Abstract:
An igniter initiates combustion in an amount of explosive that is downhole and adjacent the igniter. The igniter includes an amount of combustible material and a resistive igniter element disposed on a side of the combustible material that faces the amount of explosive. Applying a designated amount of electricity to the resistive igniter element generates sufficient heat to ignite the combustible material. Strategically initiating ignition adjacent the amount of explosive generates a flame front that travels away from the explosive, which in turn directs a flame against the explosive substantially the entire time the combustible material burns.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present disclosure relates in general to an igniter for igniting an explosive downhole, and which includes a propellant with a point of combustion adjacent the explosive. 
         [0003]    2. Description of Prior Art 
         [0004]    Explosives are sometimes used during downhole operations for the exploration and production of subterranean hydrocarbons. Perforating systems typically have shaped charges equipped with high explosive for generating a metal jet that pierces a wellbore wall to create perforations into the formation surrounding the wellbore. The shaped charges are disposed axially along the length of perforating guns that make up a perforating system. A detonation cord is usually placed adjacent each shaped charge, through which a detonation wave travels for initiating detonation of the high explosive. 
         [0005]    Packers and plugs are typically elastomeric bladder like elements that expand radially outward from a mandrel or downhole tool and into sealing contact with an inner surface of a wellbore wall. The sealing contact creates a pressure/flow barrier in the annulus between the tool and wellbore wall for isolating a designated portion of the wellbore. Some downhole packers or plugs are set with explosives that are included with the downhole tool. 
         [0006]    Igniters are usually included downhole for initiating combustion or detonation in a propellant, the detonating cord, or explosives. Typically, an igniter includes a cartridge, combustible material in the cartridge, and a resistive heating element inserted into combustible material. Electricity is supplied to the heating element via an electrode which ignites the combustible material. A flame exits the cartridge and is directed to an amount of combustible or explosive material adjacent the detonation cord or combustible material for ultimately detonating the shaped charges or setting the packer/plug. 
       SUMMARY OF THE INVENTION 
       [0007]    Disclosed herein is an example of an igniter, and a method, for initiating combustion and/or detonation of an explosive downhole. One example of the igniter includes a housing having an opening proximate the explosive, a combustible material in the housing, an igniter assembly in the opening and that is in thermal contact with the combustible material, so that when the igniter element is heated to a temperature that initiates combustion of the combustible material, a flame is generated in the housing which is directed to the explosive and that ignites the explosive. The igniter element can be made of an elongated resistive element which is made from a conductive material that is heated when current flows through the resistive element. The resistive element can be mounted on a printed circuit board. An electrode can be included in the housing for electrically coupling the resistive element with an electrical source. The explosive can be part of a perforating system or can be used for setting a packer in a wellbore. 
         [0008]    An alternative igniter for igniting an explosive downhole includes a housing having an opening, a propellant in the housing, an electrode inserted into the propellant and in electrical communication with a source of electricity, a resistive element in electrical communication with the electrode and in thermal contact with a surface of the propellant that is proximate the explosive, so that when electricity is supplied to the electrode from the source of electricity, current flows through the resistive element and heats the resistive element to a temperature that ignites the propellant, that in turn ignites the explosive. The resistive element may include an elongated electrically conductive member that is arranged in a helical configuration defined by a series of loops radially spaced away from one another. In this example, spacing between adjacent loops can be greater proximate an axis of the helical configuration. The resistive element may optionally be mounted on a printed circuit board that is set in the opening of the housing. The explosive can be used in a perforating system or for setting a packer. 
         [0009]    A method of detonating an explosive downhole is disclosed that includes providing an amount of combustible material in a housing adjacent the explosive, initiating combustion of the combustible material on an end of the combustible material that is proximate the explosive so that a flame front is created in the combustible material that travels away from the explosive that in turn generates a flame that is directed from the housing to the explosive and which ignites the explosive. The method may further include flowing current through a resistive element that is in thermal contact with the end of the combustible material proximate the explosive, wherein the resistive element becomes heated and initiates combustion of the combustible material. The explosive can be used for initiating detonation of shaped charges that create subterranean perforations, or for setting a packer in a wellbore. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]    Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
           [0011]      FIG. 1  is a side sectional view of an example of an igniter in accordance with the present invention. 
           [0012]      FIG. 2  is an axial sectional view of the igniter of  FIG. 1  taken along lines  2 - 2  and in accordance with the present invention. 
           [0013]      FIG. 3  is a side sectional view of an example of operation of the igniter of  FIG. 1  and in accordance with the present invention. 
           [0014]      FIG. 4  is a side sectional view of an example of a perforating system using the igniter of  FIG. 1  and in accordance with the present invention. 
           [0015]      FIG. 5  is a side sectional view of an example of setting a packer using the igniter of  FIG. 1  and in accordance with the present invention. 
       
    
    
       [0016]    While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF INVENTION 
       [0017]    The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. In an embodiment, usage of the term about includes +/−5% of the cited magnitude. 
         [0018]    It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
         [0019]    Shown in  FIG. 1  is a side sectional view of an example of an igniter  10  which has an elongate housing  12  and a body  14  mounted on an upper end of housing  12 . A propellant  16  is shown disposed within housing  12 . Examples of propellant  16  include any energetic material that can be burned, combusted, or otherwise decomposed, and which generates energy when burned, combusted, or decomposed. An elongate electrode  18  shown inserted through the body  14  and into the housing  12  and is set within propellant  16 . In the example of  FIG. 1 , electrode  18  is an electrically conducting element, an insulator  20  shown disposed between the electrode  18  and housing  14  to block electrical communication between electrode  18  and housing  14 . A lead  22  connects to an end of electrode  18  that protrudes outward from a side of the body  14  distal from housing  12  and which provides selective electrical communication between electrode  18  and an electrical source  23 . Examples of an electrical source include a battery, compositor, generator, and the like. 
         [0020]    Shown set in housing  12  and on an end of propellant  16  distal from body  14  is an igniter assembly  24  for initiating combustion of propellant  16 . In the embodiment shown, igniter assembly  24  is disposed in an opening  25  of the housing  12  that is distal from body  12 . Igniter assembly  24  includes a printed circuit board  26  and a resistive element  28  mounted on a surface of printed circuit board  26  facing propellant  16 . Further in the example of  FIG. 1 , the igniter assembly  24  is shown between propellant  16  and an explosive  30  adjacent an end of housing  12  distal from body  14 . The explosive  30  includes an amount of explosive material which is packed within a housing  32  that is substantially coaxial with housing  12 . Examples of explosive material include any high explosive, such as octogen (HMX), cyclonite (RDX), hexanitrostilbene HNS, to name a few; any low explosive, and any other material that can detonate when initiated. 
         [0021]    Referring now to  FIG. 2 , an axial sectional view is provided that is taken along lines  2 - 2  of  FIG. 1  and which gives a plan view of printed circuit board  26 . As shown, resistive element  28  is made from an elongate and electrically conductive resistive lead  34 . In the example, resistive lead  34  is arranged in a helical shape whose origin is proximate the axis A x  of igniter  10  ( FIG. 1 ). From the origin, the resistive lead  34  spirals radially outward towards the outer periphery of printed circuit board  26 . Also shown in  FIG. 1  is that electrode  18  rests on the resistive lead  34  so that the resistive element  28  and electrode  18  are in electrical communication. Referring back to  FIG. 2 , an optional resistive electrode  36  is provided on an end of resistive lead  34  proximate axis A x , electrode  36  is a planar element made from a conductive material. Electrode  36  defines a larger cross section than the lead  34  so that communication between electrode  18  and resistive element  28  is optimized. Alternatively, the end of resistive lead  34  opposite from resistive electrode  36  attaches to a ground G, which as shown in  FIG. 1  may include housing  12 . 
         [0022]    One example of operation of igniter  10  is shown in side sectional view in  FIG. 3  wherein the amount of electricity from electrical source  23  is supplied to electrode via lead  22 . The electricity in electrode  18  generates a flow of current through resistive lead  34  ( FIG. 2 ) which in turn heats resistive lead  34  to a designated temperature that is above the ignition temperature for the propellant  16 . As thermal energy from resistive lead  34  transfers to the propellant  16 , the resistive lead  34  and propellant are in thermal contact. By applying sufficient thermal energy to propellant  16 , the propellant  16  begins to combust and generates a flame front  38 . Arrows represent travel of the flame front  38  in a direction away from explosive  30 . As the flame front  38  moves away from the opening  25  of the housing  12 , a flame discharge  40  is created that is directed into the explosive  30 . The flame discharge  40  ignites explosive  30  to form an ignition/detonation front  42 , and which is shown travelling through the explosive  30 . One of the advantages of the present disclosure is that by igniting propellant  16  at an end, rather than in its middle, the flame discharge  40  is sustained and directed onto explosive  30  substantially during the entire time the propellant  16  is being combusted. As such, greater likelihood exists that the ignition/detonation front  42  will be created within explosive  30  and carry out its intended operation. Further in the example of  FIG. 3 , igniter assembly  24  (shown in phantom view) basically disintegrates during generation of the flame discharge  40 . Alternatively, the electrical circuit made up of the resistive lead  34  and electrode  18  can be all within electrode  18  so that connection to housing  12  is unnecessary. 
         [0023]    Referring now to  FIG. 4 , one example of using the igniter  10  in conjunction with a perforating operation is shown in a side sectional view. In this example a perforating system  44  is shown deployed in a wellbore  45  where the igniter  10  is explosively coupled with an upper end of a detonation cord  46  that extends the length of the perforating system  44 . Shaped charges  48  are provided in system  44  that are detonated by a pressure wave within detonation cord  46 . Detonating the shaped charges  48  form metal jets that create perforations  50  radially outward into a formation  52  surrounding wellbore  45 . A wireline  54  provides the electrical signal to the igniter  10  from electrical source  23  ( FIG. 1 ). Wireline  52  is shown threaded through a wellhead assembly  56  that is mounted on the opening of wellbore  45 . A surface truck  58 , which may contain the electrical source  23 , provides control of the perforating system  44  as well as a means for deploying the system  44 . 
         [0024]    Alternatively, igniter  10  is used in conjunction with a downhole tool  60  for setting a packer (or plug)  62  which projects radially outward from tool  60  and into sealing engagement with the walls of wellbore  45 . Also in the example of  FIG. 5 , the tool  60  is deployed on wireline  54  that strings through wellhead assembly  56  from surface truck  58 . As is known, igniter provides the ignition source for igniting an explosive that is used in setting packer  62 . 
         [0025]    In an example, the explosive material making up the explosive  30  detonates after being initiated, wherein a definition of detonation describes a reaction that can propagate through the material being detonated at the sound speed of the material. In a further example, detonation describes a reaction or decomposition of an explosive that, typically in response to a shock wave or heat, forms a high pressure/temperature wave. Example velocities of the high pressure/temperature wave can range from 1000 m/s to in excess of 9000 m/s. In another example, the explosive  30  can deflagrate, wherein a definition of deflagration describes a rapid auto-combustion of a material, such as an explosive. Generally, explosives that detonate are referred to as high explosives and explosives that deflagrate are referred to as low explosives. Alternate embodiments exist wherein the explosive  30  includes a propellant that combusts (or is otherwise combustible), or is replaced with a propellant. In an example, combustion describes an exothermic reaction of a material that can produce an oxide. 
         [0026]    The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.