Abstract:
Disclosed is a method and apparatus for use in initiating explosives used in application including well perforating systems. The initiator uses an air gap separating an electrically triggered semiconductor bridge plasma energy creator and a reactive foil abutting an explosive.

Description:
BACKGROUND 
     1. Technical Field 
     The present inventions relate to improved reliability and safety of electrical initiators for explosive devices. 
     2. Background Art 
     Electro-explosive initiation devices are commonly used in the aerospace, military, automotive and oil and gas industries in explosive systems to perform perforating and cutting operations. These initiation devices act as the starting element to begin the explosive sequence. 
     Conventional Semiconductor Bridge (SCB) explosive devices utilize a semiconductor bridge element in intimate contact with an energetic material as an initiator. The bridge is used to covert electrical pulse energy into thermal energy which is then used to release the chemical energy of the energetic material. When sufficient electrical impulse is applied, the semiconductor bridge vaporizes, generating a rapid release of heated particles (plasma event). At the desired electrical energy level the plasma event initiates a chemical breakdown (deflagration reaction) in the surrounding energetic material if the material is of the type that is sensitive to plasma events. 
     Common electrical hazards for electroexplosive initiators include low level stray currents and RF signals. Unless protected by circuitry or other means such as electromagnetic shields, the electrical hazards could possibly induce current flow across the semiconductor bridge causing ohmic (resistance) heating of the bridge element. If the energetic material in contact with the bridge is sufficiently insensitive, it acts as a heat sink and thus allows the bridge to “burn out” in a passive manner without initiating a plasma event. In this instance the initiator is now in the dudded condition and not able to function. If the energetic material in contact with the bridge element is sufficiently sensitive, then the simple ohmic heating (i.e., not plasma heating) could cause chemical reaction thus initiating the electro-explosive device. 
     In prior art plasma devices, such as Halliburton&#39;s Rig Environment Detonator (RED®), the semiconductor bridge is in intimate contact with an insensitive pyrotechnic material. However, to further enhance its safety, the present invention calls for the semiconductor bridge element to be separated from the energetic material regardless of whether it is sensitive or insensitive. By introducing the separation, the effects of ohmic heating are removed. The separation gap, however, causes initiation by plasma heating to become less reliable since the hot plasma particles must traverse the gap and thus undergo cooling effects. If the gap is too great, the normal “plasma mode” of initiation will fail thus leading to a dudded device. 
     While plasma gap type initiators have improved safety characteristics, separating the semiconductor bridge from the energetic material produces a less reliable initiator. In addition plasma gap type initiators are limited in that the energetic material must be of the type that is sufficiently sensitive to plasma events. 
     Therefore there is a need for an initiator with improved safety and reliability and one that can be used with a wider variety of energetic materials. 
     SUMMARY OF THE INVENTIONS 
     The present inventions provide a plasma gap type initiator that has improved reliability and can be used with insensitive energetic materials. 
     The initiator of the present invention utilizes a plasma-event-creating semiconductor bridge element that is held spaced away from the energetic material by a mechanical spacer to create a gap between the bridge element and the material. 
     Another aspect of the present invention utilizes a reactive foil material positioned in the gap abutting the energetic material. 
     According to further aspect of the present invention the reactive layer abutting the explosive material is a plasma gap type initiator comprising reactive multi-layer foil. 
     According to an additional aspect of the present invention the reactive multi-layer foil comprises mutually exothermic reactive metals formed in thin layers where the exothermic reaction is initiated by a plasma event. 
     According to further aspect of the present invention, the reactive foil comprises Nanofoil®. NanoFoil® products are composed of multiple nano-layers of nickel and aluminum. Nanofoil foil is a product supplied by Indium Corporation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawing is incorporated into and forms a part of the specification to illustrate at least one embodiment and example of the present invention. Together with the written description, the drawing serves to explain the principles of the invention. The drawing is only for the purpose of illustrating at least one preferred example of at least one embodiment of the invention and is not to be construed as limiting the invention to only the illustrated and described example or examples. The various advantages and features of the various embodiments of the present invention will be apparent from a consideration of the drawing, in which: 
         FIG. 1  is a diagram of the igniter system of the present invention illustrated in longitudinal section; 
         FIG. 2  is a diagram of another embodiment of the igniter system of the present invention illustrated in longitudinal section; and 
         FIG. 3  is a partial section view on a downhole well tool including the electro-explosive initiating system of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides an improved apparatus and method for igniting an energetic material. The present invention&#39;s particular applicability is to ignite devices used in wellbore applications. 
     Referring more particularly to the drawing, wherein like reference characters are used throughout the various figures to refer to like or corresponding parts, there is shown in  FIG. 1  one embodiment of the igniter system  10  of the present invention installed in a typical electro-explosive device. 
     The igniter system  10  is installed in an energetic device comprising a cylindrical shaped barrel  14  which has an interior chamber defined by a cylindrical inner wall  15 . The barrel  14  is open on one end  16  (illustrated on the page as the upper end) and is closed off at the other end by end wall  18  formed on a header  20 . The header  20  is attached to (or integrally formed with) the barrel  14  to form a rigid open ended housing. As will be described the open end  16  can be associated with a detonating cord of an explosive device such as a well bore perforating apparatus. In a different embodiment, it is possible for the upper end  16  of the barrel  14  to be sealed by a thin disc. The disc is sufficiently thin so as to not prevent functioning of the device. 
     Two lead wires  22  and  24  extend axially through the barrel  14 . Lead wire  22  is electrically separated from the header by an insulating sleeve  26 . A semiconductor bridge  30  is embedded in the wall  18  facing the interior of the barrel  14 . A semiconductor bridge is used to refer to a device which when pulsed with sufficient electrical energy creates a plasma event. A typical semiconductor bridge (SCB) is described in U.S. Pat. No. 4,708,060 entitled “Semiconductor Bridge (SCB) Igniter”, filed Feb. 19, 1985, issued Nov. 24, 1987, the disclosure of which is incorporated by reference into the specification of this application. The two leads  32  and  34  extending from the semiconductor bridge  30  are electrically connected to the leg wires  22  and  24 . 
     The semiconductor bridge  30  is of the type which when pulsed with a sufficient electrical energy, vaporizes, generating a rapid release of heated particles (plasma event). However, if a stray electrical pulse of lesser energy is generated across the leg wires, the semiconductor bridge will not vaporize. 
     An annular spacer ring  40  abuts the header wall  18 . The upper and lower annular faces  42  and  44 , respectively, (as illustrated in the figure) extend transverse to the axis of the ring  40 . Faces  42  and  44  are parallel extending and flat. A gap  48  is formed by the interior wall  46  of the spacer ring  40 . In the illustrated embodiment the gap  48  is empty of solid material and is referred to in the industry an “air gap”. The gap thickness can be quite small, on the order of 0.5-1.0 mm. The introduction of the gap markedly reduces the heat transfer to the energetic material that occurs during ohmic heating. 
     As pointed out above, the semiconductor bridge  30  operates such that stray current will only result in resistance heating and will not cause the device to function in the plasma mode. The air gap  48  is dimensioned such that the semiconductor bridge is separated sufficiently away from the explosive material whereby stray energy will not initiate the energetic material. In this manner the initiator system can be safely used in high stray energy environments. 
     In an alternate embodiment illustrated in  FIG. 2 , a reactive layer  50  spans the interior of the barrel  14  and abuts the wall  46  of spacer ring  40 . Conventional energetic material  60  substantially fills the interior of the barrel  14  from the open end  16  to the reactive foil  50 . In this configuration the reactive layer  50  faces the semiconductor bridge  30  embedded in the header  20  and can be ignited by the plasma event of the semiconductor bridge  30 . The term “reactive layer” as used herein refers to a thin layer of self-sustaining exothermic explosive material which requires a relatively high energy input to initiate the explosion. Reactive layers include pyrotechnic foils and the like. 
     According to one example of the present invention the reactive layer  50  comprises laminated reactive foil made by vapor-deposited alternating layers of Aluminum (Al) and Nickel (Ni) which when subjected to a heat pulse produces and self-sustaining exothermic reaction. 
     According to another embodiment the reactive layer  50  is between about 60 to 150 micrometers thick. The thickness of a layer  50  is inversely related to the gap spacing between the layer  50  and the semiconductor bridge  30 . 
     In a further example the reactive layer  50  is a laminate comprises one or more layers selected from the group consisting of nickel-aluminum, aluminum-titanium, and titanium-amorphous silicon. 
     According to another example of the present invention the layer  50  comprises a reactive foil. 
     In a further embodiment the layer  50  comprises NanoFoil® distributed by Indium Corporation. 
     According to a further example embodiment, the layer  50  comprises materials that will not ignite unless the layer is heated to at least 250 degrees C. at a rate of at least 200 degrees C/min. Further, the layer when heated below the ignition rate will anneal and lose the ability to create a self-sustaining reaction. 
     In  FIG. 3  an example of an application of the igniter of the present inventions. Numeral  110  identifies a perforating gun assembly adapted to be lowered in a well for conducting perforating operations with shaped charges. This includes a wireline  111  of substantial length which includes a current conducting member as well as a strength member. 
     The wireline  111  is connected to a cable head  112 . In turn, that is connected with a collar locator  113 . The collar locator  113  locates collars in the casing and thereby provides an electrical signal of the location of the shaped charge perforating gun assembly  110  to the surface to enable proper positioning of the apparatus in the borehole. A casing collar locator is well known in the art. 
     The apparatus further includes a firing sub  114  connected below the collar locator  113  and in turn that is connected with a firing head  115 . The firing sub and firing head combination incorporates a firing circuit (not shown). 
     The system further includes an elongate cylindrical sealed housing  121  which is closed with a bull plug  122  and which supports a number of shaped charges  117  therealong. The several shaped charges are all detonated by means of an explosive signal provided over a detonating cord  118 . 
     The detonating cord is initiated with a detonating signal from an electro-explosive initiator  10 . A wire  123  provides an electrical current flow from the firing circuit to the initiator  10 . The several shaped charges are fired to form perforations through the surrounding casing and into the adjacent formations 
     Operation of the igniter system  10  will be described when attached to a firing circuit (not shown) and installed in a down hole well tool, such as, an oilfield perforation system (not shown). Firing is initiated by applying a DC voltage across the leads of firing circuit, which causes a firing capacitor in the circuit to charge up until a fixed discharge voltage is reached. Upon reaching the discharge voltage the capacitor discharges current onto leg wires  22  and  24  causing the semiconductor bridge  30  to vaporize. Energy in the form of plasma gases are generated when the bridge  30  vaporizes. The plasma gases propagate across the gap  48  and cause the reactive foil  50  to ignite. The ignited foil  50  initiates the energetic material  60  which in turn initiates the perforating guns via a detonating cord or the like. 
     When the explosive foil  50  comprises NanoFoil®, ignition of the foil will not occur unless the semiconductor plasma heats the foil to at least 250 degrees C. in a rate of at least 200 degrees C/min. The semiconductor bridge is selected with a plasma event sufficient to create heating of the foil above the minimum. However, stray electromagnetic energy that induces current in the firing circuit will not cause the semiconductor bridge to vaporize and will instead merely result in resistance heating in the bridge. This lower rate of energy release (resistance heating) will cause the foil to remain unaffected or, at most, to anneal and loose its ability to ignite. 
     While compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods also can “consist essentially of” or “consist of” the various components and steps. As used herein, the words “comprise,” “have,” “include,” and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps. 
     Therefore, the present inventions are well adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While the invention has been depicted, described, and is defined by reference to exemplary embodiments of the inventions, such a reference does not imply a limitation on the inventions, and no such limitation is to be inferred. The inventions are capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts and having the benefit of this disclosure. The depicted and described embodiments of the inventions are exemplary only, and are not exhaustive of the scope of the inventions. Consequently, the inventions are intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects. 
     Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an”, as used in the claims, are defined herein to mean one or more than one of the element that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent(s) or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted.