U.S. patent application number 12/541827 was filed with the patent office on 2010-01-07 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 | 20100000397 12/541827 |
Document ID | / |
Family ID | 41463337 |
Filed Date | 2010-01-07 |
United States Patent
Application |
20100000397 |
Kind Code |
A1 |
Pratt; Dan W. ; et
al. |
January 7, 2010 |
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: |
Mossman, Kumar & Tyler
11200 Westheimer Rd., Suite 900
Houston
TX
77042
US
|
Assignee: |
Owen Oil Tools LP
Houston
TX
|
Family ID: |
41463337 |
Appl. No.: |
12/541827 |
Filed: |
August 14, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11405148 |
Apr 17, 2006 |
|
|
|
12541827 |
|
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Current U.S.
Class: |
89/1.15 |
Current CPC
Class: |
F42B 1/028 20130101;
F42D 3/00 20130101; E21B 43/117 20130101 |
Class at
Publication: |
89/1.15 |
International
Class: |
E21B 43/117 20060101
E21B043/117 |
Claims
1. A method for perforating a subterranean formation, comprising:
lowering a plurality of perforating modules in a wellbore, wherein
each of the plurality of perforating modules comprises: a central
rod, wherein the central rod comprises: an exterior load bearing
portion, and an interior detonation portion including a first
explosive, a second explosive at least partially surrounding the
central rod, and a liner surrounding the second explosive, wherein
the liner has a plurality of shallow concave surface indentations;
and detonating the plurality of perforating modules in the
wellbore.
2. The method of claim 1, wherein the central rod further
comprises: an axial passage.
3. The method of claim 2, wherein the central rod further
comprises: a conducting wire disposed within the axial passage.
4. The method of claim 2, wherein the axial passage is adapted to
receive a hydraulic fluid.
5. The method of claim 1, wherein the plurality of shallow concave
surface indentations each have a cavity, the cavity being defined
by a diameter and a depth, and wherein the diameter to depth ratio
is approximately not less than two to one.
6. The method of claim 5, wherein the diameter to depth ratio is
not less than six to one.
7. The method of claim 1 wherein the plurality of shallow concave
surface indentations each have a cavity, the cavity being defined
by a depth, wherein the depth is no greater than a thickness of the
liner.
8. The method of claim 1, wherein the plurality of shallow concave
surface indentations each have a cavity, and further comprising:
forming a plurality of perforations in a region adjacent to the
perforating modules, wherein the plurality of perforations extend
substantially through a cement layer into a formation a distance
from a cement face that is no greater than a diameter of the
cavity.
9. The method of claim 8, wherein the distance is no greater than
one-half of the diameter of the cavity.
10. The method of claim 8, further comprising at least partially
lining the plurality of perforations in the cement layer with a
liner material.
11. The method of claim 1, wherein the exterior load bearing
portion comprises a frangible material.
12. The method of claim 1, wherein each of the plurality of shallow
concave surface indentations is linearly contiguous with at least
another of the plurality of shallow concave surface
indentations.
13. The method of claim 1, wherein the shallow concave surface
indentations are arcuate.
14. A method for perforating a subterranean formation, comprising:
lowering a plurality of perforating modules in a wellbore, wherein
at least one of the plurality of perforating modules is separated
from another of the plurality of perforating modules by a spacer,
and wherein each of the plurality of perforating modules comprises:
a central rod, wherein the central rod comprises: an exterior load
bearing portion comprised of a frangible material, an axial passage
adapted to receive hydraulic fluid disposed along the length of the
interior load bearing portion, a wire disposed within the axial
passage, and an interior detonation portion including a first
explosive, a second explosive adapted to at least partially
surround the central rod, a liner disposed to surround the second
explosive, wherein the liner is made of a non-explosive material,
and wherein the liner has a plurality of shallow concave arcuate
surface indentations, wherein each of the plurality of shallow
concave arcuate surface indentation is polygonal, wherein each of
the plurality of shallow concave arcuate surface indentations is
configured to face substantially perpendicular to the longitudinal
axis of the wellbore, and wherein each of the plurality of concave
arcuate surface is linearly contiguous with at least another of the
plurality of shallow concave arcuate surface indentations, and a
cover disposed about the liner; and detonating the plurality of
perforating modules in the wellbore.
15. A method for perforating a subterranean formation, comprising:
lowering a perforating module into a wellbore having a casing
incased in cement, the perforating module having at least one
explosively formed penetrator forming charge and liner; positioning
the plurality of perforating modules in the wellbore and adjacent
to a substantially unconsolidated formation; perforating the casing
and cement; and perforating the formation to a distance no greater
than a diameter of the at least one explosively formed penetrator
forming charge and liner, wherein the distance measured from a
boundary between the cement and the formation.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of and claims
priority to U.S. patent application Ser. No. 11/405,148, filed on
Apr. 17, 2006.
BACKGROUND OF THE DISCLOSURE
[0002] 1. Field of the Disclosure
[0003] The disclosure relates generally to the design of
perforating tools for use in creating perforations in wellbores to
improve the flow of fluids from the wellbore.
[0004] 2. Description of the Related Art
[0005] Commercial development of hydrocarbon fields requires
significant amounts of capital. Therefore, before field development
begins, operators desire to have as much information as possible in
order to evaluate the reservoir for commercial viability. Such
information may be acquired at the seismic exploration phase,
during well construction, prior to well completion and/or any time
thereafter.
[0006] 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 of the charges results in
significant "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.
[0007] 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.
[0008] 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.
[0009] The present disclosure addresses the problems of the prior
art.
SUMMARY OF THE DISCLOSURE
[0010] The present disclosure 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.
[0011] 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.
[0012] During the detonation, the constituent components of the
module, 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, in contrast with the large
amount of debris produced by many conventional perforation
devices.
[0013] One embodiment of the disclosure includes a method for
perforating a subterranean formation, comprising: lowering a
plurality of perforating modules in a wellbore, wherein each of the
plurality of perforating modules comprises: a central rod, wherein
the central rod comprises: an exterior load bearing portion, and an
interior detonation portion including a first explosive, a second
explosive adapted to at least partially surround the central rod,
and a liner disposed to surround the second explosive, wherein the
liner is made of a non-explosive material, and wherein the liner
has a plurality of concave arcuate surface indentations; and
detonating the plurality of perforating modules in the
wellbore.
[0014] Another embodiment of the disclosure includes a method for
perforating a subterranean formation, comprising: lowering a
plurality of perforating modules in a wellbore, wherein at least
one of the plurality of perforating modules is separated from
another of the plurality of perforating modules by a spacer, and
wherein each of the plurality of perforating modules comprises: a
central rod, wherein the central rod comprises: an exterior load
bearing portion comprised of a ceramic, an axial passage adapted to
receive hydraulic fluid disposed along the length of the exterior
load bearing portion, a wire disposed within the axial passage, and
an interior detonation portion including a first explosive, a
second explosive adapted to at least partially surround the central
rod, a liner disposed to surround the second explosive, wherein the
liner is made of a non-explosive material, and wherein the liner
has a plurality of concave arcuate surface indentations, wherein
each of the plurality of concave arcuate surface indentation is
polygonal, wherein each of the plurality of concave arcuate surface
indentations is configured to face substantially perpendicular to
the longitudinal axis of the wellbore, and wherein each of the
plurality of concave arcuate surface is linearly contiguous with at
least another of the plurality of concave arcuate surface
indentations, and a cover disposed about the liner; and detonating
the plurality of perforating modules in the wellbore.
[0015] In embodiments, the plurality of shallow concave surface
indentations may each have a cavity. The cavity may be defined by a
diameter and a depth. In one arrangement, the diameter to depth
ratio is approximately not less than two to one. In another
embodiment, the diameter to depth ratio is not less than six to
one. In still other embodiments, the depth is no greater than a
thickness of the liner. The method may also include forming a
plurality of perforations in a region adjacent to the perforating
modules, wherein the plurality of perforations extend substantially
through a cement layer into a formation a distance from a cement
face that is no greater than a diameter of the cavity. In some
applications, the distance is no greater than one-half of the
diameter of the cavity. Also, the method may include at least
partially lining the plurality of perforations in the cement layer
with a liner material.
[0016] Another embodiment of the disclosure includes a method for
perforating a subterranean formation. The method may include
lowering a perforating module into a wellbore having a casing
incased in cement, the perforating module having at least one
explosively formed penetrator forming charge and liner; positioning
the plurality of perforating modules in the wellbore and adjacent
to a substantially unconsolidated formation; perforating the casing
and cement; and perforating the formation to a distance no greater
than a diameter of the at least one explosively formed penetrator
forming charge and liner, wherein the distance measured from a
boundary between the cement and the formation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] For greater understanding of the disclosure, reference is
made to the following detailed description of the embodiments of
the present disclosure, taken in conjunction with the accompanying
drawings in which reference characters designate like or similar
elements throughout the several figures of the drawings.
[0018] FIG. 1 is a side, cross-sectional view of a wellbore
containing an exemplary perforation system constructed in
accordance with the present disclosure.
[0019] FIGS. 1a and 1b illustrate a pair of alternative
constructions for perforation systems constructed in accordance
with the present disclosure.
[0020] FIG. 2 is a side, cross-section depiction of a single
perforation module of the perforation system shown in FIG. 1.
[0021] FIG. 3 is an exterior view of the module shown in FIG.
2.
[0022] FIG. 4 is a detail view of a portion of the liner of an
exemplary perforation module showing further details concerning the
indentations.
[0023] FIG. 5 is a detail view of a portion of the liner of an
exemplary perforation module showing an alternative shape for the
indentations.
[0024] FIG. 6 is a side cross-section of the portion of liner shown
in FIG. 5, taken along lines 6-6.
[0025] FIG. 7 depicts an exemplary shot pattern that is created by
the perforation module shown in FIGS. 2 and 3.
[0026] FIG. 8 illustrates an alternative embodiment for a
perforation module in accordance with the present disclosure having
triangular indentations.
[0027] FIG. 9 illustrates a further alternative embodiment for a
perforation module in accordance with the present disclosure having
square indentations.
[0028] FIG. 10 depicts a portion of the surface of the liner of a
perforation module that utilizes octagonal indentations.
[0029] FIGS. 11-15 illustrate an exemplary initiation sequence for
a single penetrator of a perforation module in accordance with one
embodiment of the present disclosure.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0030] The present disclosure relates to devices and methods for
perforating wellbores. The present disclosure is susceptible to
embodiments of different forms. These are shown in the drawings,
and herein will be described in detail, specific embodiments of the
present disclosure with the understanding that the present
disclosure is to be considered an exemplification of the principles
of the disclosure, and is not intended to limit the disclosure to
that illustrated and described herein.
[0031] FIG. 1 illustrates an exemplary perforation system 10 that
is configured in accordance with one embodiment of the present
disclosure. 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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 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.
[0036] 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. In one embodiment, a tube 51 of cardboard or a
similar material is disposed between the central rod 42 and the
high explosive body 50.
[0037] 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 disclosure. 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.
[0038] 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 and/or geometry 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.
[0039] 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. With the benefit of the
present teachings, those skilled in the art will be able to
determine optimal configurations based upon such skill and without
undue experimentation.
[0040] 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.
[0041] The shape of the cavity 60 may determine one or more
characteristics of the penetrator and may dictate how the
penetrator is formed.
[0042] Certain shapes (such as those for conventional shaped
charges) produce a Munroe effect, whereby a small fraction (10-15%)
of the liner 52 is propelled into a target to cause a narrow and
deep penetration. Under the Munroe effect, the tip of the jet
formed by a shaped charge liner collapse travels at tremendous
velocity (8-10 km/sec). A generally conical or pyramidal cavity 60
produces a jet that forms such a penetration. For the purposes of
this disclosure, a Munroe effect penetration may be defined as a
penetration wherein a depth of penetration into the formation,
i.e., a distance from a cement face, is generally six times or more
of the diameter D of the cavity 60.
[0043] Certain other shapes produce Misznay-Schardin effect,
whereby a large fraction (90-100%) of the liner 52 is propelled
into a target to cause a wide and shallow perforation. Projectiles
formed under the Misznay-Schardin effect are commonly called
Explosively Formed Penetrators (EFPs). EFPs travel much more slowly
(.about.1 km/sec.) than the jet of a conventional shaped charge. A
generally spherical, shallow curved hollow, a shallow pyramid
indentation, or a shallow concave arcuate shaped cavity 60 forms a
projectile that makes such a penetration. For the purposes of this
disclosure, "shallow" means that the cavity 60 has a diameter D to
depth De ratio of greater than two to one. In some embodiments, the
diameter to depth ratio may be six to one or greater. The diameter
of the cavity 60 may be measured across the outer perimeter 56
(i.e. diameter D), and the depth De of cavity 60 may be measured
from the apex 62 and the plane of the ridges 58. In embodiments
where the bottom of the cavity 60 is flattened, then the depth De
may be measured between the plane of the flattened area and the
plane of the ridges 58. For a non-circular shape, the diameter D
may be considered the diameter of the circle that encompasses or
circumscribes that shape. In other aspects, the term "shallow" may
include designs wherein the depth De of the cavity 60 is
approximately the same as or less than a thickness of the liner 52.
For the purposes of this disclosure, a Misznay-Schardin effect
penetration may be defined as a penetration wherein a depth of
penetration into the formation is about one-half to one diameter of
the cavity 60. In certain embodiments, the penetration may be less
than one-half times the diameter of the cavity 60.
[0044] Embodiments of this disclosure contemplate the use of a
variety of cavity 60 shapes as required to accomplish the desired
penetration.
[0045] 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.
[0046] 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.
[0047] 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).
[0048] 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.
[0049] 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.
[0050] During detonation of the perforation modules 34, 36, 38,
directional penetrators are formed by the indentations 54, 54'. 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 of the
present disclosure 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.
[0051] 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. In one
embodiment, the fifty-one (51) perforations 86 are arranged in six
horizontal rows 88a, 88b, 88c, 88d, 88e, and 88f that alternate
between eight and nine 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 may be advantageous in some circumstances.
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 may be 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 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.
[0052] Alternative to indentations having hexagonal perimeters,
other perimeter shapes may be selected, most commonly polygonal
shapes, 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.
[0053] 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.
[0054] 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.
[0055] 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 over
triangular 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.
[0056] Turning now to FIGS. 11 through 15, an exemplary initiation
sequence is illustrated for a single EFP 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 54 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). The walls of the perforation cavity may be lined with
the liner or EFP material 25 that originally formed at least part
of the liner 52 before detonation. Thus, aspects of the present
disclosure include at least partially or substantially lining a
perforation in the cement with liner material. Some unspent
residual liner or EFP material 27 may be present at the end of the
perforation.
[0057] Referring now to FIG. 15, the penetrator 110 has formed a
tunnel in the cement 20 and has proceeded a distance 21 into a
formation 16. The distance 21 may be generally measured from a
boundary 23 between the cement 20 and the formation 16. The
distance 21 may be approximately a diameter of the cavity 60 or
less. In variants, the distance 21 may be one-half or less of the
diameter of the cavity 60.
[0058] Referring now to FIG. 1, in an illustrative method of
perforating a well, the perforation system 10 may be disposed into
the wellbore 12 on the conveyance string 32. As noted previously,
the perforation system 10 may include one or more perforating
modules that are used to perforate portions of the surrounding
formation 16. The formation 16 may be loosely consolidated,
unconsolidated or un-compacted. In one aspect, such a formation
includes sediment, rocks, sand, and other granular material that
behave more as a fluid than a solid. In another aspect, the
particles making up such a formation may move relative to one
another. In still another aspect, such a formation is not
sufficiently stable to maintain a cavity or conduit formed therein.
For example, a tunnel formed into the formation would collapse soon
after formation. In a generally unconsolidated formation 16, it may
be desired to perforate the casing and the cement, but leave the
formation 16 largely untouched. For such an application, the
perforating modules 34, 36, 38 may utilize indentations configured
to utilize the Misznay-Schardin effect forming EFPs. There may, of
course, be more or fewer than three modules, depending upon the
desired length of wellbore to be perforated.
[0059] Upon the firing of the perforating modules 34, 36, and 38,
perforations will be formed in the casing and cement in a manner as
generally shown in FIG. 15. However, the unconsolidated formation
adjacent to the casing and cement are largely untouched. In
aspects, therefore, the present disclosure provides a method of
forming fluid channels in a well to produce fluids from a
subsurface hydrocarbon reservoir that resides in an unconsolidated
or substantially unconsolidated formation. In embodiments, the
perforations in the cement may be at least partially lined with
liner material.
[0060] 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 disclosure
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.
[0061] From the above, it should be appreciated that what has been
described includes, in part, a method for perforating a
subterranean formation. The method may include lowering a plurality
of perforating modules in a wellbore and detonating the plurality
of perforating modules in the wellbore. Each of the plurality of
perforating modules may include a central rod having an exterior
load bearing portion, and an interior detonation portion including
a first explosive, a second explosive at least partially
surrounding the central rod, and a liner surrounding the second
explosive, wherein the liner has a plurality of shallow concave
surface indentations. The central rod further may include an axial
passage. In one arrangement, a conducting wire may be disposed
within the axial passage. In another embodiment, the axial passage
may receive a hydraulic fluid. In embodiments, the plurality of
shallow concave surface indentations may each have a cavity. The
cavity may be defined by a diameter and a depth. In one
arrangement, the diameter to depth ratio is approximately not less
than two to one. In another embodiment, the diameter to depth ratio
is not less than six to one. In still other embodiments, the depth
is no greater than a thickness of the liner.
[0062] In arrangements, the plurality of shallow concave surface
indentations may each have a cavity. The method may include forming
a plurality of perforations in a region adjacent to the perforating
modules, wherein the plurality of perforations extend substantially
through a cement layer into a formation a distance from a cement
face that is no greater than a diameter of the cavity. In some
applications, the distance is no greater than one-half of the
diameter of the cavity. Also, the method may include at least
partially lining the plurality of perforations in the cement layer
with a liner material.
[0063] In embodiments, the exterior load bearing portion may
comprise a frangible material. Also, each of the plurality of
shallow concave surface indentations may be linearly contiguous
with at least another of the plurality of shallow concave surface
indentations. In some arrangements, the shallow concave surface
indentations may be arcuate.
[0064] From the above, it should be appreciated that what has been
described also includes, in part, a method for perforating a
subterranean formation. The method may include lowering a
perforating module into a wellbore having a casing incased in
cement, the perforating module having at least one explosively
formed penetrator forming charge and liner; positioning the
plurality of perforating modules in the wellbore and adjacent to a
substantially unconsolidated formation; perforating the casing and
cement; and perforating the formation to a distance no greater than
a diameter of the at least one explosively formed penetrator
forming charge and liner, wherein the distance measured from a
boundary between the cement and the formation.
[0065] The foregoing description is directed to particular
embodiments of the present disclosure for the purpose of
illustration and explanation. It will be apparent, however, to one
skilled in the art that many modifications and changes to the
embodiment set forth above are possible without departing from the
scope of the disclosure. Thus, it is intended that the following
claims be interpreted to embrace all such modifications and
changes.
* * * * *