1. Field of the Invention
The present invention relates to method and apparatus for performing perforating operations, and, more particularly, to performing such operations in a material which is naturally fractured or which has a low tensile strength.
2. Description of the Prior Art
For purposes of enhancing production from a subterranean formation, a perforating gun typically is lowered down into a wellbore that extends through the formation. A perforating gun comprises a plurality of radially-oriented shaped charges which are detonated to form perforations in the formation proximate the wellbore. The shaped charges typically are placed at points along a helical spiral that extends around a longitudinal axis of the perforating gun.
It is known that charge penetration into the hydrocarbon-bearing formation is a major determinant of well productivity. Extensive investigations have previously been conducted to characterize penetration, mainly into sandstone formations. Recent charge penetration experiments into coal have revealed surprising results that may be associated with the complex cleat or natural fracture system apparent in many kinds of coal. It is believed that these fractures in coal may adversely affect charge penetration performance. Specifically, such detriment may be due to shock passage through these fractures ahead of the jet. Such effects are expected to primarily occur when penetration velocity is subsonic with respect to the prevailing acoustic velocity of coal; i.e., during later stages of penetration when incoming jet velocity is lowest.
Classical hydrodynamic theory has since the 1940's been applied to the analysis shaped charge penetration. When a jet (of density ρj), traveling at velocity V penetrates a target having a density ρt, the jet-target interface will advance at a penetration velocity U. Penetration velocity is always some fraction of the incoming jet velocity; specifically:
U/(V−U)=sqrt(ρj/ρt)
The magnitude of U, relative to the prevailing local acoustic velocity (C0) of the target material, determines whether the penetration is sub- or super-sonic. If U<C0, the penetration is said to be subsonic, and the shock wave formed by the penetration event will separate from the interface and advance ahead into the target. This separated wave can alter the state of the target into which subsequent jet portions enter.
Even for jet penetration which is slightly supersonic, the shock wave may detach due to shock velocity exceeding the acoustic velocity. Furthermore, an attached shock will tend to separate from the incoming jet, if the jet itself is decelerating (as is the case with real shaped charge jets).
In accordance with the present invention, a charge is provided for a perforating gun. Such a charge, when detonated, produces a jet having a penetration velocity that will always exceed the acoustic velocity of the target material to be perforated. In one embodiment, a charge in accordance with the present invention is fabricated for use with a target material which is a naturally fractured material, e.g., coal. In another embodiment, a charge in accordance with the present invention is fabricated for use in perforating a target material which has a low tensile strength.
In accordance with the present invention, a perforating gun system is provided for use in perforating the formation material proximate a wellbore, and such a perforating gun system comprises at least one perforating gun section. Each perforating gun section in the system comprises a plurality of radially-oriented shaped charges, which, when detonated, produce jets that have penetration velocities which will always exceed the acoustic velocity of the formation material proximate the wellbore. A system in accordance with the present invention further comprises a firing head to cause said shaped charges to detonate.
In one embodiment of a perforating gun system in accordance with the present invention, each charge may for use in perforating a formation material which a naturally fractured material, e.g., coal. In yet another embodiment of a perforating gun system in accordance with the present invention, each shaped charge is for use in perforating a formation material which has a low tensile strength.
In accordance with the present invention, a method is also provided of operating a perforating gun containing a plurality of shaped charges to perforate the formation material proximate a wellbore. Such a method comprises lowering the perforating gun into the wellbore and detonating the plurality of shaped charges. A method in accordance with the present invention further comprises producing jets from said shaped charges where the jets have penetration velocities that exceed the acoustic velocity of the formation material proximate the wellbore. In one embodiment of the present invention, jets are produced which have penetration velocities that exceed the acoustic velocity of coal, while in another embodiment of the present invention, the jets from the shaped charges produce penetration velocities that exceed the acoustic velocity of material which has a low tensile strength.
In the accompanying drawings:
It will be appreciated that the present invention may take many forms and embodiments. In the following description, some embodiments of the invention are described and numerous details are set forth to provide an understanding of the present invention. Those skilled in the art will appreciate, however, that the present invention practiced without those details and that numerous variations from and modifications of the described embodiments may be possible. The following description is thus intended to illustrate and not limit the present invention.
Referring first to
With reference now to
With reference to
u/(v−u)=sqrt(ρj|ρt)
where V is the velocity of the jet, ρj is the jet density and ρt is the target density.
With reference to
With reference to
In addition to shock reflection (which is a consequence of impedance mismatch), it is believed that the compressive shock wave 32 traveling ahead of the jet-target interface 31 may be followed by a region in tension, since the velocity of the detached shock wave is greater than the penetration velocity of the jet-target interface 31. With reference to
Referring to
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