1. Field of Invention
The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a perforating system. Yet more specifically, the present invention relates to a shaped charge having a modified boss for use in a perforating gun system.
2. Description of Prior 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. 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 systems typically comprise one or more perforating guns strung together, these strings of guns can sometimes surpass a thousand feet of perforating length. In FIG. 1 an example of a perforating system 4 is shown. For the sake of clarity, the system 4 depicted comprises a single perforating gun 6 instead of a multitude of guns. The gun 6 is shown disposed within a wellbore 1 on a wireline 5. The perforating system 4 as shown also includes a service truck 7 on the surface 9, where in addition to providing a raising and lowering means, the wireline 5 also provides communication and control connectivity between the truck 7 and the perforating gun 6. The wireline 5 is threaded through pulleys 3 supported above the wellbore 1.
Perforating guns typically include a cylindrical gun strip 16 coaxially housed within a gun body 14. Shaped charges 8 are provided within the gun strip 16 and aimed generally perpendicular to the axis of the wellbore 1. FIG. 2 provides an example of a shaped charge 8 that includes a housing 18, a liner 22, a quantity of high explosive 24 inserted between the liner 22 and the housing 18, and a booster charge 26 adjacent the base of the high explosive 24.
The shaped charges 8 are typically connected to a detonating cord 27 which is affixed to the shaped charge 8 by a case slot 25 proximate to the booster charge 26. Igniting the detonation cord 27 creates a compressive pressure wave along its length that initiates the booster charge 26 that in turn ignites the high explosive 24. When the high explosive 24 is detonated, the force of the detonation collapses the liner 22 and ejects it from one end of the charge 8 at very high velocity in a pattern called a “jet” 12. The jet 12 perforates the casing and cement lining the wellbore 1 and creates a perforation 10 that extends into the surrounding formation 2.
Shaped charges 8 also include a boss 20 protruding outward from the case 18 perpendicular to the axis ASC of the case 18. The boss 20 fully circumscribes the case 18 outer circumference. A perspective example of the gun strip 16 is provided in FIG. 3 illustrating holes 28 formed through the gun strip 16 for receiving shaped charges 8 therein. The shaped charge 8 is inserted into the hole 28 until the boss 20, whose outer diameter extends past the hole 28 outer diameter, contacts the outer surface of the gun strip 16. Thus the boss 20 supports the shaped charge 8 in the gun strip 16 and vertically aligns the shaped charge 8 in the gun strip 16. However, because the boss 20 is generally planar but the gun strip 16 outer diameter is curvilinear, the entire radius of the boss 20 does not lie above the hole 28, but instead the hole 28 outer diameter is shaped to accommodate the shaped charge 8 placement. Accordingly although the shaped charge 8 outer diameter is substantially circular, the typical gun tube 16 hole 28 is not. This can be a problem in certain perforating guns employing a “high density” shot pattern. For example, FIG. 3 illustrates an example of a gun tube 16 having high density shot pattern wherein adjacent holes 28 are sufficiently close that a web portion 30 between the holes 28 is too narrow to provide adequate structural support.