 20e and 20f are used for alignment purposes, which will be explained in more detail below. A polyimide layer 22 is disposed over the top of base part 20a. One type of polyimide material, which may be used as the polyimide layer 22, is known as "Kapton". The Kapton polyimide material is manufactured by E.I. DuPont De Nemours, Incorporated (Dupont). The polyimide layer 22 must be approximately 0.025 mm (0.001 inch) in thickness; to be more specific, the layer 22 must be 0.0254mm (0.001 inch) plus or minus 0.0064 mm (0.00025 inch) in thickness. The thickness of layer 22 is an important parameter for reasons which will become apparent from the functional description of the invention set forth in the following paragraphs. A spacer 24 is deposited or disposed over the polyimide layer 22. The spacer 24 is made of a ceramic material and is approximately 0.13 mm (0.005 inch) in thickness; to be more specific, the spacer 24 is 0.13 to 0.18 mm (0.005 to 0.007 inch) in thickness. The thickness of the spacer 24 is also a very important parameter, for reasons which will also become apparent from the functional description of the invention set forth below. The spacer 24 includes a center hole 24a which is disposed immediately above the small neck section 22b2 of foil 20b. Spacer 24 also includes alignment holes 24e and 24f which are adapted to be disposed directly above dots 20e and 20f on the base part 20a. When spacer 24 is deposited or disposed over polyimide layer 22, the spacer 24 must be positioned or aligned properly over layer 22 and foil 20b, the proper alignment being achieved when alignment holes 24e and 24f are disposed immediately above dots 20e and 20f, in a concentric fashion. Spacer 24 further includes holes 24b and 24c which, when aligned properly as described above, will be positioned directly above conductors 20c and 20d in base part 20a. A cap 26 is adapted to be disposed over spacer 24, layer 22, and foil 20b, and is adapted to be threadedly connected to base part 20a. Cap 26 includes a hole 26a which is concentrically disposed directly over center hole 24a of spacer 24. An explosive pellet 26b, such as HNS, is dropped into hole 26a of cap 26, the pellet resting on spacer 24 and directly over center hole 24a. The positioning of pellet 26b directly over center hole 24a is important in implementing the bubble activated detonation in accordance with the present invention. Referring to figure 4, a side view of the EFB detonator 20 is illustrated, specifically illustrating base part 20a with conductors 20c and 20d, ceramic disc 20g, foil bridge 20b, polyimide layer 22, and spacer 24. The base part 20a includes conductors 20c and 20d, the conductors being 0.86 mm (0.034 inch) diameter pins. The ceramic disc 20g is 0.64 mm (0.025 inch) in thickness, and includes two holes adapted for receiving the conductors 20c and 20d, each hole being approximately 1.1 mm (0.044 inch) in diameter. The foil bridge 20b is approximately 4.3 µm (0.170 MILS) in thickness, plus or minus 0.13 µm (0.005 MILS). The polyimide layer 22 is approximately 0.0254 mm (0.001 inch) in thickness, plus or minus 0.0064 mm (0.00025 inch). The thickness of the polyimide layer 22 is critical and should be observed carefully to ensure that the thickness lies within the stated range. The spacer 24 is preferably made of a ceramic material and is approximately 0.13 to 0.18 mm (0.005 to 0.007 inch) in thickness. The thickness of the spacer 24 is also critical and must lie within the stated range. The center hole 24a of spacer 24 is approximately 1.0 mm (0.040 inch) in diameter. The two other holes 24b and 24c in spacer 24 which lie directly above the conductor 20c and 20d are approximately 1.5 mm (0.060 inch) in diameter. Referring to figures 5a-5b, a top view of the EFB detonator 20 of figure 3 is illustrated taken along section lines 5a-5a of figure 3. In figure 5a, the ceramic disc 20g, including dots 20e/20f, is illustrated. Two conductors 20c and 20d extend through the ceramic disc 20g, as shown more clearly in figure 5b. The conductors also extend through holes in the foil bridge 20b, and in particular, through enlarged parts 20bl of the foil bridge 20b, so as to create an electrical connection between conductor 20c, enlarged part 20b1, small neck section 20b2 of the foil bridge 20b, enlarged part 20b1, and conductor 20d. A polyimide layer 22 is disposed over the foil bridge 20b, as noted in figure 5b. When a current of sufficient magnitude and duration flows through the small neck section 20b2 of foil bridge, from conductor 20c to conductor 20d, the neck 20b2 vaporizes or explodes. This causes a turbulence to occur under polyimide layer 22 directly above the neck section 20b2 of foil bridge 20b. This turbulence will cause the polyimide layer 22, immediately above the neck section 20b2, to expand, stretch and form a bubble. The bubble will cause the detonator of the present invention to detonate. This function will be explained more clearly with reference to figures 7a-7b. Referring to figures 6a-6b, a top view of the EFB detonator 20 of figure 3 is illustrated taken along section lines 6a-6a of figure 3. In figure 6a, the spacer 24 is deposited onto, and covers the ceramic disc 20g, foil bridge 20b, and the polyimide layer 22 of figure 5a/5b. In the above paragraph, it was indicated that a bubble is formed immediately above the neck section 20b2. The shape, size, and general form of the bubble is defined by the center hole 24a of spacer 24. Since the center hole 24a is centered immediately above neck section 20b2, and immediately below the explosive pellet 26b, the carefully formed bubble, formed by center hole 24a, will detonate the pellet 26b. This function will be explained more clearly with reference to figures 7a and 7b. A functional description of the EFB detonator apparatus, for use in a perforating gun, in accordance with the present invention, is set forth in the following paragraphs with reference to figures 7a and 7b of the drawings. In figure 7a, a large current propagates through conductor 20c and into foil bridge 20b/20b1. The current flows through neck section 20b2 and into conductor 20d. The polyimide layer 22 is disposed over the foil bridge 20b, and the spacer 24, with center hole 24a, is disposed over polyimide layer 22. An explosive pellet 26b is disposed immediately above the center hole 24a of spacer 24. A detonating cord is connected to the explosive pellet for propagating a detonating wave to other parts of a system. n typical system may, for example, be a perforating gun of a well tool used in oil well boreholes. The detonating cord, in this example, would be connected to a plurality of shape charges disposed in the perforating gun. In figure 7b, the small neck section 20b2 of foil bridge 20b explodes or vaporizes when the large current of sufficient magnitude or duration flows through the neck section 20b2. When the neck section 20b2 vaporizes, a turbulence occurs in the space between the neck section 20b2 of foil bridge 20b and the polyimide layer 22. This turbulence causes the polyimide layer 22 to expand and form a bubble 22a, the bubble 22a impacting the explosive pellet 26b. The shape and size of the bubble 22a is controlled by the shape and size of the center hole 24a in the spacer 24. However, the turbulence, in the space between the neck section 20b2 and the polyimide layer 22, caused by vaporization of the neck section 20b2, is sufficient in intensity to cause the bubble to form and expand at a rapid rate, the rate of expansion of the bubble 22a being enough to cause the explosive pellet 26b to detonate when the bubble impacts the pellet 26b. When the pellet 26b detonates, the detonating cord, connected to the pellet, conducts a detonating wave to another system, such as to the shape charges in a perforating gun, used to perforate a formation of an oil well borehole. Referring to figure 8, a perforating gun including a plurality of perforators is illustrated, the perforating gun including a selective gun firing system in accordance with another aspect of the present invention, the selective gun firing system-including the EFB apparatus of figures 3-7B. In figure 8, a perforating gun 30, connected to a well truck 34 at a surface of a borehole, is lowered into a tubing 32 by wireline 30f and includes a plurality of perforators and a selective gun firing system. The plurality of perforators include perforator 30a (gun 1), perforator 30b (gun 2), perforator 30c (gun 3), and perforator 30d (gun 4), each perforator including a plurality of charges. The selective gun firing system of figure 8 is associated with each of the guns 1-4 and includes: (1) associated with gun 1 30a - a selector plug (red) (1) 30a1 (selector plug-1 30a1), an exploding Secondary Initiating Cartridge (ESIC) which includes an exploding foil bubble activated detonator (EFB) (1) 30a2 (ESIC/EFB-1 30a2), and a monoswitch (1) 30a3; (2) associated with gun 2 30b - a selector plug (green) (2) 30b1 (selector plug-2 30b1), an Exploding Secondary Initiating Cartridge (ESIC) which includes an exploding foil bubble activated detonator (EFB) (2) 30b2 (ESIC/EFB-2 30b2), and a monoswitch (2) 30b3; (3) associated with gun 3 30c - a selector plug (red) (3) 30c1 (selector plug-3 30c1), an Exploding Secondary Initiating Cartridge (ESIC) which includes an exploding foil bubble activated detonator (EFB) (3) 30c2 (ESIC/EFB-3 30c2), and a monoswitch (3) 30c3; and (4) associated with gun 4 30d - a selector plug (green) (4) 30d1 (selector plug-4 30d1), and an Exploding Secondary Initiating Cartridge (ESIC) which includes an exploding foil bubble activated detonator (EFB) (4) 30d2 (ESIC/EFB-4 30d2); there is no monoswitch associated with gun 4 30d. Each ESIC/EFB 1-4 30a2-30d2 as described above, which forms a part of the selective gun firing system, contains the exploding foil bubble activated detonator (EFB detonator 20 of figures 3-7b) described above in this specification with reference to figures 1-7b of the drawings. Each ESIC/EFB 1-4 30a2-30d2 has a positive input terminal connected to a selector plug (red) 30a1 and 30c1 and a negative input terminal connected to a selector plug (green) 30b1 and 30d1. The positive input terminal of each ESIC/EFB is connected, via a diode and capacitor network as illustrated in figure 8, to lead 20d of the EFB detonator 20 of figure 3; the negative input terminal of each ESIC/EFB is connected, via the diode and capacitor network, to lead 20c of the EFB detonator 20 of figure 3. Each monoswitch 30a3-30c3 is disposed in one of two positions, the position shown in figure 8 (hereinafter termed "position 1") and its alternate position not shown in figure 8 (hereinafter termed "position 2"). In operation, when the perforator associated with each switch detonates its charges, the particular switch changes positions from position 1 to position 2. For example, when perforator 30a detonates, monoswitch-1 30a3 changes its position from position 1 (shown in figure 8) to position 2 (not shown in figure 8). The wireline 30f lowers the perforating gun 30 into a tubing 32 disposed in a borehole, the wireline 30f being connected on one end to the perforating gun 30 and on the other end to a well truck 34. The wireline 30f, which is connected to the perforating gun 30, is also disposed within the perforating gun 30 and is connected to guns 1-4 30a-30d via monoswitches 1-3 30a3, 30b3, and 30c3. A casing collar locator (CCL) 30e is disposed within the perforating gun 30 and interconnects the wireline 30f connected to the well truck 34 to the wireline 30f connected to the guns 1-4 30a-30d. The CCL 30e detects the presence of the threaded joints disposed between sections of the tubing 32 by detecting changes in magnetic field flux at each of the threaded joints thereby determining the depth of the perforating gun 30 when the gun 30 is being lowered into the tubing 32. A functional operation of the selective gun firing system disposed in perforating gun 30 is set forth in the following paragraphs with reference to figure 8 of the drawings. Selectivity, in the context of a selective gun firing system, is a means of selectively firing one gun string at a time starting from the bottom gun (gun 1 30a). Gun 1 30a is triggered using a positive voltage, gun 2 30b is triggered using a negative voltage, gun 3 30c is triggered using a positive voltage, etc. Power in the form of a positive voltage is routed by wireline 30f from the well truck 34 to gun 1 30a (the bottom gun) via CCL 30e, gun 4 30d, gun 3 30c, gun 2 30b, and selector plug-1 30a1. The selector plug-1 30a1 energizes the positive terminal of the ESIC/EFB-1 30a2 with the positive voltage. In response, the EFB detonator in the ESIC/EFB-1 30a2 detonates the charges within the gun 1 30a. Monoswitch-1 30a3 was originally in position 1 as shown in figure 8, but changes its position to position 2 in response to detonation of the charges in gun 1 30a. However, since a positive voltage is still being propagated through wireline 30f, the charges in gun 2 fail to detonate. When the positive voltage is changed to a negative voltage by the operator at the well surface (in well truck 34), the selector plug-2 30b1 receives the negative voltage from monoswitch-1 30a3 and energizes the negative terminal of the ESIC/EFB-2 30b2 with the negative voltage thereby detonating the EFB detonator 20 in the ESIC/EFB-2 and detonating the charges in the gun 2 30b. In response to detonation of the charges in gun 2, monoswitch-2 30b3 changes its position from position 1, as shown in figure 8, to position 2 thereby allowing the negative voltage to energize the positive terminal of selector plug-3 30c1; however, since the ESIC/EFB-3 30c2 reacts to a positive voltage, and not a negative voltage, no detonation of gun 3 occurs. When the operator in well truck 34 changes the voltage from negative to positive, the EFB detonator 20 disposed within the ESIC/EFB-3 30c2 detonates the charges in gun 3 30c. In response to detonation of gun 3 30c, monoswitch-3 30c3 changes its position from position 1, as shown in figure 8, to position 2 thereby energizing the negative terminal of selector plug-4 30d1 and ESIC/EFB-4 30d2 with the positive voltage. However, no detonation of gun 4 occurs since the voltage is still positive. When the operator changes the voltage from positive to negative, the EFB detonator 20 disposed within the ESIC/EFB-4 30d2 detonates the charges in gun 4. When the charges in gun 4 30d detonate, the wireline 30f in gun 4 30d slams against the housing of the perforating gun 30, thereby creating a short circuit and ending the detonation sequence. Therefore, the selective gun firing system of figure 8 allows an operator in the well truck 34 to selectively fire a plurality of perforators disposed within a perforating gun in a predetermined sequence, such as from bottom gun (gun 1) to top gun (gun 4). The EFB detonators 20 of figures 1-7b disposed within each ESIC cartridge 30a2, 30b2, 30c2, and 30d2 of the perforating gun 30 perform more efficiently relative to the prior art EFI detonator. Referring to figures 9a-9b, a hall effect current sensor 40, disposed within the well truck 34, senses the current in the wireline 30f connected to the perforating gun 30 and to the EFB detonators 20 in the selective gun firing system. In figure 9a, a front view of the hall effect sensor 40 is illustrated. The sensor 40 comprises a 22 awg stranded, teflon insulated wire 40a wrapped around a ferrite split donut core 40b. In figure 9b, a side view of the sensor 40 is illustrated. The wires 40a are again shown as wrapped around the ferrite split donut core 40b. Two output wires 40c and 40d receive signals from the core 40b and generate output signals which represent the current in the wireline 30f. The monitoring of current in wireline 30f using a hall effect sensor 40 provides excellent current sensitivity and isolation between the sensitive wireline 30f and the instrumentation in the well truck 34. The donut core 40b forms a magnetic path for the sensor when a current carrying wire 40a is threaded through the center of the core. Current amplification is accomplished by threading the current wire 40a through the core 40b numerous times. The output is monitored using an A/D converter along with a computer and a cathode ray tube to produce a visible nomogram for wireline current as well as being stored on a nonerasable media for further evaluation. This record of current vs time, provided by the output signals present in output wires 40c and 40d of figure 9b, becomes important in diagnosing misfires and near failures even before the gunstring is out of the borehole. For scallop guns, the current provides an excellent indication for shot detection. Referring to figure 10, a firing head assembly adapted to be connected to a perforating gun is illustrated, the firing head assembly including the conventional EFI detonator, the EFI detonator including a port plug which, when removed, enables a steel rod to be inserted in the barrel of the EFI detonator thereby providing a safety barrier and a safe-arm feature for the detonator. In figure 10, the firing head assembly 50 includes a lower gun head 50a to which a perforating gun, such as the perforating gun 30 of figure 8, is connected. A conventional, prior art EFI detonator assembly 50b of figures 1, 2a and 2b is disposed within the firing head assembly 50. The EFI detonator assembly 50b includes a barrel and a hole 50b1 similar to barrel 16 and hole 16a of figures 1, 2a and 2b. A booster 50c is disposed within the firing head assembly 50 immediately below the barrel and hole 50b1 of the EFI assembly 50b. A detonating cord 50d is connected to the booster 50c, on one end, and to the charges within the perforating gun (such as gun 30). A port plug 50e is disposed through a port 50e1 in a wall of the firing head assembly 50 immediately adjacent the hole 50b1 of the EFI assembly 50b. The port 50e1 is a hole which is drilled completely through the wall of the firing head assembly 50 so as to expose the hole 50b1 of the EFI assembly 50. When the port plug 50e is removed, in accordance with one aspect of the present invention, a safety barrier 50f (shown in figure 11), which includes a steel rod, is inserted into the port 50e1 of the wall of the firing head assembly 50 and into the hole 50b1 of the EFI assembly 50b so as to completely block the hole 50b1 of the EFI assembly. This action is similar to insertion of a steel rod in hole 16a of the EFI detonator of figure 2b thereby completely blocking the hole 16a of barrel 16 in figure 2b. The steel rod functions as a safety barrier. Referring to figure 11, the safety barrier 50f including a steel rod 50fl is illustrated. In figure 11, the safety barrier 50f includes a port plug section 50f2 and the steel rod 50fl integrally connected to the port plug section 50f2. When the port plug 50e is removed from the firing head assembly 50 of figure 10, the safety barrier 50f of figure 11 is inserted in its place. When the safety barrier 50f is completely inserted, the steel rod 50fl is disposed within the hole 50bl and completely blocks the hole 50bl of the EFI assembly 50b. In operation, referring alternately to figure 10 and figures 2a and 2b, if the EFI detonator 50b of figure 10 accidentally. detonates, a flyer plate (14a of figure 2b) is sheared off from a disc (14 of figure 2a). The flyer plate (14a of figure 2b) begins to fly through a hole (16a of figure 2b) in barrel (16 in figure 2b). However, when the steel rod safety barrier is inserted into the port 50el of the wall of firing head assembly 50 and into hole 50bl of the EFI assembly 50b so as to completely block the hole 50bl of figure 10, the flyer plate (14a) hits the steel rod safety barrier and not the booster 50c of figure 10. The steel rod safety barrier provides a safe-arm feature; if the EFI detonator accidentally detonates, when the safety barrier is in place within hole 50bl as described above, an accidental detonation of the perforating gun (e.g., gun 30) cannot occur.

Claims: A detonator for detonating an explosive, the detonator comprising: a conductive foil (20b) having a bridge portion (20b2) which vaporizes when an electrical current is passed therethrough; means (20c, 20d) for passing an electrical current through the bridge portion (20b2) of the conductive foil (20b); a layer (22) of plastics material disposed on the conductive foil (20b); and a spacer layer (24) disposed on the layer (22) of plastics material, the spacer layer including a hole (24a) which is disposed directly over the bridge portion (20b2) of the conductive foil (20b) and which leads to the explosive; characterised in that the respective thicknesses of said layer (22) of plastics material and of the spacer layer (24) are selected such that vaporization of the bridge portion (20b2) of the conductive foil (20b) forces a bubble of the plastics material into the hole (24a) to impact and detonate the explosive. A detonator as claimed in claim 1, wherein the layer (22) is a polyimide layer. A detonator as claimed in claim 2, wherein said polyimide layer (22) is substantially 0.0254mm in thickness. A detonator as claimed in any preceding claim, further comprising a ceramic layer (20g) supporting said conductive foil (20b). A detonator as claimed in any preceding claim, wherein said spacer layer (24) is ceramic. A perforating gun including at least one detonator as claimed in any one of claims 1 to 5. A method of detonating an explosive, the method comprising the steps of: disposing a layer (22) of a plastics material between a conductive foil (20b) having a bridge portion (20b2) which vaporizes when an electrical current is passed therethrough and a spacer layer (24) having a hole (24a) which is disposed directly over said bridge portion (20b2) and which leads to the explosive; and passing an electrical current through the bridge portion (20b2) of the conductive foil (20b) to vaporize said bridge portion; characterised in that the respective thicknesses of said layer (22) of plastics material and of the spacer layer (24) are selected such that vaporization of the bridge portion (20b2) of the conductive foil (20b) forces a bubble of the plastics material into the hole (24a) to impact and detonate the explosive. A method as claimed in claim 7, wherein the layer (22) is a polyimide layer. A method as claimed in claim 8, wherein said polyimide layer (22) is substantially 0.0254mm in thickness. A method as claimed in any one of claims 7 to 9, further comprising the step of supporting said conductive foil (20b) on a ceramic layer (20g). A method as claimed in any one of claims 7 to 10, wherein said spacer layer (24) is ceramic.

Inventors: EDWARDS ARNOLD, HUBER KLAUS, LERCHE NOLAN, ROZEK KENNETH, SMITH EDWARD JR, EDWARDS, ARNOLD, HUBER, KLAUS, LERCHE, NOLAN, ROZEK, KENNETH, SMITH, EDWARD, JR.
Applicants: SCHLUMBERGER HOLDINGS, SCHLUMBERGER LTD, SCHLUMBERGER SERVICES PETROL, SCHLUMBERGER TECHNOLOGY BV, SCHLUMBERGER HOLDINGS LIMITED, SCHLUMBERGER LIMITED, SCHLUMBERGER TECHNOLOGY B.V., SERVICES PETROLIERS SCHLUMBERGER, (FORMERLY SOCIETE DE PROSPECTION ELECTRIQUE SCHLUMBERGER)
Application year: 1991
Main classification: F42B3
IPC codes: F42B3