U.S. patent application number 11/405148 was filed with the patent office on 2007-10-18 for high density perforating gun system producing reduced debris.
This patent application is currently assigned to Owen Oil Tools LP. Invention is credited to Manmohan Singh Chawla, Dan W. Pratt.
Application Number | 20070240599 11/405148 |
Document ID | / |
Family ID | 38603616 |
Filed Date | 2007-10-18 |
United States Patent
Application |
20070240599 |
Kind Code |
A1 |
Pratt; Dan W. ; et
al. |
October 18, 2007 |
High density perforating gun system producing reduced debris
Abstract
A perforating system has a perforating module comprising a
unitary body of explosive. The explosive is contained within a
non-explosive casing, or liner, having formed indentations and a
cover thereover. The indentations, which will transform into
explosively formed penetrators (EFP's) upon detonation, have a
perimeter shape that allows for improved packing density, e.g., a
hexagonal perimeter, which results in relatively little "dead
space" wherein no perforating penetrators are generated. In
operation, the module provides a relatively dense shot pattern and
substantially reduced amount of post-detonation debris that could
clog the perforations and/or require remedial clean-up or repeat
perforation.
Inventors: |
Pratt; Dan W.; (Benbrook,
TX) ; Chawla; Manmohan Singh; (University Park,
MD) |
Correspondence
Address: |
PAUL S MADAN;MADAN, MOSSMAN & SRIRAM, PC
2603 AUGUSTA DRIVE, SUITE 700
HOUSTON
TX
77057-5662
US
|
Assignee: |
Owen Oil Tools LP
Houston
TX
|
Family ID: |
38603616 |
Appl. No.: |
11/405148 |
Filed: |
April 17, 2006 |
Current U.S.
Class: |
102/307 ;
102/310 |
Current CPC
Class: |
E21B 43/117 20130101;
F42B 1/028 20130101; F42D 3/04 20130101 |
Class at
Publication: |
102/307 ;
102/310 |
International
Class: |
F42B 1/02 20060101
F42B001/02 |
Claims
1. An apparatus for perforating a subterranean formation
comprising: a perforation module conveyable into a wellbore drilled
into the subterranean formation, the module including at least: (a)
a body of explosive material; (b) a detonator for detonating the
body of explosive material; and (c) a liner surrounding at least a
portion of the explosive material, the liner forming a plurality of
perforating elements when the body of explosive material is
detonated.
2. The apparatus of claim 2, wherein the liner forms perforating
elements that travel in a direction substantially perpendicular to
a longitudinal axis of a wellbore.
3. The apparatus of claim 1, wherein the liner circumferentially
surrounds the body of explosive material.
4. The apparatus of claim 1, wherein the detonator comprises a
layer of primasheet.
5. The apparatus of claim 1 further comprising a support member
within the body of explosive material that connects with a
conveyance string.
6. The apparatus of claim 1, wherein the liner is formed of
tantalum.
7. An apparatus for use in perforating a subterranean formation,
comprising: a perforation module comprising a body of high
explosive material; and a liner surrounding the body of high
explosive material, the liner presenting a radially outer surface
having a plurality of indentations formed therein to be transformed
into directional penetrators upon detonation of the body of high
explosive material.
8. The apparatus of claim 7, wherein the perforation module further
comprises a substantially cylindrical cover member that radially
surrounds the non-explosive liner to provide a standoff for the
directional penetrators transformed from the indentations.
9. The apparatus of claim 7, wherein the indentations have
perimeters whose shape enables closer packing of the indentations
than would be possible if the indentations had circular
perimeters.
10. The apparatus of claim 9, wherein each indentation has a
perimeter that is one of: (i) triangular; (ii) square; (iii)
pentagonal; (iv) hexagonal; and (v) octagonal.
11. The apparatus of claim 9, wherein the indentations are arranged
in a pattern over the outer radial surface such that each
indentation is linearly contiguous with at least one other
indentation.
12. A method of perforating a subterranean formation comprising:
perforating a formation intersected by a wellbore with a plurality
of penetrators formed from one liner.
13. The method of claim 12 further comprising detonating a body of
high explosive in a wellbore to form the penetrators.
14. The method of claim 13 further comprising at least partially
surrounding the body of high explosive with the one liner.
15. The method of claim 12 further comprising forming a plurality
of indentations on the one liner.
16. The method of claim 12 further comprising directing the
penetrators into the formation along a direction substantially
normal to a longitudinal axis of the wellbore.
17. The method of claim 12 further comprising forming the
penetrators to penetrate at least a casing positioned in the
wellbore.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates generally to the design of perforating
tools for use in creating perforations in wellbores to improve the
flow of fluids from the wellbore.
[0003] 2. Description of the Related Art
[0004] Perforation guns are used within wellbore holes to increase
the permeability of the formation surrounding the wellbore. In
general, perforation guns producing greater numbers of perforations
are considered to be more effective than those producing fewer
perforations. It is therefore often desired to maximize the number
of penetrating jets within a segment of the wellbore. This may be
difficult, however, because there are limitations relating to
placement of the charges used for perforation. Standard shaped
charges have an outer housing formed of metal or another material
that encloses the high explosive charge. The shaped charge holder
has openings that have typically circular perimeters. When packing
the charges in an adjoining manner in the charge tube, interstitial
spaces are unavoidably left between the neighboring charges as a
result their shape. This packing results in "dead spaces," that is,
areas from which no perforating product, i.e., no jets, is/are
provided, between the charges, and limits the density with which
the charges can be packed.
[0005] There are a number of known styles and designs for
perforation guns. There are, for example, strip guns that include a
strip carrier upon which are mounted a number of capsule charges.
The capsule charges are individually sealed against corrosive
wellbore fluids. Also known are hollow carrier guns that have a
sealed outer housing that contains unencapsulated shaped charges.
In each case, the shaped charges are arranged such that they will
detonate in a radially outward direction to form a specific pattern
of perforations.
[0006] An alternative perforation gun design is described in U.S.
Pat. No. 5,619,008 to Chawla et al. In this design, a two-layer
liner serves to sheath discontinuous loadings of explosive
material. The liner is configured with indentations that are each
aligned with an individual loading of the explosive material. Upon
detonation of the loadings of explosive material, these
indentations act in the manner of a shaped charge, creating a
directed jet of liner material. The indentations have a circular
perimeter and are spaced apart from one another, leaving
significant "dead space" between them. Following detonation and any
resulting perforation, the housing that surrounds the charges is
not completely destroyed and forms debris. This debris is
undesirable, both because it must be removed by wireline or by
other means in a secondary operation, and because it may clog the
perforations that are formed by the perforation operation, thereby
making the perforations less effective and sometimes necessitating
repeat perforation operations. The Chawla et al. invention thus
suffers from problems relating to both "dead space" and debris
creation.
[0007] The present invention addresses the problems of the prior
art.
SUMMARY OF THE INVENTION
[0008] The present invention provides a perforating device that
produces multiple perforating penetrators from a single high
explosive charge. In one embodiment, the perforating module has a
central rod with a surrounding cylinder of high explosive. The
cylinder of high explosive is contained within a liner having
formed indentations. The liner may be of any suitable material,
such as a non-explosive material including, for example, an
elemental metal or alloy, a composite, a ceramic, a thermoplastic
or thermo set polymer, or the like. Finally, a cylindrical outer
cover is disposed about the liner. In one embodiment, the
indentations are linearly contiguous to one another. In another
embodiment, the indentations each have a perimeter that is
triangular, square, hexagonal, or octagonal and are disposed in an
adjoining fashion to one another.
[0009] In operation and as a result of detonation of the explosive
material, the module forms penetrators of liner material that
propagate into the formation in a direction that is, in one
embodiment, substantially perpendicular to the longitudinal axis of
the wellbore. The module thus is capable of providing a relatively
dense shot pattern with little or no "dead space" between the
locations from which the penetrators are formed. This results in an
effective perforation of a wellbore segment.
[0010] During the detonation, the constituent components of the
module, including in some embodiments the high explosive, the
liner, and the outer cover, are largely destroyed. As a result, the
amount of debris resulting from the detonation is reduced or
eliminated, as compared with the amount of debris produced by many
conventional perforation devices.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] For greater understanding of the invention, reference is
made to the following detailed description of the embodiments of
the present invention, taken in conjunction with the accompanying
drawings in which reference characters designate like or similar
elements throughout the several figures of the drawings.
[0012] FIG. 1 is a side, cross-sectional view of a wellbore
containing an exemplary perforation system constructed in
accordance with the present invention.
[0013] FIGS. 1a and 1b illustrate a pair of alternative
constructions for perforation systems constructed in accordance
with the present invention.
[0014] FIG. 2 is a side, cross-section depiction of a single
perforation module of the perforation system shown in FIG. 1.
[0015] FIG. 3 is an exterior view of the module shown in FIG.
2.
[0016] FIG. 4 is a detail view of a portion of the liner of an
exemplary perforation module showing further details concerning the
indentations.
[0017] FIG. 5 is a detail view of a portion of the liner of an
exemplary perforation module showing an alternative shape for the
indentations.
[0018] FIG. 6 is a side cross-section of the portion of liner shown
in FIG. 5, taken along lines 6-6.
[0019] FIG. 7 depicts an exemplary shot pattern that is created by
the perforation module shown in FIGS. 2 and 3.
[0020] FIG. 8 illustrates an alternative embodiment for a
perforation module in accordance with the present invention having
triangular indentations.
[0021] FIG. 9 illustrates a further alternative embodiment for a
perforation module in accordance with the present invention having
square indentations.
[0022] FIG. 10 depicts a portion of the surface of the liner of a
perforation module that utilizes octagonal indentations.
[0023] FIGS. 11-14 illustrate an exemplary initiation sequence for
a single penetrator of a perforation module in accordance with the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present invention relates to devices and methods for
perforating wellbores. The present invention is susceptible to
embodiments of different forms. There are shown in the drawings,
and herein will be described in detail, specific embodiments of the
present invention with the understanding that the present
disclosure is to be considered an exemplification of the principles
of the invention, and is not intended to limit the invention to
that illustrated and described herein.
[0025] FIG. 1 illustrates an exemplary perforation system 10 that
is configured in accordance with one embodiment of the present
invention. The perforation system 10 is disposed within a wellbore
12 that has been drilled through the earth 14 and a
hydrocarbon-bearing formation 16. Portions of the wellbore 12 are
cased by a steel casing 18 that is secured within the open wellbore
hole by cement 20.
[0026] The hydrocarbon-bearing formation 16 contains two
oil-bearing strata 22, 24, which are separated by a layer of water
26. A layer of water 28 also separates the lower oil stratum 24
from a stratum of gas 30. It is noted that this arrangement of
strata in formation 16 is presented only by way of example and that
those skilled in the art will recognize that the actual composition
and configuration of formations varies.
[0027] The perforation system 10 is disposed into the wellbore 12
on a conveyance string 32. The conveyance string 32 may be of any
known construction for conveying a tool into a wellbore, including
a drill pipe, wireline, production tubing, coiled tubing, and the
like. The perforation system 10 includes one or more perforating
modules that are used to perforate portions of the surrounding
formation 16. In the described embodiment, there are three
perforating modules 34, 36, 38 that are secured to one another in
series. There may, of course, be more or fewer than three modules,
depending upon the desired length of wellbore to be perforated.
Additionally, it is pointed out that there may be intermediate
sections of tubing, or subs 37 (see FIG. 1a) interposed between the
individual modules 34, 36, and 38, to provide a desired spacing
therebetween. In practice, the subs 37 are desirably non-explosive.
If desired, the modules 34, 36, and 38 may alternatively be secured
to one another so as to form an unbroken, contiguous series of
modules. FIG. 1b illustrates a further alternative perforation
system arrangement wherein the perforation modules 34, 36, and 38,
of the system are interconnected directly to one another in
series.
[0028] An exemplary individual module 40 is depicted in FIGS. 2 and
3. The module 40 is representative of each of the three modules 34,
36, and 38 shown in FIG. 1. As will be described in further detail
below, the module 40 creates a plurality of perforating penetrators
from a single explosive charge. The penetrators travel in a
direction substantially normal or orthogonal to the longitudinal
axis of the wellbore. Advantageously, this arrangement may
significantly increase shot density and simultaneously reduce the
amount of debris left in the wellbore, relative to many
conventional perforation systems. In one embodiment, the module 40
includes a support member such as a central rod 42 having upper and
lower axial ends 44, 46. The upper and lower axial ends 44, 46 are
provided with threaded connections, as is known in the art, so that
they may be secured to the conveying string 32 (see FIG. 1b) or to
an adjoining module. The central rod 42 is composed of a central
load bearing portion 41 and an outer detonation layer 43. The
load-bearing portion 41 of the central rod 42 may be a section of
pipe, rod or other load bearing structure. In one embodiment, the
load-bearing portion 41 of the central rod 42 is formed of steel.
In another embodiment, if the perforation device 10 is not to be
withdrawn from the wellbore 12 after detonation, the load-bearing
portion 41 of the central rod 42 is formed of a frangible or
combustible material that will be readily destroyed during the
detonation of the perforating device 10. Ceramic is just one
example of a suitable frangible material.
[0029] The detonation layer 43 comprises, in this embodiment, a
primasheet of a type known in the art for initiation of
detonations. The load-bearing portion 41 of the central rod 42 may
also contain an axial passage 48 along its length to contain
electrical wiring (not shown) that is necessary for initiation of
the detonation layer 43 which, in turn, results in detonation of
the body 50 of high explosive material. The detonation layer 43 may
be initiated with a control signal either manually or utilizing
some preprogrammed device. For example, suitable initiating systems
can include using electrical signals transmitted from the surface
via wiring (not shown) in the axial passage 48 to initiate
detonation cord (not shown) disposed within the axial passage 48,
by increasing hydraulic pressure in the wellbore, or by the
dropping of a drop bar (not shown) into the axial passage 48, as is
used conventionally with tubing conveyed perforation guns. Other
initiating systems can utilize timers or well bore parameter
sensitive devices (e.g., pressure, temperature, depth, etc.
Initiation systems for detonating perforating guns are known in the
art and will not be discussed in further detail.
[0030] Surrounding the central rod 42 is a substantially unitary
body 50 of high explosive material that explosively forms the
perforating penetrators using the liner 52. Suitable high explosive
materials may include, for example, conventionally-employed high
explosives such as RDX, HMX and HNS. While the size of the module
is not a critical aspect thereof, it may be convenient to configure
the module 40 such that it is a cylinder about 12 inches in length
and about 4.5 inches in diameter. However, the length and diameter
may be varied according to the dimensions of the wellbore 12 or
other factors. A tube 51 of cardboard or a similar material is
disposed between the central rod 42 and the high explosive body
50.
[0031] The liner 52 surrounds the body 50 of high explosive and is
configured to form a plurality of perforating penetrators. The
penetrators formed by the liner 52 may travel in a direction
generally perpendicular to the longitudinal axis of the wellbore,
although modifications in direction may also be achieved in other
embodiments of this invention. In one embodiment, the liner 52 may
be, in this embodiment, a cylindrical and non-explosive liner
formed of a metal, such as, for example, tantalum. Alternatively,
the liner 52 may be made from extruded copper, tungsten, steel,
depleted uranium, aluminum, or another elemental metal or alloy. In
other embodiments blends of elemental metals or alloys with
materials such as lead, graphite, and zinc stearate may also be
employed. In still other embodiments blends or alloys of aluminum
with either titanium or hafnium may be used. Additionally, a
frangible material may be used to form the liner 52 in order to
further reduce the likelihood that the formed penetrator will plug
the perforation created in the surrounding formation. Such may
include, for example, the use of pressed, sintered metallic
powders, such as those described in U.S. Pat. No. 6,012,392, which
is incorporated herein by reference in its entirety, and
metal/matrix composites.
[0032] The size, shape, velocity and other characteristics of the
perforating penetrators formed by the liner 52 may be controlled,
in part, by adjusting the surface contours of the liner 52. In one
embodiment, a plurality of linearly contiguous indentations 54 is
formed into the liner 52. As used herein, the phrase "linearly
contiguous" means that the perimeters of every indentation shares
at least one common side with an adjacent indentation. In some
embodiments a majority of each indentation is linearly contiguous
with adjacent indentations, and in other embodiments essentially
all of each indentation is linearly contiguous with adjacent
indentations. In one embodiment, each indentation 54 has an axis
that is substantially perpendicular to the exterior surface of the
liner 52, where such exterior surface is substantially parallel to
the longitudinal axis of the wellbore. In other embodiments such
indentation axis may be significantly greater or less than ninety
degrees to the exterior surface of the liner 52 and/or to the
longitudinal axis of the wellbore, in order to direct the
penetrators in a specific direction, according to the purposes and
goals of the perforation operation.
[0033] FIG. 4 depicts further details concerning one embodiment of
the indentations 54. In this embodiment, each indentation 54 has a
hexagonal outer perimeter 56 and therefore adjoins a neighboring
indentation 54 on each of its six sides, i.e., all of its six sides
are linearly contiguous with neighboring indentations 54. Because
of this fact, there are no "dead spaces" between the indentations
54 from which it is inferable that there is no area from which a
penetrator is not, or could not be, transformed. A small linear
ridge 58 is formed at each of the adjoining contact areas of the
neighboring indentations 54. A hexagonal shape for the perimeter 56
of the indentations 54 is one possible arrangement, which may offer
the additional benefit that, by approximating the shape of a
circle, a penetrator that is relatively radially uniform is, upon
detonation of the body 50 of high explosive, developed therefrom.
Additionally, the hexagonal shape of the perimeter 56 permits
relatively closer packing of the indentations 54 to form an
adjoining, interlocking honeycomb effect. As a result, the "dead
space," that is unavoidable when indentations having circular
perimeters are employed, is thereby greatly reduced or eliminated.
A further advantage of the honeycomb arrangement of the
indentations 54 is that the perforations created may, as a result,
be spaced equally in all directions, that is, in circumferential,
axial, vertical, and horizontal directions, such as to
significantly reduce the possibility of failure of the surrounding
casing 18 upon perforation. A high density of perforations may
therefore be achieved from the use of such linearly contiguous and
interlocking indentations that cover essentially the entire outer
surface area of the module 40. For example, a pattern of hexagonal
indentations that are two inches in diameter, i.e., hexagons that
can be inscribed within a two-inch diameter circle, may in some
embodiments generate a shot pattern of 51 perforations per linear
foot of the wellbore from the surface of a 4.5-inch diameter module
40. In contrast, a similarly sized, conventional carrier-type
perforating gun, using conventional shaped charges, will typically
provide only about 18 perforations per linear foot. Thus, this
embodiment illustrates a capability to increase the perforated area
by a factor of three. The size and number of hexagonal indentations
54 may be varied, depending upon factors such as the diameter of
the module 40 relative to the size of the annular space between the
perforation system 10 and the casing wall 18; the properties of the
formation in which the perforation gun is being used; the presence
or absence of fluid in the annular space; the selection of liner
material and explosive; and the like. Those skilled in the art will
be able to determine optimal configurations based upon such skill
and with, at most, routine experimentation to ensure success.
[0034] FIGS. 4, 5 and 6 show additional possible configurations for
the liner to enable formation of effective penetrators therefrom.
As illustrated therein, the indentations 54 each define a cavity
60. While the perimeter of the indentations may influence the shape
of the cavity 60, it is not necessarily determinative thereof.
Thus, in certain embodiments the shape of the cavity 60 may be of a
generally conical or pyramidal configuration, as shown in FIG. 4,
or of a generally spherical or parabolic configuration, as depicted
in FIGS. 5 and 6. The cavity 60 provides a formation distance for a
penetrator to form. The cavity 60 provides an apex 62, i.e., point
of greatest indentation, opposite the opening defined by perimeter
56. In this embodiment, the cavity 60 has six equal planar
triangular sides 70. The sides 70 adjoin one another along junction
lines 72, forming a cavity 60 that is symmetrical along certain
axes. The indentations 54 may be formed into the essentially planar
liner 52 by stamping, forging or by other known means. Thereafter,
the sheet may be formed into a cylinder by bringing opposing ends
together and then welding or otherwise connecting the ends. The
high explosive body 50 may then be cast into the space between the
liner 52 and the inner cardboard tube 51.
[0035] An alternative method for forming the high explosive body 50
is by pressing a billet to a desired length and diameter, and then
machining the billet to match the hexagonal indentations 54 at the
outer surface of the liner 52. A long axial hole is then drilled
into the center of the billet and sized to accommodate the tube 51.
As those skilled in the art are aware, a billet of high explosive
is a mass of high explosive material that has been pressed or cast
into cylindrical shape. Pressed billets can be machined to a
desired shape, while cast billets are formed to the desired shape,
such as, in this case, a cylinder with an axial passage
therethrough.
[0036] FIG. 5 illustrates an alternative design for the
indentations 54, here designated 54'. The indentations 54' still
have a hexagonal perimeter 56. However, the side surfaces defining
the cavity 60 are smooth and rounded. In side cross-section, the
cavity 60 forms a dome-like cap or parabola, as FIGS. 5 and 6,
respectively, depict. The radius and apex of each dome-like cavity
60 depend upon the liner thickness and desired formation distance,
with the goal that a penetrator may be transformed therefrom that
is optimal for creating a large perforation in the wellbore casing
18. In alternative embodiments, other cavity shapes, such as a
conical shape, may be employed.
[0037] Circumferentially surrounding the liner 52 is a cover 64
that protects the liner 52 and other parts of the module 40 from
the harsh wellbore environment. In one embodiment, the cover 64 is
a generally cylindrical construction having planar inner and outer
surfaces. The cover 64 may be formed of, for example, a
thermoplastic or thermoset polymer that is resistant to high
wellbore temperatures. The cover 64 may be relatively thin, having
a thickness of, for example, just 0.05 inch, and light in weight,
such that it will not unduly interfere with the creation of the
penetrators from the indentations 54 or 54'. In some embodiments,
an elemental metal or alloy, composite material, thermoplastic or
thermoset polymer, or glass, for example, may be used to form the
cover 64. The cover 64 overlies the adjoining ridges 58 between
neighboring indentations 54 or 54' (see FIG. 6). There is a space
disposed between the cover 64 and the ridges 58 to permit the
indentations 54, 54' to fully develop into penetrators upon
detonation. Such space may be relatively small, for example, about
5 mm. Air, at atmospheric pressure, may be trapped within the
cavities 60 of the indentations 54, 54' between the cover 64 and
the outer surface of the liner 52. The distance between the apex 62
of each indentation 54 or 54' and the outer cover 64 provides a
stand-off for each indentation 54 or 54' such that a penetrator can
more fully develop prior to contact with the well casing 18 (see
FIG. 1).
[0038] Upper and lower end caps 66, 68 (see FIG. 3) are secured to
the cover 64 and liner 52 of the module 40 and serve to help
encapsulate and protect the contents of the module 40, particularly
the explosive body 50, from fluids within the wellbore 12 prior to
detonation.
[0039] In operation, the perforation system 10 is lowered into the
wellbore 12 until the modules 34, 36, 38 of the perforation system
10 are aligned with the desired strata 22, 24, and 30,
respectively, of the formation 14. The modules 34, 36, 38 of the
perforation system 10 are then detonated to create penetrators that
perforate the casing 18, cement 20 and formation 14. Following
perforation of the formation 14, the remains of the perforation
system 10 may be removed from the wellbore 12 by pulling upwardly
on the conveyance string 32. It is anticipated that, in many
embodiments, the perforation modules 34, 36, 38 will be
substantially or totally consumed in the detonation.
[0040] During detonation of the perforation modules 34, 36, 38,
directional penetrators are formed by the indentations 54, 54'.
Because the mechanism of the creation of this type of directional
explosively formed penetrator (EFP) is well known in the art, it
will not be described here in any detail. It is noted, however,
that the detonation sequence of each module 34, 36, 38, begins at
the top end proximate to the central rod 42 and proceeds
simultaneously in axially downward and radially outward directions.
Each liner indentation 54, 54', when acted upon by the advancing
detonation wave, forms a robust EFP, which is particularly well
suited for making large and shallow perforation holes in sandy or
soft formations. While conventional shaped charges form a
relatively fast-moving, low mass jet that accomplishes the
perforation, followed by a relatively slow-moving slug that
thereafter carries the mass of the remaining charge liner but does
not take part in the actual perforation, the EFP penetrator of the
present invention carries essentially all of the mass of the liner
52 forming the indentation 54 or 54'. This means that the liner
mass effectively forms part of the penetrator and takes an active
part in the perforation, increasing the relative effectiveness
thereof. In one embodiment it has been found that the perforations
that result from indentations 54 or 54' having hexagonal perimeters
very closely approximate those created from indentations having
circular perimeters.
[0041] FIG. 7 illustrates an exemplary shot pattern that may be
formed upon detonation of the perforation module 40 within a
section 80 of the wellbore 12. FIG. 7 depicts the sidewall of the
wellbore section 80 in cylindrical projection with the upper end of
the section 80 depicted as line 82 and the lower end of the section
80 shown as line 84. The illustrated wellbore section 80 has a
length (L) of approximately one foot. There are fifty-one (51)
perforations 86 disposed within the wellbore section 80, which have
been created by penetrators formed from the indentations 54 or 54'
of the perforation module 40. In practice, those skilled in the art
frequently desire perforations having diameters, as measured at the
inner surface of the well casing, ranging from about 10 to about 22
mm, but larger or smaller perforations may alternatively be
obtained by simply varying the size of the indentations. It is
noted that the fifty-one (51) perforations 86 are arranged in six
horizontal rows 88a, 88b, 88c, 88d, 88e, and 88f of eight
perforations 86 each. Adjacent rows 88 of perforations 86 are shown
herein as horizontally staggered from one another, such that
perforations 86 in one row are located diagonal to, i.e., offset
diagonally in relation to, perforations 86 in adjacent rows. For
example, referring to FIG. 7, perforation 86b in row 88b is located
diagonal to penetrations 86a and 86c in row 88a. This staggered
pattern is frequently advantageous. Because the penetrations 86 are
more densely concentrated than perforations from conventional
shaped charge perforation devices, the staggered arrangement may
help to avoid overlapping of adjacent perforations. This is
desirable because, if there were numerous such overlaps, the
resultant effect of a linear cut in the casing 18 could
theoretically produce a casing failure, such as a casing collapse.
The staggered arrangement may therefore desirably avoid such an
undesirable event. In another embodiment, some of the indentations
may be configured of a material that does not suitably form
penetrators, in order to reduce the number of penetrators and,
therefore, the number or density of perforations obtained thereby.
Such an embodiment may be acceptable in certain applications,
wherein relatively increased amounts of post-detonation debris are
not problematic.
[0042] Alternative to indentations having hexagonal perimeters,
other perimeter shapes may be selected, desirably such that the
perimeters may be adjoined in a linearly contiguous fashion. For
example, the indentations may be configured to have triangular,
square, or octagonal perimeters. FIGS. 8 and 9 illustrate
alternative embodiments wherein such triangular and square
perimeter indentations, respectively, are used. FIG. 8 depicts an
exemplary perforation module 90 having triangular perimeter
indentations 92. As may be seen, the triangular perimeter
indentations are located in an adjacent manner such that each of
the three sides of a given perimeter borders a side of a
neighboring perimeter. Thus, "dead space" between the indentations
92 has thereby been eliminated.
[0043] FIG. 9 depicts an exemplary perforation module 94 having
square perimeter indentations 96. These indentations 96 are
arranged in several horizontally-disposed rows, e.g., 98a, 98b,
98c. Adjacent rows of indentations 96 are staggered relative to one
another, i.e., offset by half a square, such that indentations 96
in each row are located with their apices diagonal to the apices of
indentations 96 in the adjacent row.
[0044] It will be understood by those in the art that each
perimeter shape will impart some effect on the configuration of the
cavity formed by an indentation, and therefore of the penetrator
that will be formed from collapse of the cavity as a result of
detonation. Factors such as the fabrication method, and
capabilities and limitations thereof, of the liner wherein the
indentations are formed, and the material of which the liner is
composed, will desirably be taken into account when selecting the
perimeter shape and associated packing parameters. For example,
triangular and square perimeter indentations may, because of their
shape, not collapse as readily during detonation as do hexagonal
perimeter indentations in a perforation module wherein all
materials and detonation factors are the same. However,
modification of such factors may, in some embodiments, offset such
disadvantages or even turn such a tendency into an advantage.
[0045] FIG. 10 depicts a portion of an exemplary liner surface for
a perforation module wherein octagonal perimeter indentations are
used. As may be seen in FIG. 10, octagonal perimeter indentations
cannot completely cover a given area without leaving some "dead
space" between the indentations. In this aspect, their use may be
less advantageous, in some embodiments, than the use of hexagonal,
square or triangular-shaped indentations. However, octagonal
perimeter indentations may more readily approximate the collapse
sequence and penetrator transformation of indentations having a
circular perimeter, and thus may obtain an advantage overtriangular
and circular perimeter indentations in certain embodiments. FIG. 10
depicts a liner surface section 100 having a plurality of octagonal
perimeter indentations 102 that adjoin, i.e., are linearly
contiguous to, one another at four of their eight sides 104. The
remaining four sides 106 of the octagonal perimeter indentations
102 define square areas 108 as interstitial spaces. If desired, the
interstitial square areas 108 may themselves be indented, in the
manner of square indentations 96 (see FIG. 9), to provide for
additional formed penetrators.
[0046] Turning now to FIGS. 11 through 14, an exemplary initiation
sequence is illustrated for a single formed penetrator from a
perforation module 40. FIG. 11 is a cross-sectional view of the
indentation 54 prior to detonation of the perforation module 40.
The indentation 56 is formed in liner 52 that surrounds the high
explosive body 50. In this embodiment a thermoplastic cover 64
surrounds liner 52. The module 40 is disposed within a section of
wellbore casing 18 surrounded by cement 20. Fluid 57 resides in the
annular space that is between the casing 18 and the radially
exterior portion of the cover 64. FIG. 12 depicts the beginning
portion of the detonation wherein the material forming metallic
liner 52 has begun to collapse or coalesce within the space
formerly occupied by the cavity 60 of indentation 54. The cover 64
atop the indentation 54 has begun to bow outward and thin out. In
FIG. 13, the detonation process has progressed to the point where a
generally spherical penetrator 110 has been formed from the
material making up the liner 52. The casing 18 and fluid 57 are
essentially sheared through by the penetrator 110. FIG. 14 depicts
an advanced stage of the detonation with the penetrator 110 now in
a primarily plastic phase and perforating the cement 20 on its way
to the formation (not shown).
[0047] In summary of the foregoing description, those skilled in
the art will appreciate that the design of the perforation system
10 thus provides a number of advantages over conventional
perforation systems. Included among these, first, is the fact that
the linearly contiguous packing of the indentations combined with
the unitary body of high explosive produces a greater number of
perforating penetrators over a given axial length of a module 40
and reduced amount of "dead space," as compared with conventional
perforation systems using shaped charges and indentations that are
physically separated and/or have circular perimeters. The greater
number of penetrators results in a desirably greater density in the
post-detonation perforation shot pattern. Second, the invention
provides for a substantial reduction in debris formed during the
perforation operation. And third, the perforation module 40 may be
created or manufactured and customized relatively easily, without
the need for time-consuming placement and orientation of individual
shaped charges, as with conventional systems.
[0048] Those skilled in the art will recognize that numerous
modifications and changes can be made to the illustrative designs
and embodiments described herein and that the invention is limited
only by the claims that follow and any equivalents thereof.
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