Note: Descriptions are shown in the official language in which they were submitted.
CA 02569989 2007-01-03
PERFORATING CHARGE CASE
BACKGROUND
The invention generally relates to a perforating charge case.
Perforating charges are small explosive devices that are used to create holes
in an
oil well casing and tunnels into a petroleum-bearing formation for purposes of
establishing the flow of formation fluids into the casing. Referring to a
cross-sectional
view of a perforating charge 2 in Fig. 1, the perforating charge 2 typically
includes three
basic components: 1. a cup-shaped metallic case 4 that circumscribes an axis
1; 2. a
liner 6 that resides inside the case 4, is generally metallic and typically is
conical about
the axis 1; and 3. an explosive 5 that is located inside the case 4 between
the bottom of
the case 4 and the liner 6 so that the explosive 5 surrounds the liner 6 on
the liner's outer
convex surface. The perforating charge 2 may include a connector 3 to secure a
detonating cord to the perforating charge case 4.
When the explosive 5 detonates, the explosive 5 collapses the liner 6 inwardly
onto the axis I and forces the collapsed liner 6 toward the open end of the
case 4 toward
the rock formation to form a perforating jet. The perforating jet forms a hole
in the well
casing and a tunnel in the formation. Simultaneous to the formation of the
perforating jet,
the detonating explosive 5 expands the charge case 4 radially away from the
axis I until
the case 4 fragments into several pieces. The breakup characteristics of the
case 4, such
as the size and number of fragments of the case 4, depends on several factors,
primary
among these being the mechanical properties of the case material itself. The
charge case
4 typically may be fabricated from steel, but as explained below, the case 4
may also be
formed from die-cast zinc. Charge case fragments are generally termed "case
debris," or
more generally, "charge debris."
Several perforating charges may be spatially arranged in a pattern (a spiral
pattern,
for example) in a device called a perforating gun. The perforating charges are
generally
ballistically connected via a detonating cord or some other means. In general,
two types
of perforating guns exist: 1. a hollow carrier perforating gun 10 (see Fig. 2)
that typically
includes a steel pipe 12 that houses radially oriented perforating charges 14
and isolates
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the perforating charges 14 from the wellbore environment before detonation;
and 2. a
capsule gun (not shown) that is essentially a nietallic strip or similar
device onto which
the charges are attached and are exposed to the wellbore environment before
detonation.
Referring to Fig. 3, for the hollow carrier perforating gun 10, the
perforating charges 14
create several exit holes 16 in the pipe 12 when detonated. The diameter of
each exit hole
16 may be about one fourth to one half inches, depending on the
characteristics of the
perforating charges 14 and other factors.
In a vertical well, one historical advantage of the hollow carrier perforating
gun 10
is that essentially all case debris is retained inside the gun 10. Small
quantities of case
fragments may escape the hollow carrier perforating gun 10 through the exit
holes 16
immediately after charge detonation, but most debris settles to the bottom of
the gun 10.
After detonation, the hollow carrier perforating gun 10 may be retrieved from
the vertical
wellbore via a wireline or some other arrangement. Some of the case debris may
be small
enough to fit through the exit holes 16, however, no mechanism exists to expel
large
quantities of this debris through the exit holes 16. Any debris that does exit
the hollow
carrier perforating gun 10 through the exit holes 14 falls under gravity into
the "rathole"
below the gun 10 to the bottom of the wellbore.
In highly deviated or horizontal wells, however, the situation may be quite
different. In this manner, many modern completions employ "extended reach," or
very
long horizontal sections, that are perforated. Therefore, during and after
perforating, the
hollow carrier perforating gun 10 is horizontal. After perforating, the hollow
carrier
perforating gun 10 typically is retrieved to the surface after being dragged
along a
significant length of horizontal wellbore. During this retrieval, random
rotation of the
hollow carrier perforating gun 10 may occur. All this contributes to the
significantly
increased likelihood that charge debris may escape from the gun exit holes 16.
Not only
may more debris enter the wellbore than in a vertical well, the debris may
create more
significant problems than would be encountered in a vertical well. More
specifically, the
debris may "bridge" in the well casing, causing the hollow carrier perforating
gun 10 to
get stuck during its retrieval; debris may fall into (and block production
from) any
perforations on the lower side of the well casing; and any debris that does
flow toward the
surface may collect at bends or "heels" in the well casing, for example, where
a
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horizontal section of the well casing meets a vertical section. Also, any
debris that flows
i . =
throug6the well rriay cause significant damage to both downhole and surface
equipment
(valves, for example).
For purposes of addressing the debris problem, die-cast zinc may be used as a
replacement for steel as a material to form the charge case 4. In this manner,
zinc is more
effectively pulverized than steel, and any zinc debris may be dissolved with
acid
treatment. While successful overall, a number of difficulties may be
associated with zinc
charge cases: 1. for a given design, charge performance is often sacrificed;
2. for a given
sufficient exposure time, the zinc debris is known to react with certain
completion fluids
(CaC12, etc.), forming a hard cement that adversely affects the completion; 3.
the zinc
alloys typically used may lead to significant energy-liberating reactions that
lead to
observed "gun shock" and cause significant damage to completions and
equipment; and
4: zinc liquid/vapor that is deposited on the gun carrier inner wall during
carrier strain or
deformation may lead to liquid-metal embrittlement and increase the likelihood
of gun
failure (splitting of the gun, for example).
Thus, there is a continuing need for a perforating charge case that addresses
one or
more of the problems that are stated above.
SUMMARY
In an embodiment of the invention, a perforating charge case is made by a
process
that includes forming a material into a shape for the perforating charge case
and annealing
the material. -
In another embodiment of the invention, a perforating charge case is made by a
process that includes cold forming a material into a shape for the perforating
charge case.
The cold forming produces additional recrystallization nucleation sites in the
material.
After the cold forming, the material may be annealed to decrease sizes of
grains of=the
material to improve a ductility of the material to increase fragment sizes of
the perforating
charge case when an explosive that is placed inside the perforating charge
case detonates.
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78543-52D
In some embodiments of the invention, the charge
case may be formed at least partially from copper, and in
some embodiments of the invention, the charge case may be
formed at least partially from a superplastic material.
In some embodiments of the invention, there is a
shaped charge, comprising a case, at least a portion of
which is copper that is substantially free of inclusions,
oxides, defects, and fracture recrystallization nucleation
sites.
In some embodiments of the invention, there is a
method comprising: forming a shaped charge case, at least a
portion of which is copper that is substantially free of
inclusions, oxides, defects, and fracture recrystallization
nucleation sites.
Advantages and other features of the invention
will become apparent from the following description, drawing
and claims.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a cross-sectional view of a perforating
charge case of the prior art.
Fig. 2 is a perspective view of a hollow carrier
perforating gun of the prior art before detonation.
Fig. 3 is a perspective view of a hollow carrier
perforating gun of the prior art after detonation.
Figs. 4 and 7 are perspective views of charge
cases according to different embodiments of the invention.
Fig. 5 is a flow diagram depicting a technique to
make the charge case of Fig. 4 according to an embodiment of
the invention.
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CA 02569989 2008-09-09
78543-52D
Fig. 6 is a schematic diagram illustrating a portion of a hollow carrier
perforating
gun after detonation of a perforating charge according to an embodiment of the
invention.
DETAILED DESCRIPTION
Referring to Fig. 4, an embodiment 40 of a perforating charge case in
accordance
with the invention fragments into only a few, very large pieces that are more
unlikely to
escape the exit holes of a hollow carrier perforating gun than fragments that
are created
by conventional charge cases. For example, for the case of a hollow carrier
perforating
gun, Fig. 6 depicts one 74 of only a few large fragments that are formed when
a
perforating charge that uses the perforating charge case 40 detonates. As
illustrated, the
fragment 74 remains inside an outer carrier pipe 70 of the perforating gun and
thus, does
not travel through an exit hole 72 that is formed by the perforation jet.
Due to its fragmentation characteristics, the perforating charge case 40 may
offer
one or more of-the following advantages. The downhole debris, if any, that is
created by
the perforating charge case is minimized. The debris from the perforating
charge case 40
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may be recovered when the perforating gun is retrieved to the surface of the
well. The
perforating charge case 40 does not adversely affect the completion
performance. The
perforating charge case 40 does not cause the carrier pipe of a hollow carrier
perforating
gun to split. Thus, the perforating charge case 40 addresses the debris
problem with at
least the same success as zinc, without any of the accompanying drawbacks that
are
associated with zinc. Other and different advantages may be possible in
different
embodiments of the invention.
The fragmentation characteristics of the perforating charge case 40 is
attributable
to the material that forms the perforating charge case 40 and a technique
(described
below) that is used to impart the desired characteristics into the material.
More
particularly, in some embodiments of the invention, the charge case 40 is
formed from a
material that exhibits high dynamic ductility and a density equal to or
greater than that of
steel. Such ductility permits the charge case 40 to expand more than
conventional charge
cases, thereby creating fewer and larger case material fragments. These
materials may
include, as examples, alloys and tempers of copper that are processed as
described below.
As an example, the charge casing may be made from a copper or copper alloy.
For
example, some of these coppers and alloys include C 10100 oxygen free copper,
C 1 1000
electrolytic tough pitch copper, and C65500 silicon bronze. The C10100 and Cl
1000
materials are essentially pure copper (approximately 99.90% or greater
copper), with
trace amounts of additives that may enhance machining of the copper or other
properties.
Materials other than copper may be used, as noted below. In general,
regardless of the
material that is used to form the charge case, the material has a ductility
approximately
greater than 5%, in some embodiments of the invention. In other embodiments of
the
invention, the ductility may be approximately greater than 10%, may be
approximately
greater than 20%, and may be greater than 200%, as just a few examples.
The purity of the material is a significant factor that governs the dynamic
ductility
that may be achieved, as inclusions (oxygen inclusions, for example) in the
material
provide microvoid nucleation sites leading to ductile fracture, thus
decreasing the
dynamic ductility of the charge case 40.
The dynamic ductility of the above-described materials may be further
increased
by a technique 50 that is depicted in Fig. 5. In this manner, it has been
discovered that by
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forging, or cold forming (block 52), these materials, additional
recrystallization
= ~ - .
nucleation sites are established in the material. As example, a single piece
of material
may be forged to create the perforating charge case 40 in the general shape
(described in
more detail below) that is depicted in Fig. 4. The cold forming results in
less scrap
material that is left over when the charge case 40 is manufactured, thereby
resulting in a
less expensive technique to make the charge case 40, as compared to machining,
for
example.- Furthermore, the use of cold forming may result in increased
strength for the
case 40. This additional strength may be needed to withstand the loading
forces that may
be used to press a perforating charge explosive and perforating charge liner
(the explosive
and liner are not shown) into the perforating charge case 40.
As noted above, the cold forming increases the number of recrystallization
nucleation sites from which additional grains may be. formed in the case
material when
the case 40 is annealed (block 54), another part of the technique that is
depicted in Fig. 5.
As an example, the case 40 may be heated to an appropriate recrystallization
temperature
(for example, 750 to 900 Fahrenheit (F) for copper) for a time interval
approximately in
the range of 15 min. to 1 hour, depending on the particular embodiment. The
annealing
recrystalizes the casing material to impart generally finer, or smaller, sizes
to grains of the
material, as compared to the sizes of the grains of the casing material before
application
of the technique 50. The smaller grain size of the material, in turn,
increases the dynamic
ductility of the case 40 so that the case 40 does not fragment into small
pieces when the
perforating charge detonates.
In some embodiments of the invention, the explosive may be compressed into an
explosive pellet to eliminate the need to compress loose explosive powder
placed in the
charge case 40, thereby reducing the amount of loading on the charge case 40.
Experiments were conducted to examine the debris created by the charge case 40
when an explosive inside the case 40 detonates. A copper alloy was used in
these
experiments as the material for the charge case 40. The data from these
experiments
demonstrates a significant improvement in case breakup characteristics, as
compared to a
steel case (of similar design) that was not processed by the technique 50.
When screened
through standard sieves, the mass percentage of charge case debris collected
when using
the charge case 40 was significantly larger than the gun exit hole size and
had an average
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CA 02569989 2007-01-03
size that was a factor of two to three times larger than the size of fragments
formed from
> . _
steel charge cases. This data shows that the quantity of debris that is
capable of exiting
the gun is reduced by more than half. These tests neglect in-gun bridging
effects that
further reduce the amount of debris that is capable of exiting the gun carrier
when the
charge case 40 is used.
Other materials may be used in place of copper or copper alloys for the casing
material. These casing materials include other high-density ductile materials
that
generally exhibit a face-centered-cubic (FCC) crystal structure. These
materials include
numerous other copper and nickel alloys, as just a few examples. As other
examples,
nickel, lead, gold or silver may also be used as materials that, when the
technique 50 is
used, possess the dynamic ductility that is needed to cause the charge case 40
to fragment
into large pieces of debris.
Referring back to Fig. 4, as another example of a possible shape of the case
40
(although other shapes are possible), the charge case 40 may generally
circumscribe an
axis 47 and be formed from a generally circularly cylindrical section 44 (that
circumscribes the axis 47) and a generally conical section 42 (that
circumscribes the axis
47). In this manner, the smaller diameter end of the generally conical section
42 may be
closed off by an end 43 that forms the bottom of the charge case 40; and its
larger
diameter end, the generally conical section 42 transitions into the generally
circularly
section 44 at one end of the section 44. The other end of the section 44 forms
an open
end 46 that receives a case cap (not shown). As noted above, the charge case
40 may be
cold formed. For example, a forging tool that has a die in the shape that is
depicted in
Fig. 4 may be used to stamp out the shape from a piece of material.
Other embodiments of the invention are within the scope of the following
claims.
For example, the shape of the case 40 may be formed by machining the charge
case
material.
As another example of another embodiment of the invention, more than one
material may be used to form multiple layers of the charge case 40. As a more
specific
example, one material that has a relatively low dynamic ductility may be used
to
strengthen the case, and another material that has a high dynamic ductility
may be used
for purposes of increasing the fragment sizes of the case debris. In this
manner, Fig. 7
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depicts a charge case 100 that is formed from an inner material 104 (of an
inner layer)
that is contoured to the general shape of the charge case 100 and provides
structural
support for the charge case 100. The inner material 104 is surrounded by an
outer
material 102 (of an outer layer) that is contoured to the general shape of the
charge case
100 and exhibits a high dynamic ductility. The inner 104 and outer 102
materials may or
may not be bonded or laminated together. The inner 104 and outer 102 materials
may
form a composite material. -As a more specific example, the inner material 104
may be
steel, and the outer material 102 may be a copper or copper alloy (as
examples) that is
formed by the technique 50. The copper material may be thicker than the steel
material,
in some embodiments of the invention. Thus, the outer material 102-limits the
fragmentation of the inner material 104, and the inner material 104 provides
strength to
supplement the strength of the outer material 102. Other materials may be used
for the
inner 104 and outer 102 materials, and the charge case may include more than
two
material layers, in other embodiments of the invention.
As yet another example of an additional embodiment of the invention, the
charge
case may be made partially or totally out of a superplastic material. A
superplastic
material exhibits high elongation or deformation without fracturing or
breaking. The
superplastic material may be a metal (such as aluminum, copper, titanium,
magnesium, or
other light metals) or some other suitable material. Some superplastic
materials may
exhibit superplastic characteristics at about 95% to 100% of the melting
temperature of
the material. Other superplastic materials may exhibit superplastic
characteristics at other
temperature ranges, such as greater than about 50% of the melting temperature.
Thus,
depending on the desired application, the superplastic material selected may
be one that
exhibits superplastic characteristics at a desired temperature range. This
temperature
range includes the temperature that the superplastic material reaches when the
explosive
of the perforating charge detonates. In other embodiments, other highly
deformable
materials that exhibit the desired deformation characteristics ar a selected
temperature
while still maintaining structural integrity (e.g., without breaking or
fracturing) may be
used as a material for the charge case.
A superplastic material is a polycrystalline solid that has the ability to
undergo
large plastic strains prior to failure. For deformation in uni-axial tension,
elongation to
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failure in excess of 200% are usually indicative of superplasticity. For
superplastic
behavior, a material must be capable of being processed into a fine equi-axed
grain
structure that will remain stable during deformation. The grain size of
superplastic
materials are made as small as possible, normally in the range of 2 to 10
micrometers,
although materials with larger grain sizes may also exhibit superplasticity.
The superplastic material may be used to form all or part of the charge case
in
some embodiments of the invention. Furthermore, in some embodiments of the
invention, the superplastic material may be used to form all or part of a
layer of a multiple
layer charge case. Other arrangements are possible.
Other embodiments are within the scope of the claims. For example, in some
embodiments of the invention, the case charge may be cold-formed and/or
machined only
without annealing the case charge material. In this manner, sufficiently high
dynamic
ductility may be achieved without annealing the charge case material.
While the invention has been disclosed with respect to a limited number of
embodiments, those skilled in the art will appreciate numerous modifications
and
variations therefrom. It is intended that the appended claims cover all such
modifications
and variations as fall within the true spirit and scope of the invention.
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