|Publication number||US3212439 A|
|Publication date||Oct 19, 1965|
|Filing date||Nov 21, 1962|
|Priority date||Nov 24, 1961|
|Also published as||DE1181110B|
|Publication number||US 3212439 A, US 3212439A, US-A-3212439, US3212439 A, US3212439A|
|Original Assignee||Schlumberger Prospection|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (3), Referenced by (23), Classifications (12)|
|External Links: USPTO, USPTO Assignment, Espacenet|
M. REYNE Oct. 19, 1965 BLASTING CAPS CONTAINING ONLY SECONDARY EXPLOSIVE Filed NOV. 21, 1962 COMEUJ 770/V Mau/vc e Rey/7e INVENTOR United States Patent 3,212,439 BLASTING CAPS CONTAINING ONLY SECONDARY EXPLOSIVE Maurice Reyne, Bonrg-la-Reine, France, assignor to Societe de Prospection Eiectrique Schlumbcrger, S.A., Paris, France, a corporation of France Filed Nov. 21, 1962, Ser. No. 239,239
Claims priority, application France, Nov. 24, 1961,
880,063 4 Claims. (Cl. 102-28) The invention relates to blasting caps and, more particularly, to blasting caps for detonating shaped charges in a well bore.
It is known that conventional blasting caps, which contain a certain amount of explosive of the primary type such as lead-azide, are dangerous because of their sensitivity to shock forces which may cause detonation. Blasting caps are typically used to detonate a less sensitive secondary explosive such as a blasting cord comprised of cyclonite. While secondary explosives are just as powerful as the prim-aw explosives, fundamentally, they differ in being practically insensitive to shock. Therefore, according to regulations applicable to air transport, for example, detonators or blasting caps containing a primary explosive cannot be transported by airplane while secondary explosives may be transported by airplane. Since oil well services require equipment to be shifted over great distances, it is desirable to have a blasting cap which is relatively insensitive to shock and is absolutely safe and appropriate for air-lift. This safety will also be appreciated by technicians at the time of use of the blasting cap.
An object of the invention is to provide a blasting cap which contains only an explosive of the secondary type which is not particularly sensitive to shock.
According to another feature of the invention, the explosive is in the form of grains which are sized between three hundred and five hundred microns.
A detonator according to the invention comprises, within a closed container tube of appropriate thickness, a certain amount of secondary explosive and an igniter made of an electrical resistor and an inflammable explosive, and is characterized by the fact that the portion of said tube containing the secondary explosive has a bore of eight millimeters in diameter and an inner length of thirty-seven millimeters, these dimensions being determined with an accuracy within ten percent, and by the fact that said explosive is of the secondary type, with a reaction zone" whose length is less than three millimeters.
Owing to this arrangement, a confined enclosure of minimum size provides conditions under which a flame ignites a secondary explosive which ignition is transformed into a detonation. One thus achieves a safety detonator which is relatively insensitive to shock.
Secondary explosives are classified according to their sensitiueness, which is an essentially qualitative datum, yet also, and in the same order, according to the reciprocal of the length of their reaction zone. Reference may be made to The Science of High Explosive, by M. A. Cook, 1958, Reinhold Publishing Corporation, and in particular pages 123 and 124 and Table 7.3 on page 147 for further details. However, according to this study, the reaction zone of an explosive is the thickness in which the chemical reaction develops at the maximum velocity for the explosive considered. The lengths of reaction zones of various explosives are known; they were measured by means of ultra-fast photography and electronics. Thus, one knows that hexogen exhibits a reaction zone of one millimeter, Whereas coarse grained TNT exhibits a reaction zone of fifteen millimeters.
Characteristics and advantages of the invention shall a depth ranging between 5 to millimeters.
be set forth, again by the description to follow supplied merely as an example, reference being taken to drawings attached where:
FIG. 1 illustrates a cross-section of a detonator according to the invention; and
FIG. 2 illustrates a cross-section of a modified detonator according to the invention.
As shown in FIG. 1, a steel tube 10 is closed at one end 12 by a thin metal lid 14 and at its other end 16 by a conical metal liner 18. End 12 is machined down at 22 and is received in a hollow metal adapter 24 Whose upper portion is provided with a counter bore 26 into which is introduced the insulating wlad 28 of an igniter of the type described in the United States Patent No. 2,681,701, issued to M. Schlumberger. A bore 30 in adapter 24 is traversed by the igniter needle 32 filled with black powder. An electrical resistor 34 is connected across the insulating wad 28 to a conductor 36, whereas its other end is linked at 38 to the outer conducting wall of needle 32. Conductor 37 attached on the periphery of adapter 24, establishes, by means of lid 14 being in contact with the metal wall of needle 32, the return circuit for the heater current of resistor 34.
The lower end 40 of needle 32 goes through lid 14 to The interior 42 of tube 10 is filled with pentrite and compressed in the tube 10 by a ram under a low Weight (150 kilograms) inserted into end 16. Dimensions of the ex plosive are determined by the use of two sieves whose apertures are three hundred and five hundred microns. The lower portion 16 of tube 10 is machined down at 44 where a cylindrical plug 46 is attached. A nylon screw 48 extends axially into a recess 47 provided in the plug 46. An opening 50 is drilled transversely through plug 46 and a blasting cord 52 can be introduced into this opening and fixed therein between the base of plug 46 and screw 48.
If a voltage of a suitable value is applied across the extremities of conductors 36 and 37, resistor 34 heats up to a point where the black powder ignites and a flame is introduced rather deeply into the pentrite (which is in a pulverulent condition adjacent to the needle-igniter 32). This flame, in the first place, causes the pentrite to be set afire. But, since the walls of tube 10 are of a considerable thickness, say thnee millimeters, a suitable confinement is secured and as the combustion progresses within tube 10, it progressively develops into a deflagration then into a detonation which detonates the blasting cord 52.
Systematic measurements and testing were performed on a detonator as described in FIG. 1. Using various especially sensitive secondary explosives, that is explosives exhibiting reaction zone lengths helow three millimeters, such as pentrite, hexogen and tetryl, the inner diameter of tubes 10 was changed from one test to another. The critical value was determined experimentally from these tests that below eight millimeters, the detonation phenomenon developed poorly. Thereafter, the length of this same tube 10 was reduced progressively until detonation no longer occurred. The critical length was thus determined, which is approximately equal to thirtyseven millimeters. Of course, these figures represent merely an order of magnitude with an accuracy of some ten percent, and one would still obtain a suitable operation by reducing very slightly the bore of the tube While the length is being increased. In practice, only smallsized detonators offer some interest for oil field work.
Attempts were also made to use medium sensitivity secondary explosives, that is, explosives exhibiting reaction zone lengths exceeding thnee millimeters. Thus,
after having obtained satisfactory results with hexogen,
pentrite and tetryl, unfavorable results were yielded with hexyl, tolite and me linite, and even while using tube diameters notably greater than the eight millimeters mentioned above.
The grid sizes and the compression of various explosives withheld for their good working were then changed. Using two sieves exhibiting apertures either greater than three hundred microns or smaller than one hundred and fifty microns in order to select the particles of explosive according to their size, high failure rate re sulted, whereas satisfactory detonation is obtained if two sieves with apertures between three hundred and five hundred microns are used. Similarly, increasing the compression rate of the explosive beyond one hundred and fifty kilograms prevents the flame from penetrating appropriately into the mass, which in turn results in failures.
In the course of the operation of the detonator of FIG. 1, the conical liner 18 subsides under the effect of the detonation of pentrite filling volume 42, and thus improving the detonator performances still further, contributes to the detonation of the blasting cord 52. Such a shockproof detonator may be successfully employed in retrievable carrier perforators, but only in the case of shallow wells since pentrite does not perform properly at temperatures above 120 C. In order to obtain a blasting cap for use in greater depths, one would resort to hexogen which performs properly at 180 C.
Adapter 24 can also be provided with an aperture 25 to establish an opening between duct 30 of the detonator and the inner volume of the carrier perforator. Hence, if
.the carrier leaks fluid under pressure within the borehole,
such fluid would gain admittance to the blasting cap to desensitize it.
Another embodiment of a detonator according to the invention is shown in FIG. 2.
According to FIG. 2, a steel tube 54 with walls at least three millimeters thick provides a chamber 42 which is filled with hexogen under conditions of grid sizes and compression as described above. A thin-walled case 56 filled with a thermite mixture 58 such as a weakly compressed homogeneous mixture of aluminum and barium peroxide powder in roughly stoichiometric proportions (Al: 20% and BaO: 80%). An electrical resistor 60 is connected by two conductors 62 and 64 through glass bead seals 66 and 68 to an outer power supply. At the end of the detonator, both conductors 62 and 64 are mechanically secured by an insulating wad 69. Case 56 is attached to a cover 70 which rests on a shoulder 72 pro- ,vided inside the tube 54. Cover 70 is fastened by welding at 74 attaching the thinned down inner wall 76 of said tube 54. At the lower portion of chamber 42 is an amount of hexogen explosive, the chamber terminating with a bottom wall 78 approximately one millimeter thick.
-On the other side of wall 78, tube 54 is terminated by a .82, the end of the blasting cord 84 in contact with the -wall 78.
The thermite powder contained in case 56 offers particularly remarkable safety features and the cover 70 welded totube 54 and glass beads 66 and 68 provide a fluid-tight seal which also will not leak gas. Under these conditions, the combination in leak-proof receptacle of a case 56 containing thermite powder 58 and of a tube 42 of a suitable length and diameter filled with hexogen, achieves a safety detonator which is indilfcrent to shock and, moreover, indifferent to static charges, to spurious currents and to hot gases under pressure.
The invention is, of course, not limited to the embodiments described above which have merely been supplied for information purposes, but which, on the contrary, within the scope of the invention, may form the subject of various alternatives.
Thus, either with an ignition needle or with a thermite booster charge, one may use tetryl instead of hexogen or pentrite in order to achieve detonators according to the invention. But like pentrite, tetryl provides for detonators to be used at temperatures up to less than C. corresponding to wells of small depths.
One can also envisage different ways to establish the contact between the secondary explosive, mass and the blasting cord. Thus, for instance, in order to bring about the detonation of explosive charges intended to clear boring tubes, the ends of several blasting cords forming said charges are paralleled around the body of a detonator according to the invention.
Furthermore, one can, of course, use other means as setting or welding in order to fasten among them the various elements making up a detonator according to the invention.
1. A blasting cap for use with oil well perforators consisting of: a hollow steel tube with a wall thickness of at least three millimeters, an inner diameter dimension of eight millimeters and a length dimension of thirty-seven millimeters, said diameter and length dimensions being held to an accuracy of ten percent, said tube being substantially filled with a secondary explosive having a reaction zone of less than three millimeters, a closure for one end of said tube including an electrical igniter and a closure for the other end of said tube including a thin wall portion for transmitting a detonation wave therefrom.
2. The apparatus of claim 1 wherein said igniter includes a needle containing black powder, the tip of said needle extending into said explosive for a depth ranging between 5 and 10 millimeters.
3. The apparatus of claim 1 wherein said explosive has a grain size between three hundred and five hundred microns.
4. The apparatus of claim 1 wherein said explosive is selected from a group consisting of hexogen, pentrite and tetryl.
I References Cited by the Examiner UNITED STATES PATENTS 2,681,701 6/54 Schlumberger l0228 XR 2,891,477 6/59 Swanson l0228 2,981,186 4/ 61 Stresau l0228 SAMUEL FEINBERG, Primary Examiner.
BENJAMIN A. BORCHELT, Examiner,
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|International Classification||F42B3/12, F42C19/12, F42B3/195|
|Cooperative Classification||F42B3/195, F42C19/12, F42B3/124, F42B3/125|
|European Classification||F42B3/195, F42C19/12, F42B3/12F, F42B3/12D|