Note: Descriptions are shown in the official language in which they were submitted.
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ICR 93/059
CASING CONVEYED SYSTEM FOR COMPLETING A WELLBORE
Field of the
~'nvention
This invention relates to completing a well
traversing earth formation in a borehole and more
particularly to completing the well by means of perforating
devices positioned on a casing string and also by using
explosive devices located on the casing string to open
normally closed flow path members extending between the
casing string and earth formation traversed by a borehole
in which the casing string is positioned.
Backcrround of the Invention
In the process of establishing an oil or gas
well, the well is typically provided with an arrangement
for selectively excluding fluid communication with certain
zones in the formation traversed by the well to avoid
communication with undesirable fluids. A typical method of
controlling the zones with which the well is in fluid
communication is by running well casing down into the well
and then sealing the annulus between the exterior of the
casing and the walls of the wellbore with cement.
Thereafter, the well casing and cement may be perforated at
preselected locations by a perforating device or the like
to establish a plurality of fluid flow paths between the
pipe and the product bearing zones in the formation.
Unfortunately, the process of perforating through the
casing and then though the layer of cement dissipates a
substantial portion of the energy from the perforating
device and the formation receives only a minor portion of
the perforating energy.
Conventional perforating systems can be fixed
inadvertently in that they are usually armed when extended
into a wellbore and stray electrical signal or charge can
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cause premature firing which is a potential safety and
operation problem.
Conventional perforating systems are quite
expensive especially in terms of the rig time necessary to
run a perforating gun into a well. This is particularly
true for tubing conveyed systems where pipe is made up to
run a perforating gun into a high angled hole for example.
Additionally, when completing high angled or
horizontal borehole sections in a well, whether cased or in
open holes, it is often a problem getting perforating
apparatus into the high angled section because the gravity
factor may not be sufficient to assist in running the
equipment and friction between the equipment and borehole
walls or pipe further hinders such operations. For this
same reason it is difficult in many cases to run casing
into such a well, and when casing is installed, the typical
bow spring centralizers which are used are ineffective and
it is therefore difficult to center the casing in the
borehole in order to effectively cement the casing.
Accordingly, it is an object of the present
invention to provide a method and apparatus for opening a
flow path between the casing string and the formation in a
wellbore which overcomes or avoids the above noted
limitations and disadvantages of the prior art.
It is a further object of the present invention
to provide a method and apparatus for perforating a
wellbore wherein a casing string is centered in the
wellbore to provide for effective cementing of the casing
even when installed in a high angle borehole, and also
wherein perforating charges are directed into the formation
without penetrating the casing pipe and additionally
wherein the casing pipe is in an underbalanced pressure
condition.
It is yet another object of the present invention
to provide a safe and less expensive method and apparatus
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for establishing a fluid communication path between the
interior of casing pipe set in a borehole and an earth
formation by using explosives to selectively opening a
plurality of flow path devices mounted in the casing wall.
Summary of the Invention
The above and other ob j ects and advantages of the
present invention have been achieved in the embodiments
illustrated herein by the provision of an apparatus and
method for establishing a fluid communication path between
the interior of casing pipe set in a borehole and an earth
formation by using an explosive device to selectively
opening a plurality of flow path devices extended between
the casing pipe bore and the horehole wall. These
explosive devices may include detonators and associated
shaped charges which are arranged in a tubular member
extended from the casing wall. Additionally, the
detonators may be arranged to be detonated by a pressure or
shock wave originating in the casing pipe and spaced from
the tubular member.
Additionally, the charges are placed in
extendable pistons mounted in a casing string and a
pressure wave producing device is run into the casing
string in a separate operation. The casing may also then
be cemented before the charges are detonated. By placing
the charges in pistons extendable from the casing string,
the charges are directed into the formation without passing
through the casing and/or cement. The charges can be
loaded into the pistons at the well site and then the
pistons are assembled into the casing pipe to ensure a safe
operation.
In one embodiment, the system comprises a piston
for being mounted in an opening in the peripheral wall of
the pipe and for extending generally radially outwardly
from the pipe to contact the wall of the wellbore wherein
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the piston includes an explosive device therein. The
explosive device is comprised of a detonating charge which
is positioned in a plug that closes the piston to fluid
communication between the casing and formations. A rupture
disc separates the detonating charge from the casing bore.
A shaped charge is positioned in the piston adjacent the
detonating charge.
The shaped charge is assembled in a canister
which is conveniently assembled into the piston. The
piston is then assembled into the casing wall. These
operations may be performed at the well location to provide
a more safe procedure. The explosive device is run into
the borehole on the casing in an unarmed condition in that
the initiating device for initiating the detonator is run
into the casing after the casing is set in the wellbore.
A deploying device deploys the piston from a
retracted position which is generally within the maximum
exterior profile of the pipe to an extended position
wherein the piston extends generally radially from the
opening toward contact with the wall of the wellbore. Some
of the pistons will contact the wall of the wellbore and
other will not. A detonation device is then positioned in
the wellbore for detonating the explosive device in the
piston while the piston is in its deployed position against
the wall of the formation so as to perforate the formation
by an explosive proximate to the formation. The piston
when extended serves to center the pipe in the borehole and
is substantially clear of the inner bore of the pipe to
render the bore of the pipe full open.
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Brief Description of the Drawi~r~,c~s
Figure 1 is a cross-sectional view of a wellbore
traversing earth formations with a casing string arranged
therein and spaced from the walls of the wellbore by a
plurality of downhole activated pistons which are shown
being activated to an extended position and which embody
features of the present invention.
Figure 2 is an enlarged cross-sectional end view
of the casing taking along lines 2-2 in Figure 1, wherein
the centralizers are shown extended to center the casing
string in the wellbore.
Figure 3 is a cross-sectional end view similar to
Figure 2 prior to the casing being centralized and with the
downhole activated centralizers in the retracted position.
Figure 4 is an enlarged cross-sectional view of
a centralizer piston having a detonation device and shaped
charge positioned therein, with the piston shown in a
retracted or running-in position relative to the casing
wall.
Figure 5 is an enlarged cross-sectional view of
the centralizer piston of Figure 4 in an extended position
wherein the outer end of the piston is in contact with an
earth formation.
Figure 6 is a cross-sectional view of a wellbore
showing a casing centralized in a high angled or horizontal
borehole by pistons in an extended position and further
showing a pressure wave generating device positioned in the
casing by means of a tubing string.
s i a a t
Referring first to Figure 1 of the drawings, a
wellbore W is shown having been drilled into the earth
formations such as for the exploration and production of
oil and gas. The illustrated wellbore W includes a
generally vertical section A, a radial section B leading to
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a horizontal section C. The wellbore has penetrated
several formations, one of which may be a hydrocarbon-
bearing zone F. Moreover, the wellbore W was drilled to
include a horizontal section C which has a long span of
contact with the formation F of interest, which may be a
hydrocarbon-bearing zone. With a long span of contact
within a pay zone, it is likely that more of the
hydrocarbon present will be produced. Unfortunately, there
are adjacent zones which have fluids such as brine that may
get into the production stream and thereafter have to be
separated from the hydrocarbon fluids and disposed of at
additional costs. Accordingly, fluid communication with
such adjacent zones is preferably avoided.
To avoid such communication with nonproduct
bearing zones, wellbores are typically cased and cemented
and thereafter perforated along the pay zones. However, in
the highly deviated portions of a wellbore such as the
radial section B and the horizontal section C of the
wellbore, the casing tends to lay against the bottom wall
of the wellbore, thereby preventing cement from encircling
the casing and leaving a void for wellbore fluids such as
brine to travel along the wellbore arid enter the casing far
from the formation from which it is produced. In the
illustrated wellbore W, a casing string or liner 60 has
been run therein which is spaced from the walls of the
wellbore by a plurality of downhole activated pistons
generally indicated by the number 50, which serve to
centralize the casing. The downhole activated pistons or
centralizers 50 are retracted into the casing 60 while it
is being run into the wellbore as is illustrated by the
centralizers 50 in Figure 1 which are ahead of an activator
or pusher 82. Once the casing 60 is suitably positioned,
the centralizers 50 are deployed to project outwardly from
the casing as illustrated behind the activator or in
Figure 1. The centralizers 50 move the casing from the
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walls of the wellbore if the casing 60 is laying against
the wall or if the casing is within a predetermined
proximity to the wall of the wellbore W. This movement
away from the walls of the wellbore will thereby establish
an annular free space around the casing 60. The
centralizers 50 maintain the spacing between the casing 60
and the walls of the wellbore W while cement is inj ected
into the annular free space to set the casing 60. The
pistons, however, are latched in an extended position and
will thereby maintain the casing 60 centered even if the
casing is not cemented.
The centralizers 50 are better illustrated in
Figures 2 and 3 wherein they are shown ~n the extended and
retracted positions; respectively. Referring specifically
to Figure 2, seven centralizers 50 are illustrated for
supporting the casing 60 away from the walls of the
wellbore W although only four are actually shown contacting
the walls, of the weilbore W. It should be recognized and
understood that the centralizers work in a cooperative
effort to centralize the casing 60 in the wellbore W. The
placement of the centralizers 50 in the casing 60 may be
arranged in any of a great variety of arrangements.
Applicar..rs' related U.S. Patent No.- 5,228,518 describes
these arrangements 'in greater detail.
Referring again to Figures 2 and 3, the seven
illustrated centralizers 50 are evenly spaced around the
casing 60. As the casing is centralized, an annular space
70 is created around the casing within the wellbore. The
casing 60 is run into the wellbore with the centralizers 50
retracted as illustrated in Figure 3 which allows
substantial clearance around the casing 60 and permit the
casing 60 to follow the bends and turns of the wellbore W.
Such bends and turns particularly arise in a highly
,deviated or horizontal hole. With the centralizers 50
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retracted, the casing 60 may be rotated and reciprocated to
work it into a suitable position within the wellbore:
Moreover, the slim dimension of the casing 60 with the
centralizers 50 retracted (Figure 3) may allow it to be run
into wellbores that have a narrow dimension or that have
narrow fittings or other restrictions.
In Figures 2 and 3 and in subsequent figures as
will be explained below, the centralizers 50 may present
small bulbous portions 80 on the outside of the casing 60.
It is preferable not to have any dimension projecting out
from the casing to minimize drag and potential hangups
while moving the string. However, as will be discussed
below, the bulbous portions 80 are utilized in some
embodiments especially in smaller diameter casings such is
often used in horizontal holes when they are cased. It
should also be recognized that the bulbous portions 80 are
rounded to slide better along the walls of the wellbore and
that the casing string 60 will include collar sections 90
that will extend out radially farther than the bulbous
portion (see Figure 3). Thus, the collar sections 90
present the maximum outer profile of the casing string even
when the bulbous portions are present. The outward
projection of the retracted centralizers 50 being within
the maximum outer profile of the casing string 60 is
believed to minimize any problems of running the casing.
Referring again to Figure 1, a deploying device
or pusher 82 which moves from the top of the casing to its
bottom end is shown positioned within the horizontal curved
section B of the casing string. The deploying device 82 is
sized to push the pistons 50 from a retracted to an
extended position. It is noted that the centralizers or
pistons 50 behind or to the left of the pusher 82 are in an
extended position having been engaged by the tapered nose
portion 85 of the pusher. The tapered portion 85 engages
the inner ends of the pistons and pushes them outwardly as
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the piston travels until the body portion 83 has passed the
piston whereupon the piston will be fully extended and
locked into an extended position as will be hereinafter
described. The centralizers in front of the pusher 82 are
still in a retracted position and consequently the
horizontal portion C of the casing in front of the pusher
is shown lying on the bottom side of the borehole. The
upper vertical section A and radial section B are shown
centered in that the pistons 50 have been deployed to an
extended position. The activator device shown in Figure 1
is a pumpable activator or deploying device having a tail
pipe 81 which extends rearwardly from the main body portion
83 and seals the rear end of the device to the inside of
the casing so that the device may be pushed down through
the casing 60 by the application of hydraulic pressure. In
addition, the activator may be run into the casing string
on the end of a pipe string such as a drill pipe or coiled
tubing wherein force is applied to the pipe string and thus
to an activation device to engage and push out or extend
the pistons 50.
The centralizers or pistons may take many forms
and shapes as is also described in Patent No. 5, 246, 861.
In the present application, the piston or centralizer 50 is
shown in Figures 4 and 5 as including an explosive charge
for perforating formations in the borehole. Referring
first to Figure 4, the centralizer 50 has a cylindrical or
substantially cylindrical barrel portion or piston 12 which
is slidably received in a bore in button 14. The button 14
is threadedly received within a tapped hole 16 which
extends transversely through the wall of casing 60. A
bulbous or rounded outer portion 80 extends outwardly
slightly beyond the outside wall of the casing 60 but only
to provide an adequate seat for the button 14 in thin wall
smaller diameter casing and is constructed so that the
outer extension of the bulbous portion 80 does not exceed
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the maximum profile of the pipe string which would normally
be represented by the outside diameter of collars 90 in the
casing string. The button 14 has a shoulder 17 formed at
the base of the bulbous outer portion 80 that provides a
surface for seating within a mating recessed surface at the
outer end of the threaded hole 16 in the casing wall. The
shoulder 17 forms a vertical surface on the button which
fits against the mating vertical surface at the outer end
of hole 16. An O-ring 118 is arranged within a groove on
the shoulder 17 to provide a seal between the shoulder 17
and a vertical face at the end of hole 16. The button 14
is arranged so that its inner end does not extend into the
interior of the casing 60. The piston 12 is arranged for
axial movement through the button 14 from a retracted
position (Figures 3 and 4) to an extended position (Figures
2 and 5). The piston 12 and the button 14 are mounted into
casing 60 so that their axis are collinear and directed
radially outwardly with respect to the axis of the casing
60. The piston 12 includes a plug 19 secured in an
interior bore or interior bore 18 in the piston by screw
threads 22. An annular sealing ring 21 is positioned
between the plug 19 and the inner end of piston 12. The
piston 12 shown in Figures 4 and 5 also serves as a housing
for a perforating device. The plug 19 is called an initi-
ator plug in that it carries an explosive detonator device
for initiating detonation of a shaped charge in the piston.
The plug 19 does not fill the entire interior bore 18 but
is rather approximately the thickness of the casing 60.
The plug 19 further includes a rounded inner end face 25
and a flat distal end face 24. The rounded surface 25 on
the inner end of plug 19 is provided for facilitating the
use of a deploying device to push the centralizer 50 into
an extended position.
The distal end 28 of the piston 12 may be
chamfered or tapered inwardly to ease the installation of
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the piston 12 into the button 14. The piston 12 is mounted
in a central bore in the button 14 which is preferably
coaxial to the, opening 16 in the.casing 60 ahd is held in
place by a snap ring 29. The snap ring 29 is located in a
snap ring groove 31 milled in the~wall of the interior bore
of the button 14.
Piston 12 includes two radial piston grooves 32
and 33 formed in the exterior cylindrical surface of the
piston 12. The first of the two piston grooves is a
circumferential securing or locking groove 32 which is
positioned adjacent the inner end 27 of piston 12 to be
engaged by the snap ring 29 when the piston is fully
extended. The second of the two grooves is a
circumiexential retaining groove 33 positioned adjacent the
distal ev~~ 28 of the cylinder 12 to be engaged by the snap
ring 29 when the piston is i.n the retracted or running
position as shown in Figure 4. As the piston 12 is
illustrated in Figure 5 in the extended position, the snap
. ring 29 is engaged in the radial locking groove 32.
The snap ring 29 is made of a~strong resilient
material arranged to expand into the snap ring groove 31
when for~.yed outwardly and to collapse when unsupported into
the grovvE~~32 and 33 when aligned therewith. A particular
arrangement of snap ring and grooves is shown in greater
detail in Applicants' United States patent number 5,346,016
issued 13 September, 1994:
Once the casing 60 is positioned in the wellbore
for pei~nanent installation, the pistons are deployed to the
extended position. The deploying method provides a
deploying force on the inner end of each piston to overcome
the resistance of the snap ring in the retaining groove 33
and cause the snap ring 29 to ride up and out of the
retaining groove 33 whereupon the snap ring 29 is pushed up
into the ~ snap ring groove 31 within the button 14 . This
allows tr~z piston to move out into the annular space of the
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wellbore. Once the piston encounters the wellbore wall, it
will then lift the casing off of the wellbore to centralize
the casing until such time as the snap ring 29 aligns with
and expands into the locking groove 32. The pistons should
be of such a length that the pistons can be fully deployed
to the locking groove 32 while giving the maximum amount of
centralization. Once the pistons are fully deployed, the
inner surface 25 on the plug 19 will be substantially clear
of the casing bore for all practical purposes, and the
casing bore should be substantially full opened.
The button 14 further includes a sealing
arrangement to provide a pressure tight seal between the
piston 12 and the button 14. In particular, the button 14
includes two O-rings, 34 and 36, which are positioned on
either side of the snap ring 29 in O-ring grooves 37 and
38, respectively. The O-rings 34 and 36 seal against the
exterior of piston 12 to prevent fluids from passing from
one side of the casing wall to the other through the bore
of the button 14. The O-rings 34 and 36 must slide along
the exterior of the piston 12 passing the piston grooves 32
and 33 while maintaining the pressure tight seal.
The piston 12 further includes an outwardly
tapered enlarged diameter peripheral edge 39 on its inner
end 27, which edge 39 is larger than the bore in button 14
that receives the piston 12. Thus the edge 39 serves as a
stop against the button 14 to limit the outward movement of
the piston 12. The inside face of button 14 includes a
chamfered edge 41 for engaging the outwardly tapered
peripheral edge 39 on the piston when the inner end 27 of
the piston is approximately flush with the inner end face
of the button 14. Therefore, while the extended piston 12
is recessed into the button 14 and clear of the interior
bore of the casing 60, the inwardly facing rounded surface
25 of the initiator plug extends slightly into the bore of
the casing for purposes to be described so that it is
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substantially clear of the bore to render the casing bore
fully open to penait passage of the deploying device 82 or
other similar device such as packers or the like that would
be passed through the bore of a casing string.
The term full open bore within the context of oil
field terminology, encompasses a situation such as the
present wherein for all practical purposes equipment can be
moved through the bore of a pipe unrestrictedly.
Still referring to Figure 4, the inner bore 18 of
the piston 12 is shown having a shaped charge insert
installed therein. The shaped charge insert includes a
cup-shaped canister or carrier 46 which is sized to be
press fit into the bore 18 of the piston 12. A locking
compound is used to hold the canister 46 in the bore cavity
of the piston. The carrier 46 is nested against a shoulder
47 in the piston bore 18, the shoulder 47 being the end of
the threads 22 which are cut in the bore 18 of the piston
at its inner end to receive plug 19. An ignition hole 48
is formed in the inner wall 49 of the cup-shaped carrier
46. A thin metal foil 51 is placed over the outer surface
of hole 48 facing the plug 19. At the distal end of the
piston 12, an outer end cap 54 is fitted within a recessed
shoulder 55 and is held in place by its press fit and a
locking compound. A shaped charge 58 is positioned in the
canister 46 with a conical depression 59 in the distal end
of the face of the shaped charge facing outwardly. The use
of canister 46 provides a means to conveniently load the
shaped charge 58 into the piston bore 18 at the well site.
The use of preloaded canisters also provides a means to
select a variety of charges or other services which may be
loaded into the bore 18. By loading the shaped charges or
other explosive into the piston at the well site, the
presence of such explosive in the completion system can be
avoided until just prior to the casing being installed, if
need be, to assure maximum safety. Additionally, if the
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shaped charge is loaded into the piston bore 18 and pressed
therein to hold its shape and position in the bore, the
pressure required to load the charge in the bore may cause
the piston 12 to swell slightly which, in turn, may affect
its ability to be moved through the button 14 to an
extended position. The canister 46 is sized to be easily
positioned in the bore 18 and is held therein by an
adhesive or the like. The seal 54 is then installed over
the distal end of bore 18.
The shaped charge 58 is shaped at its inner end
to conform to the flat inner wall 49 of the canister. This
flat shape is designed to provide a rearwardly directed
force in the direction of the plug 19. The end 49 of the
canister engages the distal end of the plug 19 so that the
rearwardly directed force of the detonated shaped charge is
applied to the plug to strip the plug from the threaded
connection 22 and thus move the plug into the bore of the
casing pipe 60. This leaves a flowport or passage through
the cylinder 12 to provide a fluid flowpath between the
casing bore and the formation being completed. A major
portion of the explosive force generated by the shaped
charge 58 is directed toward the formation wall toward
which the piston 12 has been extended and will be effective
to penetrate any material such as cement, between the
extended piston and the fo-.~mation, and the formation
itself. If the piston 12 is fully engaged with the
formation as shown in Figure 5, all the explosive force
directed toward the distal end of the piston will be
utilized to penetrate the formation.
The opposite inner end of the piston 12 has the
plug 19 enclosing the inner end. The plug 19 has a
cylindrical recess 62 which is formed from the inner side
of the plug 19 for receiving a detonator cup 64. The cup
64 is held in place within the recess 62 by means of a
thread locking compound or the like. On the rounded outer
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surface 25 of the plug 19 and central to the plug 19, a
recess 66 is formed in the outer wall surface 25 opposite
the recess 62 on the interior of the plug 19. The recess
66 may be for example 1/8 inch in diameter and
approximately .040 inches deep to leave an integral rupture
disc portion 68 formed between the recesses 62 and 66. The
rupture disc may be on the order of .0275 inches thick.
The cup 64 which is assembled within the recess 62 has
provided within its interior bore a detonating system which
is housed in the shell or cup 64. The detonating system
may include at least one base charge 74 of a detonating
explosive composition located in the bottom of the shell 64
as shown, and a priming charge 72 of a heat sensitive
explosive composition located adjacent to the base charge.
The detonator shown includes an open volume 70 between the
priming charge 72 and the rupture disc 68. In this
application, the space between the top surface of the
priming charge 72 and the rupture disc 68 is optional and
can be any distance up to several inches of space is
available. Rupture disc 68 may be adapted by any suitable
means known int he art to seal the end of the tubular shell
64. Typical base charges that can be used are
pentaerythritol tetranitrate (PETN), cyclortrimethylene
trinitramine (RDX), cyclotetramethylene tetranitramine
(HMX), picrylsulfone, nitromannite, trinitrotoluene (TNT),
hexanitrostilbene (HNS), lead azide, and the like.
Covering the base charge is a priming charge 72 that can be
flat as shown or tapered and embedded in the base charge.
Typical priming charges are of lead azide, lead styphanate,
diazodinitrophenol, mercury fulminate and nitromannite.
Mixtures of diazodinitrophenol potassium chlorate,
nitromannite/diazodinitrophenol and lead azide/lead
styphanate also can be used. A separate layer of lead
styphanate or a layer of a mixture of lead styphanate can
be placed over lead azide. The tubular shell 64 and the
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rupture disc 68 can be aluminum, magnesium, brass or any
methal, plastic, or other suitable material.
It is noted that by installing the detonator
explosives 72 and 74 in the cup 64, the detonating device
can be conveniently assembled into the plug 19 at the well
site. This permits maximum safety in the procedure of
assembling the completion system described herein. The cup
or shell 64 can be held in place by means of an adhesive
substance or the like.
In Figure 5 of the drawings, the centralizing
piston 12 is shown having been moved to an extended and
locked position wherein the distal end 28 of the piston is
in contact with the bore hole wall. A deploying device 82
such as is shown in Figure 1 has been moved through the
interior bore of the casing string to contact the outer
surface 25 of plug 19 on the inner end of the piston. Once
the piston is extended and locked in its predetermined
fixed position as shown in Figure 5, the perforating
apparatus is now in a position to permit perforation of the
formation which the wellbore traverses. It is noted, that
alternatively the pistons 12 may be extended by the
application of hydraulic pressure to the interior of the
casing pipe string which provides a force that impinges on
the inner end of the piston to move the pistons outwardly.
It is to be noted that one particular advantage
of the apparatus described herein is that the centralizing
piston and a button 14 which guides the piston, when
provided, may be assembled within the casing string at some
time just before the casing is run into the wellbore W.
Accordingly, the handling of the casing pipe up to the
point that it is being installed in the wellbore is not
encumbered by having the explosive devices installed during
shipping and handling of the casing prior to its
installation. It is also to be noted that there is no
device present within the system thus far described to
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initiate the explosive devices within the piston so that
such handling in the configuration described above is
considered safe and will not unnecessarily endanger the
personnel who are installing the devices in the casing or
installing the casing within the wellbore.
Referring now to Figure 6 of the drawings, the
casing 60 is shown having been run into a well. The
centralizers are shown having been extended by means of a
pumpable activator device 82 such as shown in Figure 1 or
by the application of hydraulic pressure to the casing
string at the surface. Hydraulic activation is
accomplished by closing a valve at the base of the casing
string and applying the necessary activation or deploying
force required to move the pistons from the retracted
position to the extended position. Accordingly, pumps or
other pressure generating mechanism would provide the
necessary deploying force for the pistons.
Once the casing has been centralized within the
wellbore, an annulus of cement can be injected and set
around the entire outer periphery of the casing, over some
appropriate interval of casing, to seal the casing from the
formation. As suggested by the present invention, the
casing string with the centralizes system as described is
arranged so that in those portions of the wellbore where it
is desired to have a centralizing only function for the
centralizers, the centralizers are not configured so as to
provide a perforating function. However, within a zone
opposite formation F as shown in Figure 6, where it is
desirable to open the casing to permit the recovery of
fluids from the formation into the casing string and to
perforate the formation, the centralizers are likely to be
of the embodiment shown in Figures 4 and 5 which include a
shaped charge device or the like for perforating the
formation to be produced. Alternatively, if a shaped
charge is not provided, an explosive device is incorporated
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in the bore 18 of piston 12 to selectively open a flowport
through the piston 12 to provide a fluid flowpath from the
formation to the casing pipe string.
In the initial installation of the casing within
the wellbore, it is important to note that when the
centralizers are not extended the casing can be rotated and
reciprocated to work past tight spots or other
interferences in the hole. These retracted centralizers 50
also do not interfere with the fluid path through the
casing string so that fluids may be circulated through the
casing to clear cuttings from the end of the casing string.
Also the casing interior can be provided with fluids that
are less dense than the wellbore fluids, in the annular
space, causing the casing string to float. Clearly, the
centralizers 50 of the present invention permit a variety
of methods for installing the casing into its desired
location in the wellbore.
Once the casing 60 is in a suitable position, the
centralizers are deployed to centralize the casing. As
discussed above, there are several methods of deploying the
centralizers. Once the pistons are all deployed and the
snap rings have secured them in the extended fixed position
projecting outwardly toward the wall of the wellbore, the
cement may be inj ected by well known techniques into the
annulus formed by the centralizing of the casing within the
borehole.
The cement around casing 60 may be allowed to set
while the production string is assembled and installed into
the casing. It is important to note that at this point in
the process of establishing the well, the casing and
wellbore are sealed from the formation. Accordingly, there
is as yet no problem with controlling the pressure of the
formation or with loss of pressure control fluids into the
formation. In a conventional completion process, the
perforation string is assembled to create perforations in
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the casing adjacent to the hydrocarbon bearing zone.
Accordingly, high density fluids are provided in the
wellbore to maintain a sufficient pressure head against the
affect of formation pressure to avoid a blowout situation.
While the production string is assembled and run into the
well some of the wellbore fluids, in an overbalance
condition, may be forced into the formation. Accordingly,
the production string must be installed quickly to begin
producing the well once the well has been perforated.
However, with the present invention, such problems are
avoided. Once the casing is set in place, the production
string may be assembled and installed in the casing before
perforation of the formation is performed. Once the
production string is in place in the well, adequate surface
controls are in place to prevent a blowout, so that the
casing and production string can be in an underbalanced
condition. The packer 86 as shown in Figure 6 seals off
the casing string annulus between the production tubing 89
and the casing 60. This packer is set above the pistons 50
to be opened as flow paths. Thus, production may begin when
communication is established with the formation, such as by
perforation in any under balanced condition. Accordingly,
the well is brought on-line in a more controlled manner.
It is well documented that perforating underbalanced gives
higher production rates in many wells. Underbalanced
perforating is a term which describes the concept of having
a lower pressure in the well than in the adjacent
formation. When a well is perforated underbalanced the
pressure in the formation is allowed to enter the wellbore.
When a well is perforated overbalanced the pressure in the
wellbore is allowed to enter the formation. The flow of
fluids into the formation in overbalanced perforating can
damage the formation reducing permeability and later the
flow into the wellbore. Underbalanced perforating will not
cause this formation damage. It also appears that damage
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caused by the perforating itself is reduced if a well is
perforated underbalanced. Wells can be perforated
underbalanced with wireline guns but the well must be
overbalanced when the production tubing is run in the well
or a major safety problem exists. The casing conveyed
perforating described herein allows underbalanced
perforating in all types of wells and does not require that
the well ever be overbalanced because the production tubing
can be present in the well during the perforating and can
be placed immediately on production without ever having to
kill the well.
Figure 6 shows an apparatus and system for
initiating the detonators within the detonator cup 64
(Figure 5) in the pistons, in order to fire the shaped
charges and penetrate the fonaation. Firing the detonators
will also open the piston to fluid flow between the
formation and casing string. A small diameter pipe string
such as production tubing 76 or coiled tubing is run into
the interior of the casing string after the centralizers 50
are extended but before the detonators are fired. The
casing string may be in the form of a long string which
extends from the bottom of the wellbore to the surface or
in the form of a liner where the casing is required over
some specific zone in the wellbore which does not extend to
the surface. Such a liner is normally set using drill
pipe. However, the casing may or may not be cemented in
place. A detonating cord 84 may be pre-installed in the
lower end of the tubing string 76 and run into the well
with the tubing string. Alternatively, the tubing string
may be located in the casing string and then the detonating
cord is run into the tubing string. In the latter case, in
order to set the detonating cord 84 in place, the bottom of
the tubing string can be provided with a latching mechanism
93. After the tubing 76 is run into the casing string, a
sinker bar with detonating cord trailing behind, can be
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lowered into the tubing string and latched inside of the
tubing. Alternatively, a device can be pumped to the latch
93 with a detonating cord trailing. A perforating head 89
would be run at the trailing, upper end of the detonating
cord 84 to provide a means for initiating the detonating
cord. Once the tubing is run, a production packer 86 can
be set. At this time a sinker bar 91 can be dropped which
would strike the perforating head and initiate the
detonating cord. Alternatively, a wireline could be
connected with the detonating cord or perforating head in
order to initiate the detonating cord.
The detonating cord is initiated by dropping a
similar bar 91 or using an electrical wireline or as
another alternative, using a hydraulically actuated
perforating head. Once the detonating cord is initiated,
it results in the development and propagation of a pressure
wave within the pipe string 76. This pressure wave is then
communicated through the fluid in the pipe 76 and casing 60
to the plug 19 at the inner end of the cylfinders 12. If
necessary, the pipe string 76 may be centered in the casing
by means of conventional centralizers 78. Centering the
pipe string 76 in the casing string may be important in
view of the importance of propagating a pressure wave to
the cylinders 12 on all sides so that the force of this
pressure wave is sufficient to rupture the disc 68 in the
plug 19. This rupture of disc 68 will sequentially
initiate the powders 72 and 74 within the cup 64 positioned
in the plug 19. Tests have shown that initiation of the
detonator will take place without the provision of an air
space 70 in the cup 64 by locating powders adjacent to the
ruptured disc 68. The amount of pressure required to
rupture the disc is increased when the air space is
eliminated; however, detonation does take place. It is
believed that the principle behind the detonation is an
adiabatic compression within the cup 64 which is sufficient
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to initiate the powders 72, 74 therein. Therefore, it
appears to only be necessary to generate sufficient
pressure within the interior of the casing bore to cause
the rupture disc 68 to rupture which will thereby initiate
the detonator housed within the cup 64. When a shaped
charge is present in the piston 12, initiation of the
detonator is communicated through the opening 48 within the
carrier 46 to detonate the shaped charge 58. This
detonation produces a penetrating force that is directly
applied to the formation F so that all the outwardly
directed energy of the shaped charge is applied to
perforation and fracturing of the formation. Detonation of
the shaped charge 58 also removes the plug 19 and end cap
54 to open the piston 12 for fluid flow.
In the configuration shown in Figure 6, the
smaller diameter pipe 76 housing the detonating cord, may
be provided with slots or holes in the outside walls
thereof to facilitate transmission of a pressure wave
emanating from the detonating cord to the perforating
cylinders 12. However, experiments have shown that a
pressure wave may be propagated through the walls of solid
pipe which is sufficient to initiate the detonators within
the plug 19 on the cylinders 12. The system shown in
Figure 6 with a production packer 86 set in place will
permit the completion to take place with an under-balanced
fluid in the pipe string, so that upon perforation of the
formation F formation, fluids may be readily received into
the casing string through the now open cylinder 12 and from
there into the production tubing 76 for conveyance to the
surface.
In the process of perforating the formation
as described in the present invention, it is noted that the
word "penetrating" is used to describe the process for
opening a communication path into the formation. The
reason that penetrating the formation is desirable is that
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the permeability of porous reservoir rock is usually
reduced or plugged near the wellbore due to the leakage of
drilling fluids used in the drilling operation into the
first few inches of the formation material surrounding the
wellbore. This reduces permeability near the wellbore and
is referred to as skin damage. In the present perforating
technique, the shaped charges are not designed to make a
hole in the casing as in a normal perforating system, but
rather to establish communication with the reservoir rock
and to penetrate the rock itself with a fracturing and
penetrating blast that extends communication beyond the
skin damage. Whereas normal shaped charges in a
perforating system are positioned within the casing string
and must therefore progress through the fluids within the
casing string, the steel casing string wall, cement if it
is in place, and then into the skin damaged portion of the
reservoir. In the present system the shaped charge is
positioned directly against the formation and thus a much
greater portion of the energy developed by the shaped
charge is applied to the formation rock itself.
It is readily appreciated that other methods could be
used to develop a pressure wave for initiating the shaped
charge. Also, it is readily seen that a variety of
detonators might be used to initiate the explosion of the
shaped charged within the centralizing cylinder 12.
Additionally, while a casing string has been primarily
described as the device for carrying the extendible pistons
or flow path devices into the borehole, it is readily
appreciated that liners serve the same purpose and
therefore any functional substitute for a casing is
intended to be covered by this invention. Therefore, while
particular embodiments of the present invention have been
shown and described, it is apparent that changes and
modifications may be made without departing from this
invention in its broader aspects and therefore the aim in
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the appended claims is to cover all such changes and
modifications as fall within the true spirit and scope of
this invention.
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