Title:
Impulse generator and method for perforating a cased wellbore
Kind Code:
A1


Abstract:
An impulse generator (76) and method for using an impulse generator (76) to initiate a detonation in a shaped charge perforating apparatus (60) is disclosed. The shaped charge perforating apparatus (60) is adapted for use in a wellbore (62) and includes a plurality of shaped charges (86). A detonation cord (98) is operably coupled to each of the shaped charges (86). An initiator (78) is operable to detonate the detonation cord (98) upon receiving a triggering impulse. A Marx generator within the impulse generator (76) provides the triggering impulse to the initiator (78).



Inventors:
Beckman, Marvin Wayne (Weatherford, TX, US)
Application Number:
10/255387
Publication Date:
04/01/2004
Filing Date:
09/26/2002
Assignee:
BECKMAN MARVIN WAYNE
Primary Class:
Other Classes:
166/297
International Classes:
E21B43/117; E21B43/1185; H03K3/537; (IPC1-7): E21B43/11
View Patent Images:
Related US Applications:
20100018774EXCAVATOR APPARATUS FOR UNDERGROUND EXCAVATIONJanuary, 2010Furuki
20050211469Elongated coil assembly for electromagnetic borehole surveyingSeptember, 2005Kuckes et al.
20090211754WirelessTag Tracer Method and ApparatusAugust, 2009Verret et al.
20100012384Steel Tooth Bit With Scooped Teeth ProfileJanuary, 2010Luce
20050211471System and method for controlling drill motor rotational speedSeptember, 2005Zupanick
20030141115Rock core drill bitJuly, 2003Britten et al.
20100084193AIR HAMMER CORING APPARATUS AND METHODApril, 2010Livingstone
20080257606Flexible Drill ShaftOctober, 2008Teixeira et al.
20080164044Rotary drive assembly for a drill rodJuly, 2008Uelhoff et al.
20090308660AUXILIARY EQUIPMENT FOR USE WITH DRILLING APPARATUSDecember, 2009Gouws
20090314544Well Drilling and Production Using a Surface Blowout PreventerDecember, 2009Humphreys



Primary Examiner:
STEPHENSON, DANIEL P
Attorney, Agent or Firm:
LAWRENCE R. YOUST (Lawrence Youst PLLC 2200 Victory Avenue Suite 2604, Dallas, TX, 75219, US)
Claims:

What is claimed is:



1. A shaped charge perforating apparatus adapted for use in a wellbore, comprising: a plurality of shaped charges; a detonation cord operably coupled to the shaped charges; an initiator operable to initiate a detonation within the detonation cord upon receiving a triggering impulse; and a Marx generator operably associated with the initiator that is operable to generate the triggering impulse.

2. The shaped charge perforating apparatus as recited in claim 1 wherein the Marx generator further comprises an input terminal, an output terminal coupled to the initiator, a plurality of capacitors connected in series between the input terminal and the output terminal, a plurality of first surge arrester components connected between each of the capacitors in series, and a second surge arrester component positioned between the last one of the capacitors and the output terminal.

3. The shaped charge perforating apparatus as recited in claim 2 wherein the plurality of first surge arrester components and the second surge arrester component comprise gas surge arrester tubes.

4. The shaped charge perforating apparatus as recited in claim 2 wherein the plurality of capacitors comprise a ceramic material having a dielectric coefficient that increases with temperature.

5. The shaped charge perforating apparatus as recited in claim 2 wherein the triggering impulse is a discharge voltage impulse.

6. The shaped charge perforating apparatus as recited in claim 5 wherein the plurality of capacitors, the plurality of first surge arrester components and the second surge arrester component of the Marx generator comprises a plurality of stages whereby the discharge voltage impulse is proportional to the number of stages times a charging voltage applied to the Marx generator.

7. The shaped charge perforating apparatus as recited in claim 6 wherein each stage comprises one of the capacitors and one of the first or second surge arrester components.

8. The shaped charge perforating apparatus as recited in claim 6 wherein the Marx generator is operable to enter regenerative latch up upon receiving the charging voltage.

9. The shaped charge perforating apparatus as recited in claim 1 wherein the initiator is selected from the group consisting of bridge wire initiators, exploding bridge wire initiators, exploding foil initiators, percussion type initiators and pressure actuated initiators.

10. The shaped charge perforating apparatus as recited in claim 1 wherein the Marx generator is housed in a tubular housing that provides a circuit ground.

11. An impulse generator for delivering a discharge voltage impulse to an initiator that initiates a detonation within a detonation cord connected to a plurality of shaped charges positionable in a wellbore, the impulse generator comprising: an input terminal that is operable to receive a charging voltage; an output terminal coupled to the initiator, the output terminal operable to deliver the discharge voltage impulse; a plurality of capacitors connected in series between the input terminal and the output terminal; a plurality of charging resistors connected in parallel with the capacitors; a plurality of first surge arrester components connected between each of the capacitors in series; and a second surge arrester component positioned between the last one of the capacitors and the output terminal, whereby the plurality of capacitors are operable to be charged in parallel through the charging resistors and discharged in series.

12. The impulse generator for delivering a discharge voltage as recited in claim 11 wherein the plurality of first surge arrester components and the second surge arrester component comprise gas surge arrester tubes.

13. The impulse generator for delivering a discharge voltage as recited in claim 11 wherein the plurality of capacitors comprise a ceramic material having a dielectric coefficient that increases with temperature.

14. The impulse generator for delivering a discharge voltage as recited in claim 11 wherein the charging voltage is supplied from a surface location via an electric line.

15. The impulse generator for delivering a discharge voltage as recited in claim 11 wherein the plurality of capacitors, the plurality of first surge arrester components and the second surge arrester component define a plurality of stages such that the discharge voltage impulse is proportional to the number of stages times the charging voltage applied to the input terminal.

16. The impulse generator for delivering a discharge voltage as recited in claim 15 wherein each stage comprises one of the capacitors and one of the first or second surge arrester components.

17. The impulse generator for delivering a discharge voltage as recited in claim 11 wherein the initiator is selected from the group consisting of bridgewire initiators, exploding bridgewire initiators, exploding foil initiators, percussion type initiators and pressure actuated initiators.

18. The impulse generator for delivering a discharge voltage as recited in claim 11 further comprising a housing that provides protection to the plurality of capacitors, the plurality of first surge arrester components and the second surge arrester component and provides a circuit ground.

19. An impulse generator for generating a discharge voltage impulse, comprising: an input terminal that is operable to receive a charging voltage; an output terminal operable to deliver the discharge voltage impulse; a plurality of capacitors connected in series between the input terminal and the output terminal; a plurality of charging resistors connected in parallel with the capacitors; a plurality of first surge arrester components connected between each of the capacitors in series; and a second surge arrester component positioned between the last one of the capacitors and the output terminal, whereby the plurality of capacitors are operable to be charged in parallel through the charging resistors and discharged in series.

20. The impulse generator for generating a discharge voltage impulse as recited in claim 19 wherein the plurality of first surge arrester components and the second surge arrester component comprise gas surge arrester tubes.

21. The impulse generator for generating a discharge voltage impulse as recited in claim 19 wherein the plurality of capacitors comprise a ceramic material having a dielectric coefficient that increases with temperature.

22. The impulse generator for generating a discharge voltage impulse as recited in claim 19 wherein the plurality of capacitors, the plurality of first surge arrester components and the second surge arrester component define a plurality of stages such that the discharge voltage impulse is proportional to the number of stages times the charging voltage applied to the input terminal.

23. The impulse generator for generating a discharge voltage impulse as recited in claim 22 wherein each stage comprises one of the capacitors and one of the first or second surge arrester components.

24. The impulse generator for generating a discharge voltage impulse as recited in claim 19 further comprising a tubular housing that provides a circuit ground.

25. A method for perforating a well casing that lines a subterranean well, the method comprising the steps of: running downhole a shaped charge perforating apparatus including a plurality of shaped charges, a detonation cord operably coupled to the shaped charge, an initiator operable to detonate the detonation cord and a Marx generator operably associated with the initiator; supplying a charging voltage to the Marx generator to generate a discharge voltage impulse; transmitting the discharge voltage impulse to the initiator; initiating a detonation within the detonation cord; and detonating the shaped charges, thereby perforating the well casing that lines the subterranean well.

26. The method as recited in claim 25 wherein the step of supplying a charging voltage to the Marx generator to generate a discharge voltage impulse further comprises the step of triggering the Marx generator to enter regenerative latch up.

27. The method as recited in claim 25 wherein the step of supplying a charging voltage to the Marx generator to generate a discharge voltage impulse further comprises the step of executing a voltage multiplication on the charging voltage to generate the discharge voltage impulse.

28. The method as recited in claim 25 wherein the step of supplying a charging voltage to the Marx generator to generate a discharge voltage impulse further comprises supplying the charging voltage to an input terminal, charging a plurality of capacitors connected in series between the input terminal and an output terminal, overvolting a plurality of first surge arrester components connected between each of the capacitors in series and a second surge arrester component positioned between the last one of the capacitors and the output terminal and generating the discharge voltage impulse at the output terminal.

29. The method as recited in claim 28 wherein the plurality of first surge arrester components and the second surge arrester component comprise gas surge arrester tubes.

30. The method as recited in claim 28 wherein the plurality of capacitors comprise a ceramic material having a dielectric coefficient that increases with temperature.

31. The method as recited in claim 28 wherein the step of charging the plurality of capacitors connected in series between the input terminal and the output terminal further comprises the step of charging the plurality of capacitors in parallel via charging resistors.

32. The method as recited in claim 28 wherein the step of generating the discharge voltage impulse at the output terminal further comprises the step of discharging the plurality of capacitors in series.

33. The method as recited in claim 32 wherein the step of discharging the plurality of capacitors in series further comprises discharging the capacitors in series by the simultaneous spark over of the first and the second surge arrester components.

34. The method as recited in claim 25 further comprising the step of selecting the initiator from the group consisting of bridgewire initiators, exploding bridgewire initiators, exploding foil initiators, percussion type initiators and pressure actuated initiators.

35. A method for delivering a discharge voltage impulse to an initiator that detonates a detonation cord connected to a plurality of shaped charges positioned in a wellbore, the method comprising the steps of: positioning an impulse generator in a wellbore, the impulse generator including an input terminal, an output terminal coupled to the initiator, a plurality of capacitors connected in series between the input terminal and the output terminal, a plurality of first surge arrester components connected between each of the capacitors in series and a second surge arrester component positioned between the last one of the capacitors and the output terminal; applying a charging voltage to the input terminal; charging the plurality of capacitors in parallel through charging resistors; and discharging the capacitors in series by the simultaneous spark over of the first and second surge arrester components, thereby delivering the discharge voltage impulse to the initiator via the output terminal.

36. The method as recited in claim 35 further comprising the step of triggering the impulse generator to enter regenerative latch up.

37. The method as recited in claim 35 further comprising the step of executing a voltage multiplication on the charging voltage to generate the discharge voltage impulse.

38. The method as recited in claim 35 wherein the plurality of first surge arrester components and the second surge arrester component comprise gas surge arrester tubes.

39. The method as recited in claim 35 wherein the plurality of capacitors comprise a ceramic material having a dielectric coefficient that increases with temperature.

40. The method as recited in claim 35 wherein the impulse generator further comprises a housing that provides a circuit ground.

Description:

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates, in general, to perforating a cased wellbore that traverses a subterranean hydrocarbon bearing formation and, in particular, to an impulse generator for generating a discharge voltage impulse for the initiation of a detonation in order to facilitate the perforation of a cased subterranean wellbore using shaped charges.

BACKGROUND OF THE INVENTION

[0002] Without limiting the scope of the present invention, its background will be described with reference to perforating a subterranean formation with a shaped charge perforating apparatus, as an example.

[0003] After drilling a section of a subterranean wellbore that traverses a formation, individual lengths of relatively large diameter metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path for producing fluids from the producing intervals to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string, hydraulic openings or perforations must be made through the casing string, the cement and a short distance into the formation.

[0004] Typically, these perforations are created by detonating a series of shaped charges that are disposed within the casing string and are positioned adjacent to the formation. Specifically, one or more charge carriers are loaded with shaped charges that are connected with a detonating device, such as a primacord or detonation cord. The charge carriers are then connected within a tool string that is lowered into the cased wellbore at the end of a tubing string, wireline, slick line, coil tubing or other conveyance. Once the charge carriers are properly positioned in the wellbore such that the shaped charges are adjacent to the formation to be perforated, the shaped charges may be fired.

[0005] The shaped charges used to perforate the casing include high explosives and must therefore be handled with extreme caution. For example, it is imperative that the high explosives are not prematurely initiated causing the shaped charge to detonate. Accordingly, in the interest of safety, initiators that are considerably insensitive to electric current are typically used to initiate the shaped charge detonations.

[0006] For example, initiators that are operated using extremely high voltage and current have been used in order to avoid inadvertent detonation of shaped charges. Such high voltage and current initiators are substantially immune from any naturally occurring energy sources in the environment which cannot produce the voltage and current necessary to initiate these devices. Specialized electronic circuitry capable of producing the extremely high voltage and current necessary to initiate these initiators is employed to commence the detonation. Typically, the electronic circuitry includes integrated circuitry housed in a protective casing.

[0007] It has been found, however, that due to the power levels required to initiate these initiators, the existing electronic circuitry has several drawbacks. In order to produce the necessary electric current and voltage, the existing semiconductor based electronic circuitry is bulky and expensive. Moreover, a protective casing must be employed to ensure the functionality of the circuitry at elevated downhole temperatures and pressures. In addition, to be cost effective, the protective casing must enable the electronic circuitry to survive the firing of the shaped charges such that the electronic circuitry and its protective casing may be reused.

[0008] In order to survive the detonation of the shaped charges, conventional designs place the electronic circuitry at or near the top of the string away from the initiator. A specialized coaxial cable or flat cable having a low resistance and low inductance connects the electronic circuitry to the initiator in order that the high voltage and current pulse may be fully conducted. The specialized cabling necessary to conduct the voltage impulse adds further engineering and expense to the existing detonation schemes. Accordingly, it has been found that the useful life of these systems makes their use uneconomical due to the likelihood of damage caused during the perforation process.

[0009] Therefore a need has arisen for a system and method for generating a discharge voltage impulse for the initiation of a detonation that is insensitive to electrical currents and which prevents accidental triggering of the detonation. Further, a need has arisen for such a system and method that is inexpensive enough to be suitable for single use operations and that is compact enough to be easily lowered into a wellbore along with a perforating gun string. Additionally, a need has arisen for such a system and method that is capable of withstanding the great temperatures and pressures in a downhole environment prior to its operation.

SUMMARY OF THE INVENTION

[0010] The present invention disclosed herein comprises a shaped charge perforating apparatus and a method for perforating a cased wellbore that are capable of generating a discharge voltage impulse for the initiation of a detonation using an initiator that is insensitive to electrical currents. The apparatus of the present invention is compact enough to be easily lowered into a wellbore, is able to withstand downhole temperatures and pressures and, due to its inexpensive construction, the apparatus is expendable.

[0011] The shaped charge perforating apparatus of the present invention achieves these results by employing a Marx generator to create a voltage impulse which is applied to the initiator. The shaped charge perforating apparatus comprises multiple shaped charges and a detonation cord operably coupled to each of the shaped charges. The initiator is operable to initiate a detonation within the detonation cord upon receiving a triggering impulse such as the discharge voltage impulse from the Marx generator.

[0012] The Marx generator comprises an input terminal, an output terminal coupled to the initiator and a series of capacitors connected in series between the input terminal and the output terminal. Multiple surge arrester components are connected between the capacitors in series, and another arrester component is positioned between the last one of the capacitors and the output terminal. The capacitors are operable to be charged in parallel via the charging resistors and discharged in series.

[0013] The Marx generator has multiple stages whereby the discharge voltage impulse is proportional to the number of stages times a charging voltage that is applied to the Marx generator. Upon receiving a sufficient charging voltage, the Marx generator is operable to enter regenerative latch up. In one embodiment, each stage comprises one capacitor and one surge arrester component. In another embodiment, the surge arrester components may include gas surge arrester tubes. Additionally, the capacitors may include a ceramic material having a dielectric coefficient that increases with temperature.

[0014] In a further embodiment, the present invention is directed to a method for delivering a discharge voltage impulse to an initiator that initiates a detonation in a detonation cord connected to a series of shaped charges positioned in a wellbore. The method includes the steps of positioning an impulse generator in a wellbore, applying a charging voltage to the input terminal, charging the plurality of capacitors in parallel through the charging resistors and discharging the capacitors in series by the simultaneous spark over of the surge arrester components, thereby delivering a discharge voltage impulse to the initiator via the output terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which:

[0016] FIG. 1 is schematic illustration of an offshore oil and gas platform operating a shaped charge perforating apparatus of the present invention;

[0017] FIG. 2 is a side view partially cut away of a shaped charge perforating apparatus adapted for use in a wellbore of the present invention;

[0018] FIG. 3 is a cross sectional view of an exploding foil type initiator as used in the shaped charge perforating apparatus of the present invention;

[0019] FIG. 4A is a top plan view of an exploding foil or slapper initiator for use in the shaped charge perforating apparatus of the present invention;

[0020] FIG. 4B is a top plan view of a second exploding foil or slapper initiator for use in the shaped charge perforating apparatus of the present invention;

[0021] FIG. 4C is a top plan view of a third exploding foil or slapper initiator for use in the shaped charge perforating apparatus of the present invention;

[0022] FIG. 4D is a top plan view of a fourth exploding foil or slapper initiator for use in the shaped charge perforating apparatus of the present invention;

[0023] FIG. 5 is a cross sectional view of a bridgewire type initiator as used in the shaped charge perforating apparatus of the present invention;

[0024] FIG. 6 is a side view partially in cross section of a Marx generator for generating a discharge voltage impulse in accordance with the teachings of the present invention; and

[0025] FIG. 7 is a circuit diagram of an impulse generator circuit electrically coupled to an initiator for generating a discharge voltage impulse and initiating a detonation in accordance with the teachings of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention.

[0027] Referring initially to FIG. 1, a shaped charge perforating apparatus adapted for use in a wellbore operating from an offshore oil and gas platform is schematically illustrated and generally designated 10. A semi-submersible platform 12 is centered over a submerged oil and gas formation 14 located below sea floor 16. A subsea conduit 18 extends from deck 20 of platform 12 to wellhead installation 22 including blowout preventers 24. Platform 12 has a hoisting apparatus 26 and a derrick 28 for raising and lowering pipe strings.

[0028] A wellbore 36 extends through the various earth strata including formation 14. Casing 38 is cemented within wellbore 36 by cement 40. When it is desired to perforate casing 38 adjacent to formation 14, a shaped charge perforating apparatus 42 is lowered into casing 38 via electrical line 44. Thereafter, an electric signal is sent to an impulse generator 46 which, in turn, sends a discharge voltage impulse to initiator 48. Initiator 48 initiates the detonation of the shaped charges that are disposed within shaped charge perforating apparatus 42. Upon detonation, perforations are created that extend outwardly through casing 38, cement 40 and into formation 14.

[0029] Even though FIG. 1 depicts a vertical well, it should be noted by one skilled in the art that the shaped charge perforating apparatus of the present invention is equally well-suited for use in wells having other geometries such as deviated wells, inclined wells or horizontal wells. Also, even though FIG. 1 depicts an offshore operation, it should be noted by one skilled in the art that the shaped charge perforating apparatus of the present invention is equally well-suited for use in onshore operations.

[0030] Referring now to FIG. 2, therein a shaped charge perforating apparatus 60 is illustrated positioned in a wellbore 62 that penetrates formation 64. A casing 66 lines wellbore 62 and is secured in position by cement 68. A fluid such as drilling fluid (not shown) fills the annular region between shaped charge perforating apparatus 60 and casing 66. An electric line 70 is coupled to shaped charge perforating apparatus 60 at a cable head 72. A collar locator 74 is positioned below cable head 72 to aid in the positioning of shaped charge perforating apparatus 60 in wellbore 62. An impulse generator 76 is coupled to collar locator 74 and is in electrical communication with the surface via electric line 70.

[0031] An initiator 78 is mechanically and electrically coupled to impulse generator 76. Upon receiving a triggering impulse, initiator 78 is operable to initiate the detonation of shaped charge perforating apparatus 60. Initiator 78 may be a bridgewire initiator (BWI), an exploding bridgewire initiator (EBWI), an exploding foil initiator (EFI), a percussion type initiator, a pressure actuated initiator or the like.

[0032] Shaped charge perforating apparatus 60 includes a perforating gun 80 having a carrier 82 made of a cylindrical sleeve having a plurality of recesses, such as recess 84, defined therein. Radially aligned with each recess 84 is a respective one of a plurality of shaped charges, such as shaped charge 86. Each of the shaped charges includes an outer housing, such as housing 88 of shaped charge 86, and a liner, such as liner 90 of shaped charge 86. Disposed between each housing and liner is a quantity of high explosive.

[0033] The shaped charges are retained within carrier 82 by a support member 92 which includes an outer charge holder sleeve 94, an inner charge holder sleeve 96. In this configuration, outer charge holder sleeve 94 supports the discharge ends of the shaped charges, while inner charge holder sleeve 96 supports the initiation ends of the shaped charges. Disposed within inner tube 96 is a detonation cord 98, such as a primacord, which is used to detonate the shaped charges. In the illustrated embodiment, the initiation ends of the shaped charges extend across the cental longitudinal axis of perforating gun 80 allowing detonation cord 98 to connect to the high explosive within the shaped charges through an aperture defined at the apex of the housings of the shaped charges.

[0034] Each of the shaped charges is longitudinally and radially aligned with a recess 84 in carrier 82 when perforating apparatus 60 is fully assembled. In the illustrated embodiment, the shaped charges are arranged in a spiral pattern such that each shaped charge is disposed on its own level or height and is to be individually detonated so that only one shaped charge is fired at a time. It should be noted, however, by those skilled in the art that alternate arrangements of shaped charges may be used, including cluster type designs wherein more than one shaped charge is at the same level and is detonated at the same time, without departing from the principles of the present invention.

[0035] In an operational embodiment, to detonate the shaped charges, an electrical signal, i.e. a charging voltage, is sent from the surface to impulse generator 76 via electric line 70. Alternatively, a downhole battery or other power source operably associated with impulse generator 76 may provide the electric signal to the impulse generator 76. After receiving the charging voltage, impulse generator 76 executes a voltage multiplication increasing the voltage and current of the charging voltage to generate a discharge voltage impulse. The discharge voltage impulse is transmitted to initiator 78 which, in turn, initiates a detonation within detonation cord 98 such that the shaped charges are fired.

[0036] FIG. 3 presents an EFI 102 that includes an upper cap portion 104 having molded therein two electric conductors 106 and a slapper or initiator foil 108. Upper cap portion 104 may be formed of a molded plastic or the like. A flyer 110 is placed over initiator foil 108. In one embodiment, flyer 110 comprises a thin disk constructed from an insulating material such as a plastic.

[0037] A barrel 112 is placed over flyer 110 to sandwich flyer 110 tightly against initiator foil 108. A housing 114 contains a pressed pellet 116 of a secondary explosive material which is in intimate contact with barrel 112. It should be noted that the secondary explosive material can be handled more safely than conventional primary explosive material because it is less sensitive to shock and, accordingly, other external stimuli are less capable of causing premature detonation. An air gap 120 is positioned within pressed pellet 116. A seal boot 122 forms crimps 124 that connect a detonation cord 126 to EFI 102. Seal boot 122 has an internal, hollow, axial passage and external crimps 124 formed by crimping the shell around a detonation cord 126. Detonation cord 126 is connected to a set of shaped charges deployed in the shaped charge perforating apparatus as best seen in FIG. 2.

[0038] Typically, a high voltage, high intensity current electrical impulse is supplied to electrical conductors 106 by the impulse generator in a manner to sufficiently vaporize or cause initiator foil 108 to be exploded or vaporized. In response to the pressure generated by the vaporization gases, flyer 110 penetrates barrel 112 in the fashion of a projectile. Flyer 110 propels down housing 114 until it sufficiently impacts secondary explosive 118, detonating secondary explosive 118 which, in turn, detonates detonation cord 126 connected to the string of shaped charges. It should be understood by those skilled in the art that the EFI may have design modifications. For example, the EFI may have one electric conductor instead of two.

[0039] FIG. 4A illustrates an exemplary configuration of an initiator foil 130 of the type that may be employed in the EFI illustrated in FIG. 3. A narrow neck 132 is positioned in initiator foil 130 which comprises a thin sheet of conductive material such as aluminum or copper, for example. Once the critical breakdown voltage is reached, current flows across narrow neck 132 which causes initiator foil 130 to explode or vaporize. FIG. 4B illustrates a second exemplary configuration of an initiator foil 134 of the type that may be employed in the EFI illustrated in FIG. 3. A narrow neck 136 and a control gap 138 are employed to facilitate the explosion or vaporization of initiator foil 134. Similarly, FIG. 4C illustrates a third exemplary configuration of an initiator foil 140. A control gap 142 facilitates the vaporization of initiator foil 140. FIG. 4D illustrates a fourth exemplary configuration of an initiator foil 144 having narrow necks 146, 148 to facilitate vaporization.

[0040] FIG. 5 presents an BWI 150 that includes a housing 152 having positioned therein an electric conductor 154 and a bridgewire ignitor 156 that extends from electric conductor 154 into a pressed pellet 158 of a primary explosive material. A pressed pellet 162 of a secondary explosive material is positioned against pressed pellet 158 of primary explosive material. A seal boot 166 forms crimps 168 that connect a detonation cord 170 to BWI 150. Seal boot 166 has an internal, hollow, axial passage and external crimps 168 formed by crimping the shell around detonation cord 170.

[0041] In an operational embodiment, a charging voltage is supplied to electrical conductor 154 by the impulse generator in a manner to sufficiently cause bridgewire ignitor 156 to ignite. In response to the heat produced by ignited bridgewire ignitor 156, primary explosive material 158 is detonated, which, in turn, detonates secondary explosive material 162 and initiates the detonation in detonation cord 170, which runs into the shaped charge perforating apparatus to fire the shaped charges.

[0042] FIG. 6 illustrates a Marx generator 180 for generating a triggering impulse in accordance with the teachings of the present invention. Capacitors 182-200 are connected in series between an input terminal 202 and an output terminal 204. Input terminal 202 includes a piece of coaxial cable or other connector and receives the charging voltage from an electric line or other downhole power source. Output terminal 204 includes a piece of coaxial cable or short air gap and transmits a discharge voltage impulse to the initiator.

[0043] Preferably, capacitors 182-200 comprise a ceramic material having a dielectric coefficient that increases with temperature. It should be understood by those skilled in the art, however, that any device which consists essentially of two conductors, such as parallel metal plates, insulated from each other by a dielectric and which introduce capacitance into a circuit, stores electrical energy, blocks the flow of direct current and permits the flow of alternating current to a degree dependent on the capacitors' capacitance and the current frequency is within the teachings of the present invention.

[0044] Multiple surge arrester components 206-222 are connected between each of capacitors 182-200 in series and an additional surge arrester component 224 is positioned between capacitor 200 and output terminal 204. Preferably, surge arrester components 206-224 comprise gas surge arrester tubes. Typically, gas surge arrester tubes have a switching time below 50 nanoseconds and exceptionally high peak currents that produce steep voltage or current pulses of a few microseconds duration. Moreover, gas surge arrester tubes are inexpensive and capable of only one-time use. Accordingly, the disposable qualities of the surge arrester tubes are well suited for downhole use in close proximity to explosives. Alternatively, surge arrester components 206-224 may comprise fast-recovery hydrogen spark gaps or any gas-filled switching spark gap device that is characterized by a very steep current rise during the breakdown of the switch. The exact values of the capacitors and resistors will depend on the desired voltage and current of the discharge voltage impulse.

[0045] A tubular housing 226 cabins capacitors 182-200 and surge arrester tubes 206-224. In one embodiment, tubular housing 226 provides a circuit ground. Additionally, tubular housing 226 protects capacitors 182-200 and surge arrester components 206-224 from downhole temperatures and pressures. In this regard, the construction of Marx generator 180 is ideal for downhole use. Marx generator 180 includes no temperature sensitive components such as the semiconductor components of existing downhole impulse generators. Accordingly, the lack of temperature sensitive components translates into an inexpensive housing having a small profile that can be easily positioned downhole.

[0046] In operation, the charging voltage is applied to Marx generator 180 and capacitors 182-200 are charged through charging resisters as shown and explained below with reference to FIG. 7. When capacitors 182-200 are fully charged, surge arrester component 206 is broken down from overvoltage. This effectively places capacitors 182 and 184 in series which overvolts the next surge arrester component 208, which places capacitors 182, 184 and 186 in series. This erecting continues until the discharge voltage impulse is transmitted to the initiator.

[0047] Referring now to FIG. 7, an impulse generator circuit 240 for generating a discharge voltage impulse is electrically coupled to an initiator 242. Circuit 240 includes a series of capacitors 244-262 positioned in parallel with charging or isolation impedances illustrated as resistors 264-302. Resistors 264-302 may be carbon-composition resistors for example. It should be understood by those skilled in the art, however, that resistors 264-302 may comprise any device or material that offers an opposition to the flow of current, equal to the voltage drop across the element divided by the current through the element.

[0048] A series of switching elements illustrated as gas surge arrester tubes 304-320 are positioned in series with capacitors 244-262. The last gas surge arrester tube 322 acts as a peaking switch to isolate circuit 240 from the load, i.e. initiator 242, until capacitors 244-262 are completely charged and circuit 240 enters regenerative latch up. Additionally, a ground 324 is provided. It should be understood by those skilled in the art that the circuit may be constructed of various types of components and, in particular, the construction may include components not enumerated herein. Similarly, other criteria within the circuit may be varied.

[0049] In an operational embodiment, a charging voltage is applied to input terminal 326. Capacitors 244-262 are charged in parallel through charging resistors 264-302. The bank of capacitors 244-262 is charged up to the voltage that is applied to input terminal 362. Once capacitors 244-262 become charged, circuit 240 enters regenerative latch up at which time, capacitors 244-262 are discharged in series by the simultaneous spark over of gas surge arrester tubes 304-322. The first gas surge arrester tube 304 is triggered to arc at the desired time which causes a regenerative firing of all of the remaining gas surge arrester tubes 306-322 in a very short duration producing an impulse which has an extremely fast rise time. This effectively places all of capacitors 244-262 in series across the output load, i.e. initiator 242. The duration of the resulting discharge voltage impulse is dependent upon the load resistance and the value of the stage capacitance. In the illustrated embodiment, the stacked array of capacitors 244-262 and surge arrester tubes 304-322 form a series of ten stages wherein each stage comprises a capacitor and a surge arrester tube such as capacitor 244 and surge arrester tube 304. Circuit 240 performs a voltage multiplication on the charging voltage to generate the discharge voltage impulse. The discharge voltage impulse is proportional to the number of stages times the charging voltage applied to circuit 240. The number of stages, however, may vary as a design choice that depends upon the required voltage amplification. A discharge voltage impulse is thereby discharged to initiator 242.

[0050] While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.