You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of priority to PCT/US2006/061251 filed 27 Nov. 2006, which is incorporated herein by reference in its entirety for all purposes. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     In a drilled well, representative samples of rock are often cored from the formation using a hollow coring bit and transported to the surface for analysis. To collect these core samples, a number of coring methods may be used, including conventional coring and sidewall coring. With conventional coring, the drillstring is first removed from the wellbore and then a rotary coring bit with a hollow interior for receiving the cut core sample is run into the well on the end of the drillstring. Sidewall coring, on the other hand, involves removing the core sample from the bore wall of the drilled well. There are generally two types of sidewall coring tools, rotary and percussion. Rotary coring is performed by forcing an open, exposed end of a hollow cylindrical coring bit against the wall of the bore hole and rotating the coring bit against the formation. Percussion coring uses cup-shaped percussion coring bits, called barrels, that are propelled against the wall of the bore hole with sufficient force to cause the barrel to forcefully enter the rock wall such that a core sample is obtained within the open end of the barrel. The barrels are then pulled from the bore wall using connections, such as cables, wires, or cords, between the coring tool and the barrel as the coring tool is moved away from the lodged coring bit. The coring tool and attached barrels are finally returned to the surface where core samples are recovered from the barrels for analysis 
     In a typical percussion coring tool, an explosive device is used to propel the barrel from the tool into the surrounding formation. This explosive device is usually electrically fired, meaning an electrical current is used to initiate the explosion. Because these explosive devices are electrically initiated, they may be inadvertently initiated by stray voltage, static charge buildup, and radio frequency energy. In populated areas, sources of radio frequency may include CB radio, cellular telephones, radar, microwaves used for special communication and heat generation, conventional radio signals, power lines, high power amplifiers, high frequency electrical transformers, coaxial cables, etc. With respect to locations offshore, another source of radio frequency is powerful land-based transmitters used to communicate with equipment located on offshore platforms. Given the vast number of stray radio frequency sources, shutting these sources down temporarily so that sidewall percussion coring may be performed is impractical, if not impossible, particularly in congested areas near land-based oil and gas fields. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more detailed description of the present invention, reference will now be made to the accompanying drawings, wherein: 
         FIG. 1  is cross-sectional view of one embodiment of a voltage activated igniter; 
         FIG. 2  is a schematic illustration of the electrical circuit for the voltage activated igniter depicted in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of one embodiment of a core gun comprising a voltage activated igniter; 
         FIG. 4  is an end view of the core gun depicted in  FIG. 3 ; and 
         FIGS. 5A to 5D  depict a typical sequence for removing a core sample using a sidewall percussion coring tool comprising the voltage activated igniter depicted in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of a sidewall percussion coring tool comprising a voltage activated igniter and its method of use will now be described with reference to the accompanying drawings. In the drawings and description that follow, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawing figures are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. 
     Embodiments of the sidewall percussion coring tool and methods disclosed herein may be used in any type of application, operation, or process where it is desired to perform sidewall percussion coring service. Moreover, the tool and its methods of use are susceptible to embodiments of different forms. Specific embodiments are described in detail and are shown in the drawings, with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. Any use of any form of the terms “connect”, “engage”, “couple”, “attach”, or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the elements unless specifically noted and may also include indirect interaction between the elements described. The various characteristics mentioned above, as well as other features and characteristics described in more detail below, will be readily apparent to those skilled in the art upon reading the following detailed description of the embodiments, and by referring to the accompanying drawings. 
       FIG. 1  illustrates a cross-sectional view of a representative voltage activated igniter  100  comprising a housing  105  having a bore  110  therethrough, an explosive charge  115 , a bleeder resistor  120 , a capacitor  125 , a semiconductor bridge (SCB)  130 , and a spark gap  135  for protecting the igniter  100  against accidental initiation. The SCB  130  and the spark gap  135  are connected by a pair of electrically conductive wires  140 ,  145  to a means (not shown) for introducing an electrical charge into the SCB  130  The electrical charge is introduced to the SCB  130  by applying positive DC voltage across the leads  150  using any suitable means in the art, such as but not limited to, electrical wiring run downhole from the surface or a battery. The housing  105  at one end is sealed with a seal cap  155  and, surrounding that, a pressure seal boot  160 . In other embodiments, the seal cap  155  may be replaced with a radio frequency attenuator  163 . At the opposite end of the igniter  100 , a venting tube  160  is inserted into and extends from the explosive charge  115 . An end seal cap  165  acts as a barrier between the explosive charge  115  and the surrounding environment. 
     The housing  105  of the voltage activated igniter  100  includes a bore  110  therethrough, the diameter being sufficient to permit inclusion of an SCB  130  within the bore  110 . The thickness of the housing wall varies, typically ranging from 0.075″ to 0.125 inches thick. The housing  105  is comprised of substantially any material of high impedance, such as, for example, aluminum, steel, stainless steel, brass, and rigid plastics. Regardless of the housing  105  material, it must be suitable for high temperature applications, i e., temperatures up to 400 degrees Fahrenheit or above. 
     The explosive charge  115  may be introduced into the housing  105  as a powder and thereafter compressed by application of, for example, a ram to the explosive  115  at the end  170  of the housing  105 . The explosive charge  115  comprises any suitable explosive material known in the art, such as but not limited to, granular cyclotetramethylene tetranitramine (HMX), hexanitrostilbene (HNS), bis(picrylamino) trinitropyridine (PYX), trinitrotrimethylenetriamine (RDX) and mixtures thereof. The end  170  of the housing  10  is sealed by a thin metal or plastic disk that is pressed into place or by a thin layer of epoxy to provide a seal  165  on the exposed end of the explosive  115  in the bore  110  of igniter  100 . 
     The SCB  130  is positioned within the housing  105  such that it will be in contact with or at least close proximity to the explosive charge  115 . Preferably, the SCB  130  is positioned such that it will be in contact with the surface of the explosive charge  115  exposed in the bore  110 . The SCB  130  may be any suitable, commercially available semiconductor bridge in a size capable of insertion within the housing  105 . Suitable SCBs are available from, for example, Thiokol Corporation, Elkton, Md. and SCB Technologies, Inc., Albuquerque, N. Mex. The SCB  130  may be activated by any suitable electrical charge, including but not limited to, an electrical charge of approximately 173 volts at an amperage of approximately 0.010 amps. It is to be understood, however, that other SCBs suitable for initiating the deflagration reaction with the explosive charge  115  in the igniter  100  may be used. 
     The SCB  130  is connected by an electrically conductive wire  175  to a spark gap  135 . The spark gap  135  protects the igniter  100  against accidental initiation by an electrostatic discharge, stray voltage, radio frequency energy, or other unintended sources of electrical current. The spark gap  135  has a voltage threshold, for example, 150 to 158 volts, before passage of an electrical charge to the SCB  130  occurs. This prevents accidental initiation by unintended electrical charges below the threshold. Spark gaps  135  are available with various ratings, and igniters  100  may be prepared using different spark gaps  135  to permit controlled initiation of individual or multiple explosive charges in response to different electrical charges transmitted from an electrical source. Suitable spark gaps  135  are available from, for example, Reynolds Industries, Okyia, and Lumex Opto. 
     The SCB  130  and spark gap  135  are provided with electrically conductive wires  140 ,  145  that provide an electrical connection that extends outside the housing  105 . At the connection end  173  of the igniter  100 , the housing  105  may be sealed with plastic resins or similar materials  155  that bond to the housing  105  to seal the various components within the housing  105 . The electrically conductive wires  140 ,  145  pass through the seal cap  155 , leaving the leads  150  exposed for application of an electrical charge. Alternatively, the housing  105  may be sealed by insertion of a radio frequency attenuator  163 , in lieu of the seal cap  155 , having passageways therethrough to allow the wires  140 ,  145  to extend from the housing  105 . A radio frequency attenuator  163  may reduce the strength of any radio signal present to a level whereby the signal is incapable of accidental initiation of the igniter  100 . Suitable radio frequency attenuators  163  include the MN 68 ferrite device available from Attenuation Technologies, La Plata, Md. 
       FIG. 2  depicts an electrical circuit for the voltage activated igniter  100  comprising the spark gap  135  connected to the SCB  130  by the electrically conductive wire  175 , the capacitor  125 , the bleeder resistor  120 , and the explosive charge  115 . The explosive charge  115  includes a pyrotechnic  180  and a secondary explosive  185  in contact with the SCB  130  The capacitor  125  is utilized to store electrical energy sufficient to pass through the spark gap  135  and initiate the SCB  130 . The bleeder resistor  120  is used to slowly drain the capacitor  125  in the event the capacitor  125  is partially charged during an interrupted firing of the igniter  100  Typically, the capacitor  125  is selected to provide a capacitance of 3.5 mF, while the bleeder resistor  120  provides a 10,000 to 20,000 ohm resistance. Although  FIG. 2  illustrates a single capacitor  125  and a single resistor  120 , one skilled in the art may readily appreciate that multiple capacitors of varied capacitances and/or multiple resistors of varied resistances may be employed to perform these same functions. Moreover,  FIGS. 1 and 2  depict illustrations for only one embodiment of a voltage activated igniter. One skilled in the art may readily appreciate that various other combinations of the disclosed components, e.g. explosive materials, SCBs, and spark gaps, may be utilized to produce the same result, namely a voltage activated igniter that is immune to stray voltage, static discharge buildup, and radio frequency energy. 
       FIGS. 3 and 4  depict cross-sectional and end views, respectively, of a sidewall percussion coring tool  200  that utilizes at least one voltage activated igniter  100  to propel at least one barrel  215  into the surrounding formation. In some embodiments, including those depicted by  FIGS. 3 and 4 , the sidewall percussion coring tool  200  is a core gun. The tool  200  utilizes one or more voltage activated igniters  100  to ignite one or more quantities of core load explosive  210 . Once ignited, the core load explosive  210  detonates, propelling the core barrel  215  into the surrounding formation. The at least one voltage activated igniter  100  is positioned inside cavity  190  within the tool body  195 . Leads  150  extend from the outer end of the igniter  100  and may be attached to electrical wiring (not shown) used to apply an electrical charge to the igniter  100 . The connector end  173  of the igniter  100 , including the leads  150  and any attached electrical wiring, is sealed by an outer seal  205 . 
     The core barrel  215 , which will be propelled into the surrounding formation to collect a core sample, is seated on the core explosive load  210  The core barrel  215  includes the barrel shaft  220  through which a slot  225  passes, a seal plug  230 , and a seal plug retainer pin  235 . A core barrel retainer cable  240  passes through slot  225  of the barrel shaft  220 . Each end of the core barrel retainer cable  240  is wrapped multiple times around and attached to a cable retainer pin  245 , which is securely fastened to the tool body  195 . The seal plug  230  provides a means of sealing the cable  240  within slot  225  at the base of the barrel shaft  220 , while the seal plug retainer pin  235  locks the seal plug  230  to the barrel shaft  220 . When the core load explosive  210  detonates, the core barrel  215  is propelled into the formation while remaining tethered to the tool body  195  by the core barrel retainer cable  240  and the cable retainer pins  245 . 
       FIGS. 5A through 5D  schematically depict one embodiment of a sequence of operations wherein the sidewall percussion coring tool  200 , comprising multiple voltage activated igniters  100 , is used to collect core samples.  FIG. 5A  depicts one representative sidewall percussion coring service environment comprising a coiled tubing system  300  on the surface  305  and one embodiment of a sidewall percussion coring tool  200  being lowered into a wellbore  310  on coiled tubing  315 . The coiled tubing system  300  includes a power supply  320 , a surface processor  325 , and a coiled tubing spool  330 . An injector head unit  335  feeds and directs the coiled tubing  315  from the spool  330  into the wellbore  310 . Although this figure depicts the use of coiled tubing  315  to lower the sidewall percussion coring tool  200  within the wellbore  310 , one skilled in the art may readily appreciate that any similar means, for example, wireline, may be used. 
       FIG. 5B  depicts the sidewall percussion coring tool  200 , shown in  FIG. 5A , at the desired position in the wellbore  310  after run-in is complete. In this position, the igniters  100  are activated to propel the core barrels  215  into the surrounding formation  340 , wherein each igniter  100  ignites the explosive charge  115  contained within it and subsequently detonates the core load explosive  210  in contact with it via a venting tube  160  to propel a single core barrel  215 . 
     Firing of each igniter  100  is accomplished by applying positive DC voltage across its leads  150 . In some embodiments, the DC voltage source may be electrical wiring run from the surface  305  into the wellbore  310  along with and attached to the tool  200 . In other embodiments, the DC voltage source may be a battery(s) attached to or housed within the tool  200 . As the positive DC voltage is applied to the leads  150 , the capacitor  125  charges until a threshold level is reached, for example, between 130 and 160 volts, at which point the fixed voltage gap breaks down. Upon gap discharge, current flows through the SCB  130 , causing it to vaporize. Vaporization of the SCB  130  generates plasma gases that ignite the pyrotechnic  180 . The burning pyrotechnic  180 , in turn, causes a deflagration reaction to begin in the secondary explosive  185 . Hot gases resulting from burning of the pyrotechnic  180  and the secondary explosive  185  of the explosive charge  115  pass through the venting tube  160  to ignite and subsequently detonate the core load explosive  210 . Upon detonation of the core load explosive  210 , the core barrel  215  is propelled into the formation  340 . As shown in  FIG. 5C , a single core barrel  215  is depicted as having been propelled into the formation  340 . One skilled in the art may readily appreciate that a single, multiple, or all core barrels  215  housed within the sidewall percussion coring tool  200  may be deployed into the formation  340  in the same fashion. 
     As depicted in  FIG. 5D , the sidewall percussion coring tool  200  and attached core barrels  215  may be removed from the wellbore  310  by retracting the coiled tubing  315 . As the coiled tubing  315  is retracted and the tool  200  is pulled towards the surface  305 , the core barrel retainer cable  240  remains securely fastened both to the core barrel  215  and the tool  200 , thereby pulling the core barrel  215  from the formation  340  wall. Once extracted from the formation  340 , each core barrel  215  contains a core sample of the formation  340 , which may retrieved from the core barrel  215  for analysis after the tool  200  reaches the surface  305 . 
     While various embodiments of and methods of using a sidewall percussion coring tool comprising at least one voltage activated igniter have been shown and described herein, modifications may be made by one skilled in the art without departing from the spirit and the teachings of the invention. The embodiments described are representative only, and are not intended to be limiting. Many variations, combinations, and modifications of the applications disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is defined by the claims which follow, that scope including all equivalents of the subject matter of the claims.

Summary:
An apparatus and methods for sidewall percussion coring service are disclosed. In some embodiments, the side-wall percussion coring tool includes a voltage activated igniter, explosive material, and a core barrel in communication with the explosive material, wherein activation of the igniter causes detonation of the explosive material to propel the core barrel from tool. Some method embodiments for performing sidewall percussion coring service using the disclosed sidewall percussion coring tool include positioning the tool within a wellbore, activating the voltage activated igniter housed within the tool, detonating the explosive material within the tool with the voltage activated igniter, propelling a core barrel from the tool into the surrounding formation by detonation of the explosive material, retrieving the core barrel from the formation, and removing the tool from the wellbore.