US3358780A

US3358780A - Cumulative shaped charges

Info

Publication number: US3358780A
Application number: US457978A
Authority: US
Inventors: A A Venghiattis
Original assignee: Dresser Industries Inc
Current assignee: Dresser Industries Inc
Priority date: 1965-05-24
Filing date: 1965-05-24
Publication date: 1967-12-19
Anticipated expiration: 1984-12-19

Description

Dec. 19, 1967 A. A. VENGHIATTIS 3,358,780

CUMULATI VE SHAPED CHARGES Filed May 24, 1965 INVENTOR. ALEXIS AVENGHIATTIS BY d/g M7 41 ATTORNEY United States Patent 3,358,780 CUMULATIVE SHAPED CHARGES A. A. Venghiattis, Houston, Tex., assignor to Dresser Industries, Inc, Dallas, Tex., a corporation of Delaware Filed May 24, 1965, Ser. No. 457,978 3 Claims. (Cl. 1754.5)

ABSTRACT OF THE DISCLGSURE This invention relates to lined shaped charges and more particularly, to a novel method and apparatus for increasing the depth of penetration of the jet formed by a lined shaped charge.

In general, a lined shaped charge is comprised of a block of explosive material having a conical cavity formed in its front face. A liner of non-explosive material covers the face of the cavity. Upon detonation of the shaped charge, the explosive forces traveling in the direction of the liner transform part of the liner material into a high velocity, particle laden stream which travels along the axis of the cavity. This stream is commonly called the jet and is capable of penetrating many inches of steel.

Lined shaped charges have been used extensively by the petroleum industry for perforating operations. The jet penetrates the steel casing lining the borehole, the cement sheath surrounding the casing, and also forms a generally cylindrical hole in the formation. It is very desirable that the hole formed in the formation be as deep as possible so that the fluid in the formation may drain into the borehole. Accordingly, it has always been a prime aim in the lined shaped charge art to increase the penetrating capabilities of shaped charges.

It is well known in the art of explosives that the depth of penetration of a lined shaped charge is proportional to the length of the jet produced. One way to increase the length of the jet is to deepen the conical cavitythat is, to decrease the apex angle of the cavity. However, if the included angle is decreased below 25, the Monroe elfect disappears and the charge explodes like a petard. Another method of increasing the length of the jet of a lined shaped charge is to build a charge of a larger caliber. In creasing the caliber produces not only a deeper hole, but also one of a larger diameter. However, although the idea of making a larger caliber shaped charge to increase the depth of penertation sounds relatively simple, it presents many serious drawbacks. For example as the caliber of the charge increases, the size and weight of the weapon firing the charge has to be correspondingly increased. Also, the weight of the explosive of the charge increases roughly as the cube of the caliber, bringing about serious manufacturing problems in general. In the petroleum industry in particular, where lined shaped charges are often shot from within a hollow carrier gun, the problems of resistance of materials become rapidly insurmountable. Moreover, it has been determined that increasing the diameter of the hole in the formation above a required minimum does not correspondingly improve the well flow index.

Accordingly, it is an object of the present invention to provide a novel method whereby the depth of penetration of a formation surrounding a borehole is substantially 3,358,780 Patented Dec. 19, 1967 increased without increasing the caliber of the lined shaped charge used in the perforating operation.

It is another object to provide a lined shaped charge unit in which the depth of penetration of the jet formed by the unit is substantially greater than the depth of penetration of prior art lined shaped charges of the same caliber.

In accordance with the present invention, a plurality of lined shaped charges are coaxially aligned and are then detonated in timed sequence so that the jet of a later detonated shaped charge closely follows the jet of the preceding detonated charge. It has been found that by practicing the above method, the depth of penetration resulting from the correlative jets is substantially increased over the depth of penetration resulting from the jet of a charge of corresponding caliber.

A lined shaped charge unit of the present invention is generally formed of a pair of lined shaped charges which are coaxially aligned in spaced relation in a common carrier. Means are provided for detonating the shaped charges sequentially. The detonating means may be formed of a common booster having two lengths of Primacord attached to the charges. The lengths of Primacord are controlled so that the detonation of the second charge occurs after such a time interval that the jet of the later detonated charge closely follows the jet of the first detonated charge. It has been found that by sequentially detonating the shaped charges in such a manner that the tip of the jet of the second detonated charge closely follows the tail of the jet of the first detonated charge, the depth of penetration is substantially increased. One reason for this may be that the preceding jet opens the way for the second jet, thus enabling it to follow without having to overcome the resistance of the material forming the target.

It is a further object of the present invention to provide a method of increasing the depth of penetration of a lined shaped charge unit by aligning a number of spaced shaped charges coaxially and then detonating the charges sequentially so that the jet of a sequentially detonated charge closely follows the jet of a prior detonated charge.

It is a further object to provide a lined shaped charge unit comprised of two lined shaped charges which are coaxially aligned in spaced relation in a common carrier and provided with detonating means which sequentially detonates the two shaped charges in such a manner that the jet of the second detonated charge closely follows the jet of the first detonated charge, there being substantially no interference between the formation of the two jets.

Further objects and objects relating to details of manufacture and use of the method and apparatus are set forth in the detailed description about to follow in which the present invention, both as to its organization and manner of operation, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic view of a shaped charge unit in accordance with the present invention showing the relationship of spacing and length of detonating means.

FIG. 2 is a cross sectional view of a borehole having a perforator incorporating a shaped charge unit of the present invention.

FIG. 3 is a schematic view showing the relationship of the two jets after detonation of the second charge.

FIG. 4 is an end view of the shaped charge unit showing one method of attaching the detonating means.

FIG. 5 is a view similar to FIG. 4 showing an alternative method of attaching the detonating means.

It has been found that by coaxially aligning a plurality of shaped charges in spaced relation and then detonating the shaped charges in timed sequence with the jet from a prior detonated charge preceding the jet of a later detonated charge without interfering with the formation of the jet of the later detonated charges and yet at the same time, having the tip of the jet of the later detonated charge closely follow the tail of the prior detonated charge, that a greater depth of penetration results.

As previously pointed out, the depth of penetration resulting from the detonation of a lined shaped charge is directly related to the length of the jet produced by the shaped charge and the length of the jet depends to a large extend upon the caliber of the charge. With the apparatus according to the present invention, the depth of penetration is substantially increased without correspondingly increasing the caliber of the charge.

In accordance with the apparatus of the present invention, a plurality of lined shaped charges are coaxially aligned in spaced relation. The charges are then detonated in timed sequence whereby the tail of the jet produced by a priorly detonated charge is closely followed by the tip of a subsequently detonated charge. It appears that the jet of the initially detonated charge clears the way for the jet of the succeeding detonated charge. Such action results in the initial travel of the succeeding jet having a free flight, thereby permitting the succeeding jet to conserve its energy until it contacts the end of the hole in the formation formed by the prior jet. Since the succeeding jet has almost all of its energy intact until it reaches the deepest point of penetration resulting from the prior jet, it will continue its flight and form an additional hole in the formation. Such action results in increased penetration.

As previously pointed out, the common types of liners used with lined shaped charges result in a slug or carrot which follows the main stream of the jet. The carrot usually lodges in the end of the hole formed by the jet and is detrimental to flow. Efforts have been directed to eliminating the carrot. One such method is fully disclosed in my copending application S.N. 159,092, now US. Patent No. 3,255,659. In accordance with this application, the liner for the shaped charge is comprised of finely divided metal particles which cohere to one another and to the cavity solely by pressure. With such liner, the carrot is virtually eliminated.

Although it is felt that the present method will improve the depth of penetration of any type of lined shaped charge, it is preferable that the liner be non-carrot forming so that the succeeding jet does not have to penetrate the carrot before it engages new target material. If the liner of the shaped charge is carrot forming, it may be desirable to divert the carrot by means well known in the art, and to delay the detonation of the succeeding charge until the carrot has been diverted.

One way of detonating the shaped charges in timed sequence is to use two commonly ignited lengths of Primacord having one attached to each charge.

Assuming that two lined shaped charges having noncarrot forming liners are used, the charges may be spaced a distance d with a length l of Primacord to the forward charge and l' of Primacord to the rearward charge such that,

In such a situation there will be critical factors:

(1) The coaxiality of the two charges, (2) The distance d, (3) Al being greater than Zero,

which can all be controlled without excessive difiiculty. The distance d must be such as to prevent the excitation of the rearward charge (for example, through the air gap between the two charges) before the rearward charge has been detonated by its Primacord and before its jet has been formed. Al must provide the time lag required for the first jet to be completely formed before the beginning of the second one. The values of d and Al are, of course, specific to each type of charge. In the case of one charge which takes approximately 20 to 25 microseconds for the formation of the jet, d would be of an order of millimeters and Al of millimeters. It should be pointed out that two charges combined in accordance with this invention would have an overall diameter smaller than one and three-quarter inches and a total load of about 35 grams of explosive, while a single charge that would give the same depth penetration would have a diameter of at least three and one-half inches with a load of approximately 150 grams of explosive.

Reference will now be made to the drawings, and in particular to FIG. 2 where a shaped charge unit 10 of the present invention is depicted. As can be seen, the shaped charge unit 10 is in the form of a capsule chargethat isthe shaped charge unit is enclosed in an individual pressure resistant housing 12. A number of such capsule units may be attached together by a pair of structural members 1414 to form what is commonly called in the art, a shaped charge capsule gun. If desired, a number of shaped charge units 10 may be enclosed in a pressure resistant housing which is known in the art as a hollow carrier gun. In either event, the shaped charge unit 10 is lowered into a borehole 16 by means of a cable (not shown) and positioned at the appropriate depth in the borehole 16 opposite the formation which it is desired to have perforated.

The shaped charge unit 10 is formed of a pair of shaped charges 20-22 which are coaxially aligned in a common carrier 24. A spacer 25 separates the two charges 20-22 by the distance d (see FIG. 1) maintaining the shaped charges 20-22 in spaced relation.

The two shaped charges 20-22 may be similar in construction and size. However, if desired, the forward charge may be designed to produce a large hole in which the outgoing portion of the first jet would not interfere with the ingoing portion of the second jet. Each one of the shaped charges is formed of a cup-shaped case 26 having an axial aperture 28. Positioned in the cup-shaped case 26 is a block of high explosive 30 cast or otherwise consolidated or pelletized to proper density, as is well known in the art, and may be formed of the type of high explosive substances normally used in lined shaped charges such as RDX explosive (desensitized cyclotrimethylenetrinitramine). The block of explosive 30 has a conical cavity 32 formed in its front face. The block of explosive 30 has an axial aperture 34 which is smaller in diameter than the aperture 28 of the case 26 whereby a portion 36 of the block of explosives is exposed by the aperture 28 at the rear of the case 26.

Covering the face of the conical cavity 32 is a liner 37 formed of non-explosive material. Upon detonation of the shaped charge, the detonation wave progresses from the point of initiation, progressively encountering the cavity 32 and the liner 37. A collapsing and progressive disintegration of the liner 37 transforms the material of the liner 37 into a high velocity, particle laden jet which travels along the axis of the cavity 32. A portion of the most common liners is not disintegrated and follows bebind the faster jet stream as a relatively slow moving slug or carrot. It is desirable that a non-carroting forming liner such as that disclosed in my copending application S.N. 159,092, now US. Patent No. 3,255,659, be used to eliminate the carrot. As previously specified, such a liner is generally comprised of finely divided copper particles which cohere to one another and to the cavity solely by pressure.

Detonating means are provided to detonate the charges 20-22 in timed sequence. One such means is shown in FIG. 1 where it can be seen that a booster 38 is connected to two pieces of Primacord 40-42. Primacord 40 is attached to the exposed portion 36 of the forward shaped charge 20 and Primacord 42 is attached to the exposed portion 36 of the rearward shaped charge 22. Primacord has a rather consistent precise time of detonation; therefore, the two lengths of Primacord 40-42 are so adjusted that the detonation of Primacord 40 reaches the exposed portion 36 of the forward charge 20 and detonates it prior to the time that the detonation of the Primacord 42 reaches the exposed portion 36 of the rearward charge 22. The lengths of the Primacord may be as previously discussed.

FIGS. 4 and 5 show alternate methods of attaching the Primacord to the exposed portion 36 of the explosive 30 to cause excitation of the charge.

It has been found that if a non-carrot forming liner is utilized, the timing sequence can be such that the rearward charge is detonated relative to the forward charge so that the tip of the jet formed by the rearward charge closely follows the rear of the jet of the forward charge. By having the aperture 34 in the forward charge 20, a path is cleared for the jet formed by the rearward charge 22 thus eliminating the necessity of the second jet expending any of its energy piercing the case of the forward charge.

The cumulative action of the jets is illustrated in FIG. 3. As can be seen, the shaped charge unit is positioned in the borehole 16 having a steel casing 50, a cement sheath 52 and the surrounding formation 54. The object of a perforator in oil field operations is to pierce the steel casing 50, cement sheath 52 and produce a hole 56 in the formation 54. It is desirable that the hole 56 extend as deep as possible to provide the best drainage of the formation reservoir. Also, the deeper the hole, the better the results from fracturing and other well treating processes. As can be seen, the jet from the first jet 58 pierces the steel casing 50, cement sheath 52 and produces the hole 56 in the formation 54. The action of the first jet 58, in effect, clears a path for the second jet 60. The initial portion of the travel of the second jet 60 is in free flight and its energy is conserved until it contacts the bottom of the hole formed by the first jet. Therefore, the second jet expends its energy in forming a further hole 62 (shown in dotted lines in FIG. 3) in the formation resulting in a deeper penetration. FIG. 3 also illustrates the effect resulting from designing the front charge to produce a larger diameter hole than the rear charge. As can be seen, the jet 58 is larger in diameter than the jet 60. Therefore, the outgoing portion of the jet stream from the front charge does not interfere with the ingoing portion of the jet of the rear charge and more effectively clears a path for the second jet.

As can be seen from the foregoing, the cumulative effect of having two spaced coaxially aligned lined shaped charges detonated in timed sequence produces a deeper penetration of a formation without increasing the caliber of the charges used.

While a preferred embodiment of the invention has been illustrated and described, it is to be understood that the invention is capable of modifications such as come within the scope of the appended claims.

I claim:

1. A shaped charge unit for perforating a well casing comprising:

a pair of coaxially aligned, lined shaped charges,

an axial aperture in at least the forward of said charges;

a carrier for said shaped charges, said carrier maintaining said shaped charges in coaxial alignment;

means maintaining the shaped charges in spaced relation in said carrier;

means for detonating said shaped charges formed of a common booster and two pieces of Primacord, one piece of Primacord extending to each shaped charge, the length of each Primacord being such that the detonation of the rear charge is retarded a sufiicient length of time so that the jet of the rear charge begins to form as the jet of the forward charge is about to be completed, whereby the tip of the jet of the rear charge closely follows the end of the jet of the first charge.

2. A shaped charge unit for comprising:

a pair of shaped charges;

a carrier for said shaped charges, said carrier coaxially aligning said shaped charges;

means maintaining the shaped charges in spaced relation in said carrier;

means for detonating said shaped charges formed of a common booster and two pieces of Primacord, one piece of Primacord extending to each shaped charge, the length of each Primacord being such that the detonation of the rear charge is retarded a sufiicient length of time so that the jet of the rear charge begins to form as the jet of the forward charge is about to be completed, whereby the tip of the jet of the rear charge closely follows the end of the jet of the front charge.

3. A shaped charge unit for comprising:

a forward shaped charge and a rear shaped charge, said forward shaped charge being designed to produce a larger diameter hole than said rear shaped charge, said forward shaped charge having an axial aperture to facilitate passage for the jet from said rear shaped charge;

means maintaining the shaped charges in spaced relation in said carrier;

means for detonating said shaped charges in time sequence such that the detonation of the rear charge is retarded a suificient length of time so that the jet of the rear charge begins to form as the jet of the forward charge is completed, whereby the tip of the jet of the rear charge closely follows the end of the jet of the first charge.

perforating a well casing perforating a well casing References Cited UNITED STATES PATENTS 2,843,041 7/ 1958 Stewart -35 2,946,283 7/1960 Udry 175-45 2,984,307 5/1961 Barnes 1754.6 X 3,215,074 11/ 1965 Robinson et a1. 175-4.5

FOREIGN PATENTS 999,974 10/ 1 France. 1,002,092 10/ 1951 France. 1,231,003 4/ 1960 France.

CHARLES E. OCONNELL, Primary Exlaminer. DAVID H. BROWN, Assistant Examiner,

Claims

2. A SHAPED CHARGE UNIT FOR PERFORATING A WELL CASING COMPRISING: A PAIR OF SHAPED CHARGES; A CARRIER FOR SAID SHAPED CHARGES, SAID CARRIER COAXIALLY ALIGNING SAID SHAPED CHARGES; MEANS MAINTAINING THE SHAPED CHARGES IN SPACED RELATION IN SAID CARRIER; MEANS FOR DETONATING SAID SHAPED CHARGES FORMED OF A COMMON BOOSTER AND TWO PIECES OF PRINACORD, ONE PIECE OF PRIMACORD EXTENDING TO EACH SHAPED CHARGE, THE LENGTH OF EACH PRIMACORD BEING SUCH THAT THE DETONATION OF THE REAR CHARGE IS RETARDED A SUFFICIENT LENGTH OF TIME SO THAT THE JET OF THE REAR CHARGE BEGINS TO FORM AS THE JET OF THE FORWARD CHARGE IS ABOUT TO BE COMPLETED, WHEREBY THE TIP OF THE JET OF THE REAR CHARGE CLOSELY FOLLOWS THE END OF THE JET OF THE FRONT CHARGE.