WO1999053263A2

WO1999053263A2 - Deflagration to detonation choke

Info

Publication number: WO1999053263A2
Application number: PCT/US1999/001330
Authority: WO
Inventors: Jerry D. Motley
Original assignee: Halliburton Energy Services, Inc.
Priority date: 1998-01-29
Filing date: 1999-01-22
Publication date: 1999-10-21
Also published as: WO1999053263A3

Abstract

A detonator (10) is described which employs an explosive choke (26) along an explore column to cause conditions which are highly conducive to detonation. The detonator comprises a cylindrical casing (12) which contains a bore and an enclosed pair of coaxial explosive column portions - a larger diameter column (28) and a smaller diameter column (30). The two column portions are joined at a choke (26) or diametrical reduction in the size of the bore. The detonator also includes an initiator assembly (16). The inventive detonator (ensures) reliable deflagration to detonation activity in high temperature explosives such as HNS, PYX, BRX and NONA. A deflagration to detonation method which features the initiation of deflagration in a portion of a confined, larger diameter explosive as desribed. Deflagration occurs along the length of the larger diameter column portion until an explosive choke is reached wherein the larger diameter column portion is transitioned into a smaller diameter coaxial column portion.

Description

DEFLAGRATION TO DETONATION CHOKE

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to explosive detonators which are substantially insensitive to thermal changes, impacts and electrostatic discharge. The invention also relates to methods of reliably causing detonation of a secondary explosive column. 2. Description of the Related Art Explosive detonators are used to initiate larger secondary explosions. Blasting caps are perhaps the most widely known type of detonator. Explosive devices that are used in a hostile environment, such as within an oil well borehole, must be insensitive to high temperatures, impacts and static electrical discharges. Such detonators are required for the operation of perforating guns and casing cutters among other devices which are used in downhole systems. One example of a detonator useful for initiating downhole explosions is described in U.S.

Patent 5,503,077, entitled Explosive Detonation Apparatus which is assigned to the assignee of the present invention. This patent is incorporated herein by reference.

Because detonators used in downhole environments must be resistant to high temperatures, temperature resistant explosives must be used within the detonator. Such "high temperature" explosives are typically resistant to temperatures up to 375° to 525°F. High temperature explosives of this type include HNS (hexanitrostilbene), PYX (2, 6 - Bis (picrylamino) -3, 5 - dinitropyridine), BRX (1, 3, 5 - trinitro - 2, 4, 6, tripicrylbenzene) and NONA (nonanitroterphenyl). Unfortunately, these high temperature explosives are relatively insensitive as compared to "low temperature" explosives, such as RDX (cyclotrimethylene trinitramine), HMX (cyclotetramethylene tetranitramine) and PETN (pentaerythritol tetranitrate). Thus, it is difficult to reliably cause high temperature explosives to jump from burn (deflagration) to detonation over a short interval, such as within a small, confined tube. Therefore, an arrangement that increases the reliability of such a jump is desirable.

SUMMARY OF THE INVENTION A novel detonator is described which employs an explosive choke along an explosive column to cause conditions which are highly conducive to detonation. The detonator comprises a cylindrical casing which contains a bore and an enclosed pair of coaxial explosive column portions - a larger diameter column portion and a smaller diameter column portion. The two column portions are joined at a choke, or diametrical reduction in the size of the bore. The detonator also includes an initiator assembly. A method is also described for the reliable creation of detonations from a deflagration condition. The described methods feature the initiation of deflagration in a portion of a confined, larger diameter explosive column. Deflagration occurs along the length of the larger diameter column portion until an explosive choke is reached wherein the larger diameter column portion is transitioned into a smaller diameter coaxial column portion. The inventive detonator is particularly well suited for ensuring reliable deflagration to detonation activity in high temperature explosives such as HNS, PYX, BRX and NONA. This technique can also be used with the "lower temperature" explosives such as PETN, RDX and HMX in order to create shorter length transitions from deflagration to detonation.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic longitudinal cross-section of an exemplary detonator constructed in accordance with the present invention.

Figure 2 depicts a cross-section of the exemplary detonator of Figure 1 following detonation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 depicts an exemplary detonator assembly 10 that is adapted to receive an electrical signal and then detonate in order to initiate a larger secondary explosive. The detonator 10 includes a tubular casing 12 which is typically formed of aluminum, steel, brass, rigid plastics or the like in which is capable of withstanding exposure to a temperature of about 500°F. for a period of at least one hour without structural failure. The casing 12 defines a chamber 14 at a first end of the casing 12. Disposed within the chamber 14 is an ignition device 16 which typically comprises a semi-conductor bridge (SCB) of a type which is known in the art. Alternatively, the ignition device 16 may comprise a non-electrical initiator such as a percussion primer.

A pair of electrical wires 18 extends from the ignition device 16 toward a source of electrical power. Although not shown in Figure 1 , it should be understood by those of skill in the art that the first end of the casing 12 is sealed off in a manner such that the ignition device 16 is sealed against the introduction of fluids and external matter. The usual technique for such sealing involves the placement of epoxy or a plastic or metal disk within the first end.

A second end of the casing 20 features a connector 20 to which is affixed a secondary or explosive booster. Typically, the booster charge is crimped onto the connector 20. The casing 12 defines a first, expanded diameter bore 22, located proximate the chamber 14, and a second, reduced diameter bore 24 which adjoins the first bore 22 and is coaxial with it. Choke 26 is the point at which the first bore 22 transitions into the second bore 24. The choke 26 is a reduction in the diameter of the bore. It is preferred that the reduction in diameter be by an amount which is approximately 25-30% of the original diameter of the first bore 22 as this amount of reduction appears at present to optimize the conditions needed for creation of a high density plug of the type that will be described.

An exemplary set of preferred dimensions for the detonator 10 are provided below:

Length of casing 12: 2.90"

Length of chamber 14: 0.40" Diameter of chamber 14: 0.25"

Length of first bore 22: 1.20"

Diameter of first bore 22: 0.152"

Length of second bore 24: 1.3"

Diameter of second bore 24: 0.116"

It is noted that these dimensions and sizes are exemplary only and are not intended to limit the invention described herein.

A composite column of high temperature explosive is confined within the casing 12 as will now be described. A first column portion 28 of high temperature explosive is confined within the first bore 22 so as to abut the ignition device 16 and extends to the choke 26. The first column portion 28 is electrically or otherwise operationally connected with the ignition device 16, in a manner known in the art so that actuation of the ignition device 16 will cause ignition, or deflagration of the first column portion 28. A second column portion 30, also composed of high temperature explosive extends from the first column portion 26 toward a secondary explosive charge (not shown) which is affixed to the connector 20 of the casing 12. As with the first end of the casing, the second end is also sealed against the external environment. The explosive contained in both the first and second column portions 28, 30 is compacted to a maximum bulk density of approximately 1.0 grams/cc. The preferred high temperature explosive for placement in the columns 28, 30 is a mixture of 75% HNS and 25% NONA. HNS is coarser and more granular than NONA, which typically has the consistency of talcum powder. Thus, the use of a high proportion of HNS permits the explosive mixture to be disposed within the casing 12 such that the mixture provides for an explosive column of relatively consistent uniform density and, thus, greater workability. Other combinations of explosives, such as HNS and BRX work equally well, as well as other pure explosives. It is presently preferred, for workability reasons, that the amount of HNS exceed the amount of NONA or BRX in the mixture. In operation, an electrical charge is created across the SCB of the ignition device 16 via wires 18. Deflagration or ignition occurs through the first column portion 28 moving away from the ignition device 16. Deflagration is accompanied by a compression wave which acts upon the explosive in the first column portion 28 by creating friction, heat and rapid consolidation, or compression, of the explosive. The choke 26 acts as a restriction and shock reflector which encourages compression of the explosive. As the larger diameter first column portion 28 of explosive is rapidly compressed against the choke 26 by the hot gasses of the compressive shock wave, the explosive within the first column portion 28 is compacted into a high density plug. The now high density plug is then extruded through the choke 26 and into the smaller diameter second bore 24. As the extrusion occurs, the explosive crystals within the high density plug begin to shear and, along with the friction created at the choke by the extrusion, causes detonation of the second column portion 30 to occur from the point of the choke 26 forward toward the connector 20.

Localized shock reflections also occur at the interface of the choke, thus promoting the deflagration-to-detonation transition. Shock reflections occur as the propagating shock wave encounters the choke 26 and is at least partially bounced back toward the center of the casing 20. These reflections of the propagating shock wave results in an increased pressure within the casing which enhances and promotes a detonation.

Fig. 2 depicts a detonator assembly 10 after a detonation has occurred. The casing 12 has become deformed as a result of initiation of the explosives contained within. The first bore 22 has become radially expanded just forward of the location of the choke 26, as shown at 32. The radial expansion 32 has resulted from creation of the high density plug within that section of the first bore 22. The second bore 24 is also radially expanded. However, the radial expansion of the second bore 24 is conical in shape with the smallest diameter portion of the cone located proximate the choke 26 and the largest diameter portion of the cone located proximate the connector 20. This conical configuration is indicative of an explosion which has occurred along the length of the second bore 24.

It has been found that a detonator constructed substantially as described will function reliably to cause a detonation to deflagration reaction along a relatively short column length. This permits a detonator constructed in accordance with the present invention to be used in downhole tools such as perforating guns and casing cutters even where the borehole diameter is relatively restricted.

It is to be recognized that the principle of the deflagration-to-detonation choke is independent of the means of ignition. This disclosure describes ignition by use of an electrically- activated semiconductor bridge (SCB), but ignition can also be achieved by other sources such as hot-wires, thin films or percussion primers. While the invention has been described with respect to preferred embodiments thereof, those of skill in the art will recognize that it is not so limited numerous changes and modifications can be made without departing from the scope and spirit of the invention.

Claims

CLAIMSWhat is claimed is:

1. An electrical detonator comprising: a casing; a column of explosive charge disposed within the casing, said column having a first column portion with a first cross-sectional diameter and a second column portion having a second cross-sectional diameter which is smaller than the first diameter.

2. The detonator of claim 1 further comprising an ignitor which is operably associated with the first column portion of the column of explosive charge to cause ignition thereof.

3. The detonator of claim 1 wherein the column of explosive charge is substantially comprised of a high temperature explosive.

4. The detonator of claim 1 wherein the column of explosive charge is a high temperature explosive selected from the group of explosives consisting essentially of HNS, BRX NONA or

PYX.

5. The detonator of claim 1 wherein the column of explosive charge is a low temperature explosive selected from the group of explosives consisting essentially of PETN, RDX and HMX.

6. The detonator of claim 1 wherein the column of explosive charge comprises a mixture formed from HNS and NONA, wherein the amount of HNS in the mixture exceeds the amount ofNONA.

7. The detonator if claim 6 wherein the high temperature explosive comprises a mixture of approximately 75% HNS and approximately 25% NONA.

8. The detonator of claim 6 wherein the high temperature explosive comprises a mixture formed from HNS and BRX, wherein the amount of HNS in the mixture exceeds the amount of

BRX.

9. The detonator if claim 8 wherein the high temperature explosive comprises a mixture of approximately 75% HNS and approximately 25% BRX.

10. The detonator of claim 3 wherein the high temperature explosive substantially comprises HNS.

11. The detonator of claim 3 wherein the high temperature explosive substantially comprises BRX.

12. The detonator of claim 3 wherein the high temperature explosive substantially comprises NONA.

13. The detonator of claim 3 wherein the high temperature explosive substantially comprises PYX.

14. The detonator of claim 1 wherein the second cross-sectional diameter is 25%-30% smaller than the first cross-sectional diameter.

15. The detonator of claim 2 wherein the ignitor comprises a semiconductor bridge.

16. The detonator of claim 2 wherein the ignitor comprises a percussion primer.

17. A method of detonating a column of explosive, comprising the steps of: a) confining a first column of explosive within a first bore having a first diameter; b) confining a second column of explosive within a second bore having a second diameter which is less than the first diameter, the second bore being adjacent to and coaxial with the first bore; c) igniting a portion of the first column of explosive.

18. The method of claim 17 further comprising the step of extruding a portion of the explosive through a choke.

19. The method of claim 18 further comprises the step of allowing shock reflections at the choke to enhance or promote detonation.