Ballistic systems having an impedance barrier

A shaped charge assembly for use in a perforating gun that comprises a shaped charge holder, two or more shaped charges, slots formed to receive the shaped charges, and an impedance barrier disposed between adjacent shaped charges. The impedance barrier can comprise a gap formed in the shaped charge holder, where the impedance barrier runs across the shaped charge holder. The impedance barrier can be a void formed on the surface of the shaped charge holder, or can include shock attenuating material therein, such as wood, cork, cotton, polymeric materials, and combinations thereof, to name a few.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a ballistic system including an impedance barrier. Yet more specifically, the present invention relates to a perforating gun system whose shaped charges are held in a medium, and where a gap is formed within the medium between each adjacent shaped charge.

2. Description of Related Art

Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore, and the casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.

Perforating systems40typically comprise one or more perforating guns42strung together, these strings of guns42can sometimes surpass a thousand feet of perforating length. Included with the perforating guns are shaped charges44that typically include a housing, a liner, an initiator, and a quantity of high explosive inserted between the liner and the housing. A detonating cord46attached to each shaped charge sequentially actuates the initiator within each shaped charge. The perforating systems are generally lowered into a wellbore on wireline or tubing48where the initiation of the perforating gun detonation is transmitted through the wireline or tubing48. Firing heads50are typically included on the perforating guns for receiving the transmitted detonation signal from the surface52and in turn igniting the detonation cord46.

When the high explosive within the shaped charge44is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge at very high velocity in a pattern called a “jet”54. The jet penetrates the casing, the cement and a quantity of the formation56thereby forming a conduit58by which the hydrocarbons entrained within the formation may be drained into the wellbore for production at the wellbore surface.

In addition to the perforating jet formed by detonation of the shaped charges, the charges also produce shock waves that emanate into the formation and along the perforating gun42. These shock waves can be transmitted onto other shaped charges prior to or during their detonation and interfere with the trajectory of the perforating jet54. This jet interference can in turn create curved perforations and reduce the overall depth of the perforations58. Curved and shorter perforations present an undesired condition since this can reduce the production capability of hydrocarbon bearing formations. Therefore, there exists a need for an apparatus and method capable of impeding the shock and/or vibration transmitted between shaped charges.

BRIEF SUMMARY OF THE INVENTION

The present invention involves a shaped charge assembly comprising, a shaped charge holder, bores formed on the shaped charge holder, and an impedance barrier disposed between each bore formed in the shaped charge holder. The shaped charge holder may be a perforating gun tube, a perforating gun body, and a shaped charge carrier. The impedance barrier can be comprised of a void formed in the shaped charge holder. Moreover, an impedance material can be disposed in the void where the impedance material might consist of wood, cork, rubber, cotton, plastic, polymeric materials, wool, foam, other shock absorbing materials, and combinations thereof. The void may comprise a groove formed along the outer surface of the shaped charge holder. The impedance barrier may optionally comprise a series of rings axially disposed along the length of the shaped charge holder. The impedance barrier might instead be comprised of a spiral pattern interconnected with axial grooves or alternatively might comprise a spiral pattern formed on the shaped charge holder. Shaped charges can also be disposed within the bores of the shaped charge assembly.

Also included with the shaped charge assembly can be a firing head, a detonating cord, and an actuating member. The actuating member could be a wireline conveyance member or a tubing conveyed member.

An alternative embodiment of a shaped charge assembly is included with this invention. The alternative embodiment comprises a shaped charge holder, bores formed on the shaped charge holder, shaped charges disposed in each bore, and an impedance barrier formed between each bore formed in the shaped charge holder. The impedance barrier of this alternative embodiment attenuates the shock wave imparted during detonation of each shaped charge and prevents the shock wave produced by one shaped charge from affecting the performance of other shaped charges. The shaped charge holder can be a perforating gun tube, a perforating gun body, or a shaped charge carrier. The impedance barrier of the alternative embodiment can be comprised of a void formed in the shaped charge holder. Also in the alternative embodiment, an impedance material can be disposed in the void. The impedance material can be wood, cork, rubber, cotton, plastic, polymeric materials, wool, foam, other shock absorbing materials, and combinations thereof. The void can comprise a groove formed along the outer surface of the shaped charge holder. The impedance barrier of the alternative embodiment may comprise a spiral pattern formed on shaped charge holder. The alternative shaped charge assembly can further comprise shaped charges disposed within the bores. This embodiment of a shaped charge assembly can further comprise a firing head, detonating cord, and an actuating member. The actuating member can be a wireline conveyance member or a tubing conveyed member.

DETAILED DESCRIPTION OF THE INVENTION

The present device disclosed herein addresses the problem of shock wave interference in ballistics systems by providing an impedance barrier between the shock producing sources.FIG. 1ademonstrates an embodiment of a shaped charge assembly having an impedance barrier as disclosed herein. The shaped charge assembly5ofFIG. 1acomprises a shaped charge holder12with bores8formed thereon with an impedance barrier10positioned between the bores8. The shaped charge holder12can be any device used to hold and retain shaped charges, such as a gun body, gun tube, or any other type of carrier used for carrying and holding shaped charges. The shaped charge holder12may alternatively be a unibody type, such as a single piece or single body. The bores8on the shaped charge holder12should be formed to receive and hold therein the perforating shaped charges. Accordingly, when fully assembled, the shaped charge assembly would further include shaped charges within the bores8and the presence of the impedance barrier would isolate these shaped charges from the shock waves produced by other shaped charges. Moreover, the impedance barrier as disclosed herein is capable of isolating shaped charges from other transient shock waves that might be transmitted along a perforating gun system.

As shown in the embodiment ofFIG. 1a, the bores8are generally aligned along the length of the shaped charge holder12. Thus to provide an isolating function between the bores8, the impedance barrier10is situated between each of the bores8in a series of rings formed along the length of the shaped charge holder12. However the pattern of the impedance barrier10is not limited to the annular form ofFIG. 1a, but can include any configuration necessary for isolating shaped charges from the shock of other shaped charges. The shaped charge holder12cofFIG. 1dalso has bores8aligned along its length, however the corresponding impedance barrier10chas a spiral or helical formation along the outer surface of the charge holder12c.

Alternative embodiments illustrating other impedance barrier configurations are shown inFIGS. 1band1c. InFIG. 1ba shaped charge holder12is shown where the bores8are disposed in a staggered arrangement along the length of the shaped charge holder12. The resulting shape of the impedance barrier10ais a series of interlocking grooves for isolating adjacent shaped charges from one another. Similarly, the shaped charge carrier12ofFIG. 1calso includes a staggered bore pattern, here however the shaped of the impedance barrier10chas the form of a helical26that spirals along the length of the shaped charge holder12. Interconnecting verticals24axially connect the helical26to form a lateral barrier between bores8that are disposed at roughly the same axial location on the shaped charge holder10bbut that are radially spaced apart.

As shown in a cut-away view inFIG. 2, the impedance barrier can comprise a groove11formed on the outer surface of the shaped charge holder12. The groove11can be etched, cut, or forged into the holder12. The cross sectional contour of the impedance barrier10is not limited to the rectangular shape as shown inFIG. 2, but can have other profiles such “U”-shaped, triangular, or oval. The barrier10however should comprise some form of discontinuity of material for terminating and/or absorbing any energy waves that might be transmitted along the length of the charge holder12. Moreover, the barrier need not be open at the outer surface of the holder12, but instead can be a void formed within the body of the holder12beneath its surface. As shown inFIG. 2Aattenuating type materials1can be included within the groove11A to form the impedance barrier10A of the holder12A. The materials1can be wood, cork, rubber, cotton, wool, plastic, polymeric materials, foam, other shock absorbing materials, or combinations thereof.

FIG. 3depicts a perspective exploded view of an embodiment of a perforating device4comprising ends18, a charge carrier22, a washer20, shaped charges14, and an optional orienting weight16. The charge carrier22is used for holding and retaining the associated shaped charges14prior to and during detonation of the shaped charges14. Similar to the shaped charge holders12ofFIGS. 1a-1d, the charge carrier22includes bores8formed therein perpendicular to the axis30of the charge carrier22. The bores8extend through the charge carrier22, where the inner peripheries of the bores8are profiled to match the profile of the outer periphery of the shaped charges14. Accordingly each bore8engagingly receives a shaped charge14within its inner periphery and retains the shaped charge14therein prior to and during use. While the bores8shown are aligned at roughly the same radial location on the charge carrier22, the bores8can be formed at any radial location on the carrier22. As with many perforating systems, the shaped charges14can be positioned within the perforating device4to detonate at all radial locations around the charge carrier22. An embodiment of the impedance barrier10dis shown on the charge carrier22between each bore8. Here the impedance barrier10dis a series of grooves cut or formed perpendicular to the axis30of the charge carrier22.

The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. For example, the invention described herein is applicable to any shaped charge phasing as well as any density of shaped charge. Moreover, the invention can be utilized with any size of perforating gun. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.