Ammunition and explosives – Blasting – Detonation wave modifying
Reexamination Certificate
2000-02-28
2002-04-30
Nelson, Peter A. (Department: 3641)
Ammunition and explosives
Blasting
Detonation wave modifying
C102S306000, C102S476000
Reexamination Certificate
active
06378438
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of shaped charges for use in perforating gun systems.
BACKGROUND OF THE INVENTION
Installation of an oil or gas well involves fixing a tubular steel casing in cement in an underground bore. Holes are subsequently created in the steel casing and cement in order to gain access to the surrounding formation, i.e., oil or gas deposit. Such holes are generally created through a process known as perforation using a perforating gun. A well may also need to be re-perforated from time to time, for example, if the flow of oil or gas into the well becomes impeded by debris. It is also common practice to re-perforate a depleted well which is to be abandoned. Such wells are often sealed by introduction of cement between the casing and well bore to minimize leakage of remaining oil or gas. Re-perforation is carried out to open existing channels in order to improve infusion of the cement. Such specialized perforating guns are sometimes referred to in the industry as “channel finder guns”.
In general, a perforating gun includes an elongate member having several explosive charges spaced along the member. There is a detonation cord running between and connected to the charges. The charges are generally arranged along the length of the gun to explode radially outwardly in different directions into the formation. Such a charge, known as a “shaped charge”, includes a case, often conically shaped, which contains explosive material sealed in the case interior by a liner. When detonated, the charge delivers explosive forces to penetrate into the formation.
Efforts continue to be made to improve various aspects of the perforation process. For example, Renfro describes in U.S. Pat. No. 5,509,356 (Apr. 23, 1996), the specification of which is incorporated herein by reference, a shaped charge with an improved liner. The charge includes a housing made of a pulverable material such as a ceramic. The shaped charge includes explosive powder and a binder and the powder is pre-compressed to at least 95% of its total maximum density. The liner is generally hemispherical, but decreases in thickness from the central area to its perimeter, or skirt.
Hasselman describes in U.S. Pat No. 5,522,319 (Jun. 4, 1996), the specification of which is incorporated herein by reference, another effort to provide an improved shaped charge for oil well perforation in which a generally hemispherically shaped liner is thicker at its central region than towards its edge. This apparently improves the coherence, stability and mass distribution of the jet produced such that a tip velocity comparable to that of a conical shaped charge is obtained. The case housing of the charge is not described in detail, but is illustrated to be of uniform thickness and, being of thickness of the same order of the liner, is presumably metal. Hasselman states that the case material and configuration are of little importance to the overall design optimization, although they do have an effect and need to be included in any detailed design.
Willis et al. describe in U.S. Pat No. 5,564,499 (Oct. 15, 1996), the specification of which is incorporated herein by reference, a method and device for slotting well casing and scoring surrounding rock to facilitate hydraulic fractures. The device shown includes linear charges arranged lengthwise in the gun and concentrically about the center axis of the gun. This arrangement is described as creating a linear aperture in the well casing in which the charges are exploded.
Conventional perforating guns, however, have been found not to be entirely satisfactory when used to perforate a well in preparation for introduction of cementitious material to seal the casing to the well bore, i.e., when used as channel finder guns. The degree of perforation by such guns is sometimes insufficient to adequately open channels which can lead to insufficient infusion of the material into areas surrounding the casing. This can lead to an inadequate seal which permits leakage of formation contents, typically water, gas, or oil from between the casing and the formation. The invention disclosed herein has been found to be particularly useful in the context of this situation, although its application is not limited to such.
GENERAL DESCRIPTION OF THE INVENTION
The present invention involves a shaped charge assembly which produces a superior performance when used as part of a perforating gun to be used as a channel finder gun. The improved performance stems from the use of a shaped charge assembly in which the charge is configured to produce a jet which is substantially non-circular in cross section (in relation to the axial direction of travel of the jet) in which the assembly includes a base that is relatively rigid with respect to the liner of the assembly.
According to a preferred embodiment, the shaped charge assembly includes a linear shaped charge.
Thus, in one broad aspect, the invention is a shaped charge assembly having: a relatively rigid base that has a cavity; a relatively flexible liner; and a shaped charge located in the cavity, wherein the charge is in a shape and the liner located such that, upon explosion of the charge, the liner is propelled along an axial direction of thrust in the formation of a jet having a cross sectional shape which is non-circular.
The cavity of the base can have an opening at a top end of the base and preferably at least a portion of the liner is located in the opening of the base and the charge is shaped such that, upon explosion thereof, the jet travels along a major axis through the opening of the base.
Preferably, the shaped charge is a linear charge.
The base can be of milled steel, stamped metal, or cast metal, etc.
The exposed, or outer surface part of the liner can be concave.
The liner (or the portion of the liner in the opening of the base) can be “V”-shaped and inwardly concave with respect to the top end of the base, or the liner can be “W”-shaped and be generally inwardly directed with respect to the top end of the base.
Particularly, interior surfaces of the base can define a longitudinal trough extending between first and second ends of the base, there can be first and second walls extending between the first and second ends, respectively, third and fourth walls extending between the first ends of the first and second walls, and the second ends of the first and second walls, respectively, and the base can include a pair of apertures therethrough for contacting detonator wires with the charge.
In a particular embodiment, illustrated below, the floor of the cavity has a central ridge extending upwardly toward the liner.
In certain embodiments, there are substantially upright interior side walls extending between the first and second ends of the base.
It is possible for the interior floor surface of the base to be generally concave upward.
In particular embodiments, interior surfaces of the base define a longitudinal trough extending between first and second ends of the base and the liner is concave inward with respect to the cavity such that the shaped material within the cavity has a substantially constant transverse cross section between the first and second ends.
There can be metal foil at the respective first and second ends, to seal the charge in the assembly.
Usually, the base and liner are each of metal.
Usually, the base has a floor having a thickness at least twice that of the liner. The base can have a floor having a thickness at least three times the thickness of the liner. The base can have a floor having a thickness at least four times the thickness of the liner. The base can have a floor having a thickness at least five times the thickness of the liner. The base can have a floor having a thickness at least six times the thickness of the liner.
Preferably, the base has walls having interior surfaces which define the cavity and the walls are relatively thick with respect to the thickness of the liner.
The explosive charge material can be compressed directly against inner surfaces of the base. The liner and base can be form
Lussier Michael Norman
Lussier Norman Gerald
Hunt John C.
Nelson Peter A.
Prime Perforating Systems Limited
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