PERFORATING GUN WITH VARIABLE FREE GUN VOLUME
TECHNICAL FIELD
The present disclosure relates generally to equipment
utilized and operations performed in conjunction with a
subterranean well and, in an embodiment described herein,
more particularly provides a perforating gun with a variable
free gun volume.
BACKGROUND
It is well known that a pressure reduction can be
experienced in a wellbore when well fluid rushes into void
spaces of a perforating gun after the perforating gun is
fired. Unfortunately, however, this pressure reduction can
be too large, creating an excessively underbalanced
condition which can break down a perforated formation near
the wellbore, leading to production of sand, etc.
For this reason and others, it would be advantageous to
be able to selectively vary a free gun volume of a
perforating gun.
SUMMARY
In carrying out the principles of the present
disclosure, improvements are provided to the art of well
perforating. One example is described below in which a free
gun volume of a perforating gun can be increased or
decreased, based on a desired pressure reduction in a
wellbore following detonation of the perforating gun.
Another example is described below in which a material is
flowed about perforating charges in the perforating gun, to
thereby reduce the free gun volume.
In one aspect, this disclosure provides to the art a
method of adjusting a pressure reduction to occur in a
wellbore following firing of at least one perforating gun.
The method can include determining a desired free gun volume
which corresponds to a desired pressure reduction in the
wellbore resulting from firing of the perforating gun; and
varying a free gun volume of the perforating gun until the
free gun volume is substantially the same as the desired
free gun volume.
This method can be performed separately for each
perforating gun or set of perforating guns used to perforate
multiple formation intervals.
In another aspect, this disclosure provides to the art
a well system which can include at least one perforating gun
positioned in a wellbore, the perforating gun comprising
multiple perforating charges and a free gun volume, and the
free gun volume being reduced by presence of a flowable
material about the multiple perforating charges.
These and other features, advantages and benefits will
become apparent to one of ordinary skill in the art upon
careful consideration of the detailed description of
representative embodiments of the disclosure hereinbelow and
the accompanying drawings, in which similar elements are
indicated in the various figures using the same reference
numbers .
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a representative partially cross-sectional
view of a well system and associated method which can embody
principles of the present disclosure.
FIG. 2 is a representative partially cross-sectional
view of a perforating gun which may be used in the well
system and method of FIG. 1 .
FIG. 3 is a representative graph of free gun volume vs.
dynamic underbalance .
FIG. 4 is a representative partially cross-sectional
view of another configuration of the well system.
DETAILED DESCRIPTION
Representatively illustrated in FIG. 1 is a well system
10 and associated method which can embody principles of the
present disclosure. In the example depicted in FIG. 1 , a
perforating gun 12 is installed in a wellbore 14 lined with
casing 16 and cement 18. The perforating gun 12 is used to
form perforations 20 extending through the casing 16 and
cement 18, so that communication is established between the
wellbore 14 and an earth formation 22 surrounding the
wellbore.
Perforating charges 24 (not visible in FIG. 1 , see FIG.
2 ) in the perforating gun 12 are detonated to form the
perforations 20. Following the detonation of the
perforating charges 24, there is a reduction in pressure in
the wellbore 14 due to fluids in the wellbore flowing into
the now-perforated gun 12.
In one unique aspect of the system 10, a free gun
volume of the perforating gun 12 can be selectively varied,
so that a predetermined desired pressure reduction in the
wellbore 14 will follow detonation of the perforating
charges 24. The free gun volume is the volume in the
perforating gun 12 into which the well fluid flows following
detonation of the perforating charges 24.
This free gun volume is typically sealed at atmospheric
pressure when the perforating gun 12 is assembled at
surface. By varying the free gun volume, the pressure
reduction in the wellbore 14 can be selectively tailored to
particular wellbore circumstances (e.g., different fluids,
pressures, temperatures, etc.), to particular formation
characteristics (e.g., extent of consolidation, desired
debris removal, etc.), to other well equipment (e.g., to
prevent adversely affecting a packer, etc.), and/or for
other purposes.
At this point it should be pointed out that the well
system 10 and method as depicted in the drawings and
described herein is merely one example of a wide variety of
different well systems and methods which can incorporate the
principles of this disclosure. Therefore, it should be
understood that those principles are not limited in any
manner to the details of the well system 10 and method, or
of any of their components.
Referring additionally now to FIG. 2 , an example of a
perforating gun 12 which can be used in the well system 10
and method is representatively illustrated. Of course, the
perforating gun 12 can also be used in other well systems
and methods, as well.
The perforating gun 12 includes a generally tubular
outer body 26, the perforating charges 24 and, in this
example, a generally tubular charge carrier 28. A
detonating cord 30 transfers a detonation train along the
length of the perforating gun 12.
FIG. 2 depicts only a small axial section of the
perforating gun 12. Although two perforating charges 24 are
shown in FIG. 2 , any number and/or arrangement of
perforating charges may be used in other examples. The
charge carrier 28 is not necessarily tubular in form, since
other shapes of charge carriers (e.g., sheet metal, formed
wire, strips, plastics, molded, cast, etc.) can be used in
other examples.
It is also not necessary that all of the components of
the perforating gun 12 are separately constructed. Instead,
any or all of the components could be integrated with any
other components. It is not necessary for all of the
components of the perforating gun 12 described herein to be
present in a perforating gun which comes within the scope of
this disclosure.
The perforating gun 12 has a free gun volume 32 which
will be occupied by fluid from the wellbore 14 following
detonation of the perforating charges 24. The free gun
volume 32 is reduced, as depicted in FIG. 2 , by addition of
a material 34 into the perforating gun 12.
By reducing the free gun volume 32, a pressure
reduction in the wellbore 14 following firing of the
perforating gun 12 will also be reduced. This is due to the
fact that fluid from the wellbore 14 will have less volume
to occupy in the perforating gun 12 after the charges 24 are
detonated.
The material 34 is preferably flowable about the
components of the perforating gun 12, for ease of
installation. The material 34 could be in granular, powder,
fluid, or other form. The material 34 preferably has the
capability to flow through small openings and fill voids in
the outer body 26.
If in powder form, moisture is preferably avoided,
however if the material 34 comprises sodium chloride, some
moisture from humidity during assembly of the perforating
gun 12 can be permitted. If magnesium chloride is used in
the material 34, however, moisture is preferably avoided.
The material 34 is preferably dispersible after the
perforating operation, so that it does not pose a possible
hindrance to future operations. The material 34 could, for
example, be dissolvable in the well fluid. When the
material 34 is dispersed, it preferably does not adversely
affect the formation 22, or any components of the well
(e.g., via corrosion, etc.).
If the well fluid is aqueous, the material 34 could be
at least partially water-dissolvable. Suitable waterdissolvable
materials can include NaCl, KC1, MgCl 2, CaCl2,
etc. NaCl, KC1 and CaCl2 in particular are heat resistant,
with melting points well above 300 degrees C .
If the well fluid comprises a hydrocarbon fluid, the
material 34 could be at least partially dissolvable in the
hydrocarbon fluid. Suitable materials can include rosemary
extract powder, etc.
A cover 36 can be positioned over the outer ends of the
charges 24, to thereby prevent the material 34 from getting
into an interior 38 of each charge. Exclusion of the
material 34 from the interior 38 of the charge 24 allows an
optimum jet to be formed in the interior of the charge when
its explosive is detonated. Suitable materials for the
covers 36 can include aluminum, aluminum foil, plastics,
sheet metal, etc.
In one method of using the material 34, a desired
pressure reduction in the wellbore 14 is determined based on
characteristics of the formation 22 (e.g., the formation
structure, type, extent of consolidation, porosity,
permeability, etc.), dimensions of the various components,
fluids in the wellbore, etc. A desired free gun volume can
then be determined, based on the desired pressure reduction.
The perforating gun 12 can be assembled with the
perforating charges 24, charge carrier 28 and detonating
cord 30, leaving a free gun volume 32 in the interior of the
outer body 26. Then the free gun volume 32 can be reduced
by adding the material 34 to the interior of the body 26.
The free gun volume 32 is reduced until it matches the
desired free gun volume to produce the desired pressure
reduction in the wellbore 14.
Of course, other methods may be used in keeping with
the principles of this disclosure. In another example, the
perforating gun 12 could initially have the material 34
therein, and then the material could be removed from the
interior of the body 26 to thereby increase the free gun
volume to a desired level.
Referring additionally now to FIG. 3 , a graph of free
gun volume vs. desired dynamic underbalance is
representatively illustrated. In this example, it can be
seen that, as the free gun volume increases, the dynamic
underbalance (pressure differential from the formation 22 to
the wellbore 14) also increases.
The dynamic underbalance increases when more pressure
reduction is produced following firing of the perforating
gun 12. Therefore, the dynamic underbalance can be
controlled by controlling the pressure reduction in the
wellbore 14 following firing of the perforating gun 12.
However, it should be clearly understood that it is not
necessary for the free gun volume and the dynamic
underbalance to be related as depicted in FIG. 3 , and it is
not necessary for an underbalance to be created in other
examples. The pressure reduction could result in less
overbalance in some examples, rather than resulting in an
underbalance.
Referring additionally now to FIG. 4 , another
configuration of the well system 10 is representatively
illustrated. In this configuration, the wellbore 14 is
generally horizontal, but the wellbore could extend in any
direction in other examples.
Multiple intervals 22a, b are penetrated by the wellbore
14. These intervals 22a, b are isolated from each other in
the wellbore 14 by packers 40. Multiple perforating guns 12
are to be used for perforating the respective multiple
intervals 22a, b .
The intervals 22a, b could be different zones of the
same earth formation 22, or they could be intervals of
separate formations. If the intervals 22a, b have different
characteristics, it may be advantageous to tailor the
perforating operation, so that optimum pressure levels are
achieved in the wellbore 14 adjacent each of the intervals.
For example, it may be advantageous to produce
different pressure levels in the wellbore 14 adjacent the
interval 22a, as opposed to pressure levels in the wellbore
adjacent the interval 22b. Even if it is desired to produce
the same pressure levels in the wellbore 14 adjacent both of
the intervals 22a, b , different characteristics of the
perforating guns 12, other components in the well, length of
the intervals, etc., may require that the free gun volumes
of the perforating guns be varied in order to achieve the
desired pressure levels.
The methods described herein permit the free gun
volumes of the perforating guns 12 to be individually
varied, so that desired pressure reductions are produced
following firing of the perforating guns. This allows an
enhanced degree of customization of the perforating
operation, so that optimum results can be more easily and
economically achieved.
Although only one perforating gun 12 is depicted in
FIG. 4 for each of the intervals 22a, b , it will be
appreciated that any number of perforating guns could be
used for any of the intervals. Where only one perforating
gun 12 is shown in FIGS. 1 & 4 , any other number, spacing,
type, etc., of perforating guns may be used.
It may now be fully appreciated that the above
disclosure provides advancements to the perforating art in
the form of a method of adjusting a pressure reduction to
occur in a wellbore 14 following firing of at least one
perforating gun 12. The method can include determining a
desired free gun volume which corresponds to a desired
pressure reduction in the wellbore 14 resulting from firing
of the perforating gun 12, and adjusting a free gun volume
32 of the perforating gun 12 until the free gun volume 32 is
substantially the same as the desired free gun volume.
Adjusting the free gun volume 32 can include adjusting
a volume of material 34 in the perforating gun 12.
The method can include positioning a cover 36 on a
perforating charge 24, thereby isolating the material 34
from an interior 38 of the perforating charge 24.
The material 34 can be at least partially dispersible
in well fluid. The material 34 may be at least partially
dissolvable in well fluid.
The material 34 can be dissolvable in water or
hydrocarbon fluid.
The at least one perforating gun 12 may comprise
multiple perforating guns 12. The determining step can
include determining an individual desired free gun volume
for each of the perforating guns 12.
Also provided by this disclosure is a method of
perforating multiple formation intervals 22a, b . The method
can include determining a first desired free gun volume for
a first one of the perforating guns 12; varying a free gun
volume 32 of the first perforating gun 12 until the first
perforating gun free gun volume 32 is substantially the same
as the first desired free gun volume; determining a second
desired free gun volume for a second one of the perforating
guns 12; and varying a free gun volume 32 of the second
perforating gun 12 until the second perforating gun free gun
volume 32 is substantially the same as the second desired
free gun volume.
The above disclosure also provides a well system 10 to
the art. The well system 10 can include at least one
perforating gun 12 positioned in a wellbore 14, the
perforating gun 12 comprising multiple perforating charges
24 and a free gun volume 32. The free gun volume 32 can be
reduced by presence of a flowable material 34 about the
multiple perforating charges 24.
The well system of claim 14, wherein each perforating
charge has a cover which excludes the material from an
interior of the perforating charge.
It is to be understood that the various embodiments of
the present disclosure described herein may be utilized in
various orientations, such as inclined, inverted,
horizontal, vertical, etc., and in various configurations,
without departing from the principles of the present
disclosure. The embodiments are described merely as
examples of useful applications of the principles of the
disclosure, which is not limited to any specific details of
these embodiments .
Of course, a person skilled in the art would, upon a
careful consideration of the above description of
representative embodiments of the disclosure, readily
appreciate that many modifications, additions,
substitutions, deletions, and other changes may be made to
the specific embodiments, and such changes are contemplated
by the principles of the present disclosure. Accordingly,
the foregoing detailed description is to be clearly
understood as being given by way of illustration and example
only, the spirit and scope of the present invention being
limited solely by the appended claims and their equivalents.
WHAT IS CLAIMED IS:
1 . A method of adjusting a pressure reduction to
occur in a wellbore following firing of at least one
perforating gun, the method comprising:
determining a desired free gun volume which corresponds
to a desired pressure reduction in the wellbore resulting
from firing of the perforating gun; and
varying a free gun volume of the perforating gun until
the free gun volume is substantially the same as the desired
free gun volume.
2 . The method of claim 1 , wherein varying the free
gun volume further comprises varying a volume of material in
the perforating gun.
3 . The method of claim 2 , further comprising
positioning a cover on a perforating charge, thereby
isolating the material from an interior of the perforating
charge .
4 . The method of claim 2 , wherein the material is at
least partially dispersible in well fluid.
5 . The method of claim 2 , wherein the material is at
least partially dissolvable in well fluid.
6 . The method of claim 2 , wherein the material is
dissolvable in water.
7 . The method of claim 2 , wherein the material is
dissolvable in hydrocarbon fluid.
8 . The method of claim 1 , wherein the at least one
perforating gun comprises multiple perforating guns, and
wherein the determining step further comprises determining
an individual desired free gun volume for each of the
perforating guns.
9. A method of perforating multiple formation
intervals, the method comprising:
determining a first desired free gun volume for a first
one of the perforating guns;
varying a free gun volume of the first perforating gun
until the first perforating gun free gun volume is
substantially the same as the first desired free gun volume;
determining a second desired free gun volume for a
second one of the perforating guns; and
varying a free gun volume of the second perforating gun
until the second perforating gun free gun volume is
substantially the same as the second desired free gun
volume .
10. The method of claim 9 , wherein varying the first
perforating gun free gun volume further comprises
introducing a volume of material into the first perforating
gun .
11. The method of claim 10, further comprising
positioning a cover on a perforating charge, thereby
isolating the material from an interior of the perforating
charge .
12. The method of claim 10, wherein the material is at
least partially dispersible in well fluid.
13. The method of claim 10, wherein the material is at
least partially dissolvable in well fluid.
14. The method of claim 10, wherein the material is
dissolvable in water.
15. The method of claim 10, wherein the material is
dissolvable in hydrocarbon fluid.
16. A well system, comprising:
at least one perforating gun positioned in a wellbore,
the perforating gun comprising multiple perforating
charges and a free gun volume, and
the free gun volume being reduced by presence of a
flowable material about the multiple perforating charges.
17. The well system of claim 16, wherein the material
is at least partially dispersible in well fluid.
18. The well system of claim 16, wherein the material
is at least partially dissolvable in well fluid.
19. The well system of claim 16, wherein the material
is dissolvable in water.
20. The well system of claim 16, wherein the material
is dissolvable in hydrocarbon fluid.
21. The well system of claim 16, wherein the at least
one perforating gun comprises multiple perforating guns, and
wherein each perforating gun has a corresponding free gun
volume .
22. The well system of claim 16, wherein each
perforating charge has a cover which excludes the material
from an interior of the perforating charge.