U.S. patent application number 11/532929 was filed with the patent office on 2007-02-15 for controlling transient pressure conditions in a wellbore.
This patent application is currently assigned to Schlumberger Technology Corporation. Invention is credited to Lawrence A. Behrmann, Kenneth R. Goodman, Andrew J. Martin, Wanchai Ratanasirigulchai.
Application Number | 20070034369 11/532929 |
Document ID | / |
Family ID | 34911167 |
Filed Date | 2007-02-15 |
United States Patent
Application |
20070034369 |
Kind Code |
A1 |
Ratanasirigulchai; Wanchai ;
et al. |
February 15, 2007 |
Controlling transient pressure conditions in a wellbore
Abstract
A method and apparatus for use in a wellbore includes running a
tool string to an interval of the wellbore, and activating a first
component in the tool string to create a transient underbalance
pressure condition in the wellbore interval. Additionally, a second
component in the tool string is activated to create a transient
overbalance pressure condition in the wellbore interval, or vice
versa.
Inventors: |
Ratanasirigulchai; Wanchai;
(Shanghai, CN) ; Behrmann; Lawrence A.; (Houston,
TX) ; Martin; Andrew J.; (Aberdeen, GB) ;
Goodman; Kenneth R.; (Cypress, TX) |
Correspondence
Address: |
SCHLUMBERGER RESERVOIR COMPLETIONS
14910 AIRLINE ROAD
ROSHARON
TX
77583
US
|
Assignee: |
Schlumberger Technology
Corporation
Sugar Land
TX
|
Family ID: |
34911167 |
Appl. No.: |
11/532929 |
Filed: |
September 19, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10710564 |
Jul 21, 2004 |
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11532929 |
Sep 19, 2006 |
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|
10667011 |
Sep 19, 2003 |
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10710564 |
Jul 21, 2004 |
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|
10316614 |
Dec 11, 2002 |
6732798 |
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|
10667011 |
Sep 19, 2003 |
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|
09797209 |
Mar 1, 2001 |
6598682 |
|
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10316614 |
Dec 11, 2002 |
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60186500 |
Mar 2, 2000 |
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60187900 |
Mar 8, 2000 |
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60252754 |
Nov 22, 2000 |
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Current U.S.
Class: |
166/63 ;
166/55.1 |
Current CPC
Class: |
E21B 49/087 20130101;
E21B 37/00 20130101; E21B 43/117 20130101; E21B 2200/04 20200501;
E21B 43/11 20130101; E21B 49/08 20130101; E21B 43/04 20130101; E21B
21/00 20130101; E21B 43/26 20130101; F42D 5/045 20130101; E21B
43/119 20130101; E21B 37/08 20130101; E21B 43/1195 20130101; E21B
21/085 20200501; F42B 3/02 20130101 |
Class at
Publication: |
166/063 ;
166/055.1 |
International
Class: |
E21B 43/117 20070101
E21B043/117 |
Claims
1. An apparatus comprising: a propellant having a plurality of
cavities; and explosive devices mounted in the cavities.
2. The apparatus of claim 1, wherein the explosive devices comprise
shaped charges mounted in the cavities.
3. The apparatus of claim 1, further comprising a detonating cord
ballistically connected to the explosive devices, wherein
activation of the detonating cord causes detonation of the
explosive devices and initiation of the propellant.
4. An apparatus comprising: a strip; explosive devices mounted on
the strip; and propellant inserts positioned between the explosive
devices.
5. The apparatus of claim 4, wherein the explosive devices comprise
shaped charges.
6. The apparatus of claim 4, further comprising a detonating cord
ballistically connected to the explosive devices and the propellant
inserts.
7. The apparatus of claim 4, wherein the explosive devices are
arranged in a spiral pattern.
8. A tool string comprising: a first component activatable to
create a transient underbalance pressure condition in a wellbore
interval proximal the tool string; and a second component
activatable to create a transient overbalance pressure condition in
the wellbore interval.
9. The tool string of claim 8, wherein the first component
comprises a carrier containing explosive devices, wherein
activation of the explosive devices causes openings to be created
in the carrier to enable the communication of wellbore pressure
into a low-pressure chamber of the carrier to create the transient
underbalance pressure condition in the wellbore.
10. The tool string of claim 8, wherein the second component
includes a propellant that is initiated to generate high-pressure
gas in the wellbore interval to create the transient overbalance
pressure condition.
11. The tool string of claim 10, wherein the propellant includes
cavities, and the second component comprises explosive devices
mounted in the cavities.
12. The tool string of claim 11, wherein the explosive devices
comprise shaped charges.
13. The tool string of claim 13, wherein the second component
includes a carrier having a plurality of explosive devices, and the
propellant is included in the carrier.
14. The tool string of claim 8, wherein the second component
includes a propellant and a pressure chamber to receive
high-pressure gas generated by initiation of the propellant.
15. The tool string of claim 8, wherein the second component
further comprises a rupture element adapted to rupture the rupture
element by the pressure in the pressure chamber.
16. The tool string of claim 15, wherein the second component
further comprises a vent sub having one or more openings to release
high-pressure gas from the pressure chamber.
17. An apparatus comprising: a propellant; a pressure chamber, the
pressure chamber to receive high-pressure gas released by
initiation of the propellant; and a rupture element to be ruptured
by pressure greater than a predetermined level in the pressure
chamber.
18. The apparatus of claim 17, further comprising a vent sub to
vent high pressure in the pressure chamber in response to rupture
of the rupture element.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a divisional of U.S. Ser. No. 10/710,564, filed Jul.
21, 2004, which is a continuation-in-part of U.S. Ser. No.
10/667,011, filed Sep. 19, 2003, which is a continuation-in-part of
U.S. Ser. No. 10/316,614, filed Dec. 11, 2002, now U.S. Pat. No.
6,732,798, which is a continuation-in-part of U.S. Ser. No.
09/797,209, filed Mar. 1, 2001, now U.S. Pat. No. 6,598,682, which
claims the benefit of U.S. Provisional Application Ser. Nos.
60/186,500, filed Mar. 2, 2000; 60/187,900, filed Mar. 8, 2000; and
60/252,754, filed Nov. 22, 2000. Each of the referenced
applications is hereby incorporated by reference.
TECHNICAL FIELD
[0002] The invention relates to improving reservoir communication
within a wellbore.
BACKGROUND
[0003] To complete a well, one or more formation zones adjacent a
wellbore are perforated to allow fluid from the formation zones to
flow into the well for production to the surface or to allow
injection fluids to be applied into the formation zones. A
perforating gun string may be lowered into the well and the guns
fired to create openings in casing and to extend perforations into
the surrounding formation.
[0004] The explosive nature of the formation of perforation tunnels
shatters sand grains of the formation. A layer of "shock damaged
region" having a permeability lower than that of the virgin
formation matrix may be formed around each perforation tunnel. The
process may also generate a tunnel full of rock debris mixed in
with the perforator charge debris. The extent of the damage, and
the amount of loose debris in the tunnel, may be dictated by a
variety of factors including formation properties, explosive charge
properties, pressure conditions, fluid properties, and so forth.
The shock damaged region and loose debris in the perforation
tunnels may impair the productivity of production wells or the
injectivity of injector wells.
[0005] One popular method of obtaining clean perforations is
underbalanced perforating. The perforation is carried out with a
lower wellbore pressure than the formation pressure. The pressure
equalization is achieved by fluid flow from the formation and into
the wellbore. This fluid flow carries some of the damaging rock
particles. However, underbalance perforating may not always be
effective and may be expensive and unsafe to implement in certain
downhole conditions.
[0006] Fracturing of the formation to bypass the damaged and
plugged perforation may be another option. However, fracturing is a
relatively expensive operation. Moreover, clean, undamaged
perforations are required for low fracture initiation pressure (one
of the pre-conditions for a good fracturing job). Acidizing,
another widely used method for removing perforation damage, is less
effective in removing the perforation damage, or for treating sand
and loose debris left inside the perforation tunnel. Additionally,
having undamaged perforations implies a better matrix or acid
fracture job in a carbonate formation.
[0007] A need thus continues to exist for a method and apparatus to
improve fluid communication with reservoirs in formations of a
well.
SUMMARY
[0008] In general, a method and apparatus for use in a wellbore
includes running a tool string to an interval of the wellbore, and
activating a first component in the tool string to create a
transient underbalance pressure condition in the wellbore interval.
A second component in the tool string is activated to create a
transient overbalance pressure condition in the wellbore
interval.
[0009] In general, according to another embodiment, a method and
apparatus for use in a wellbore includes running a tool string to
an interval of the wellbore, and activating a first component in
the tool string to create a transient overbalance pressure
condition in the wellbore interval. A second component in the tool
string is activated to create a transient underbalance pressure
condition in the wellbore interval.
[0010] Other or alternative features will become apparent from the
following description, from the drawings, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 illustrates a tool string for applying transient
underbalance and/or overbalance pressure conditions in a wellbore
interval, according to some embodiments.
[0012] FIG. 2 is an exploded view of a portion of the tool string
of FIG. 1.
[0013] FIG. 3 illustrates a perforating gun according to an
embodiment of the invention.
[0014] FIG. 4 illustrates a tool according to another embodiment of
the invention.
[0015] FIGS. 5-7 are timing diagrams to illustrate generation of
transient underbalance and overbalance pressure conditions in a
wellbore.
[0016] FIGS. 8 and 9 illustrate tools according to other
embodiments for creating a transient underbalance condition.
[0017] FIG. 10 illustrates a tool for generating a controlled,
transient overbalance condition, according to an embodiment.
DETAILED DESCRIPTION
[0018] In the following description, numerous details are set forth
to provide an understanding of the present invention. However, it
will be understood by those skilled in the art that the present
invention may be practiced without these details and that numerous
variations or modifications from the described embodiments may be
possible.
[0019] As used here, the terms "up" and "down"; "upper" and
"lower"; "upwardly" and "downwardly"; "upstream" and "downstream";
"above" and "below" and other like terms indicating relative
positions above or below a given point or element are used in this
description to more clearly described some embodiments of the
invention. However, when applied to equipment and methods for use
in wells that are deviated or horizontal, such terms may refer to a
left to right, right to left, or other relationship as
appropriate.
[0020] According to some embodiments of the invention, transient
overbalance and underbalance pressure conditions are generated in a
wellbore to enhance communication of formation fluids with the
wellbore. The well operator is able to control a sequence of
underbalance and overbalance conditions to perform desired cleaning
and/or stimulating tasks in one or plural wellbore intervals in a
well.
[0021] There are several potential mechanisms of damage to
formation productivity and injectivity due to perforation. One may
be the presence of a layer of low permeability sand grains (grains
that are fractured by explosive shaped charge) after perforation.
As the produced fluid from the formation may have to pass through
this lower permeability zone, a higher than expected pressure drop
may occur resulting in lower productivity. The second major type of
damage may arise from loose perforation-generated rock and charge
debris that fills the perforation tunnels. Debris in perforation
tunnels may cause declines in productivity and injectivity (for
example, during gravel packing, injection, and so forth). Yet
another type of damage occurs from partial opening of perforations.
Dissimilar grain size distribution can cause some of these
perforations to be plugged (due to bridging, at the casing/cement
portion of the perforation tunnel), which may lead to loss of
productivity and injectivity.
[0022] To address these issues, pressure in a wellbore interval is
manipulated in relation to the reservoir pressure to achieve
removal of debris from perforation tunnels. The pressure
manipulation includes creating a transient underbalance condition
(the wellbore pressure being lower than a formation pressure) or
creating an overbalance pressure condition (when the wellbore
pressure is higher than the reservoir pressure) prior to detonation
of shaped charges of a perforating gun or a propellant. Creation of
an underbalance condition can be accomplished in a number of
different ways, such as by use of a low pressure chamber that is
opened to create the transient underbalance condition, the use of
empty space in a perforating gun to draw pressure into the gun
right after firing of shaped charges, and other techniques
(discussed further below).
[0023] Creation of an overbalance condition can be accomplished by
use of a propellant (which when activated causes high pressure gas
buildup), a pressurized chamber, or other techniques.
[0024] The manipulation of wellbore pressure conditions causes at
least one of the following to be performed: (1) enhance transport
of debris (such as sand, rock particles, etc.) from perforation
tunnels; (2) achieve near-wellbore stimulation; and (3) perform
fracturing of surrounding formation.
[0025] In accordance with some embodiments of the invention, the
sequence of generating underbalance and overbalance pressure
conditions is controllable by a well operator. For example, the
well operator may cause the creation of a transient underbalance,
followed by a transient overbalance condition. Alternatively, the
well operator may start with a transient overbalance condition,
followed by a transient underbalance condition. In yet another
scenario, the well operator can create a first transient
underbalance condition, followed by a larger transient underbalance
condition, followed by a transient overbalance condition, and so
forth. Any sequence of transient underbalance and overbalance
pressure conditions can be set by the user, in accordance with the
needs of the well operator.
[0026] FIG. 1 illustrates a tool string 100 that has been lowered
into an interval of a wellbore 102. The tool string 100 is carried
into the wellbore 102 by a carrier structure 104, such as a
wireline, slickline, coiled tubing, or other carrier structure. The
tool string 100 includes several components, including a first
component 106 (referred to as an "underbalance pressure creating
component") for generating a transient underbalance pressure
condition in the wellbore 102, a second component 108 (referred to
as an "overbalance pressure creating component") to generate a
transient overbalance pressure condition, and a perforating gun 110
for creating perforations into surrounding formation 112. Note that
the perforating gun 110 can be combined with either of the
underbalance pressure creating component 106 or the overbalance
pressure creating component 108. In other implementations, the
perforating gun 110 can be omitted or replaced with another
tool.
[0027] The first component 106 can be activated first to create the
underbalance pressure condition, followed by activating the second
component 108 to create the overbalance pressure condition. In some
scenarios, the second component 108 can be activated while the
underbalance pressure condition is still present. Conversely, the
second component 108 can be activated first to create the
overbalance pressure condition, followed by activating the first
component 106 to create the underbalance pressure condition. In
some scenarios, the first component 106 can be activated while the
overbalance pressure condition is still present.
[0028] As used here, a "component" can refer to either a single
module or an assembly of modules. Thus, for example, an
underbalance pressure creating component can include a low pressure
module (such as an empty chamber), a second module containing
explosive devices, and other modules (such as connector modules to
connect to other parts of a tool string). The modules may be
separate items or integrated into a single tool.
[0029] To create an underbalance pressure condition in the wellbore
interval, the well operator provides a control signal (which can be
an electrical signal, optical signal, pressure pulse signal,
mechanical signal, hydraulic signal, and so forth) to cause
activation of the underbalance pressure creating component 106.
Once the underbalance condition is created in the wellbore
interval, a downhole task (such as a perforating task) is
performed. Next, the well operator may cause the overbalance
pressure creating component 108 to generate an overbalance
condition in the wellbore interval. The overbalance condition may
cause creation of a sufficient pressure to cause fracturing or
other stimulation of the surrounding formation (such as after
perforation tunnels have been extended by the perforating gun 110
into the formation 112).
[0030] Although the following describes some specific embodiments
of components, the present invention can use other components and
methods to achieve the desired result. FIG. 2 illustrates a
component 200 that is usable with the tool string 100 depicted in
FIG. 1. The component 200 can be any of a selected one of the
component 106, 108, or 110 in the tool string 100 of FIG. 1. The
component 200 includes an upper head assembly for attaching to
another part of the tool string above the component 200, and a
lower head assembly 204 for attaching the component 200 to a
portion of the tool string below the component 200. Between the
upper and lower head assemblies 202 and 204 is attached a carrier
206.
[0031] The carrier 206 is a hollow housing that is capable of
receiving either a propellant loading tube 208 or a standard
loading tube 210. The standard loading tube 210 is capable of
carrying shaped charges that are mounted at positions corresponding
to openings 212 in the loading tube 210. When activated, the shaped
charges cause perforating jets to fire through respective openings
212. In the illustrated embodiment, the loading tube 210 has a
generally cylindrical shape. In other embodiments, the loading tube
210 can have other shapes, including non-cylindrical shapes.
[0032] The propellant loading tube 208 is a propellant pre-cast to
a cylindrical shape (according to one example implementation) or
another shape. The propellant has cavities for receiving shaped
charges 214. Thus, in effect, the propellant is a loading tube that
has cavities for carrying shaped charges 214. In such an
arrangement, the loading tube is formed of the propellant instead
of more conventional metal housings. If the propellant loading tube
208 is provided in the carrier 206, then firing of the shaped
charges 214 also causes activation of the propellant. Burning of
the propellant causes high pressure gas to build up.
[0033] In operation, a detonating cord (or other type of detonator)
is ballistically coupled to the shaped charges 214 of the
propellant loading tube 208. The detonating cord or other detonator
is also ballistically coupled to the propellant. A firing head
causes initiation of the detonating cord (or other detonator) which
in turn causes initiation of the propellant and the shaped charges
214. The shaped charges 214, once fired, shoots out perforating
jets that blast corresponding holes through the carrier 206. The
perforating jets extend through any casing or liner that lines the
wellbore 102, and further extends perforations into the surrounding
formation 112. At this time, after firing of the shaped charges
214, the propellant continues to burn, which causes buildup of high
pressure gas in the wellbore interval. The buildup of high pressure
gas causes an overbalance condition to be created in the wellbore
interval.
[0034] The burning of the propellant can cause pressure to increase
to a sufficiently high level to fracture the formation. The
fracturing allows for better communication of reservoir fluids from
the formation into the wellbore or the injection of fluids into the
surrounding formation.
[0035] In an alternative embodiment, instead of shaped charges 214
that can extend perforating jets through surrounding casing/liner
and formation, smaller shaped charges can be used that have
sufficient energy to blow holes through the carrier 206 (but does
not cause the perforation of the surrounding casing/liner in
formation). In this case, perforations are not created in the
formation 112--instead, openings are created in the carrier 206 to
enable burning of the propellant to cause buildup of pressure to
achieve an overbalance condition. In this alternative embodiment,
the shaped charges are referred to as "punchers" or "puncher
charges" since the charges are able to punch through the carrier
206 without cutting through the surround liner or casing.
[0036] Shaped charges in the standard loading tube 210 are
similarly activated by a detonating cord or other detonator to
cause generation of perforating jets that extend through the
openings 212 of the loading tube 210. The perforating jets also
create openings in the carrier 206. The difference is that a
propellant is not burned in the standard loading tube 210 so that
buildup of gas pressure does not occur with the activation of the
shaped charges in the loading tube 210.
[0037] FIG. 3 illustrates a different arrangement of a perforating
gun 300, which can be used as perforating gun 110 in FIG. 1. The
perforating gun 300 includes a carrier strip 302 on which are
mounted shaped charges 304. As depicted, the shaped charges 304 are
arranged in a spiral pattern. A detonating cord 306 extends along
the length of the perforating gun 300 in a generally spiral path to
enable the detonating cord 306 to be ballistically connected to
each of the shaped charges 304.
[0038] In the embodiment of FIG. 3, the shaped charges 304 are
capsule shaped charges, which include sealed capsules for housing a
shaped charge within each sealed capsule. The capsule shaped
charges 304 do not have to be carried within a sealed gun carrier
housing (such as carrier 206 in FIG. 2), but rather, the capsule
shaped charges can be exposed to wellbore fluids.
[0039] In addition, propellant elements 308 in the form of inserts
are provided in spaces available between capsule shaped charges 304
and around capsule charges 304. The propellant elements 308 are
initiated in response to a detonation wave traveling through the
detonating cord 306. Here again, activation of the shaped charges
304 also causes activation of the propellant inserts 308 to cause
buildup of high pressure gas and creation of an overbalance
condition in the wellbore interval.
[0040] FIG. 4 illustrates a tool string according to another
embodiment of the invention. The tool string 400 of FIG. 4 includes
several sections 402A, 402B, 402C, 402D, and 402E. The section 402A
includes a control module 404, and a gun and propellant module 406.
The gun and propellant module 406 includes both shaped charges and
propellant elements. For example, the gun and propellant module 406
can either be the perforating gun 300 of FIG. 3 or the propellant
loading tube 208 installed in the carrier 206 of FIG. 2.
[0041] The second section 402B includes a control module 408 and a
perforating gun 410. In the second section 402B, a propellant is
not provided. However, the perforating gun 410 can be designed to
have a relatively large amount of empty space within the
perforating gun 410. The empty space (space other than the shaped
charges, the main core, and other components of the perforating gun
410) is initially sealed from the wellbore pressure. Upon firing of
the shaped charges, openings are formed in the sealed housing of
the perforating gun 410. Following shaped charge detonation, hot
detonation gas fills the internal chamber of the gun 410. If the
resultant detonation gas pressure is less than the wellbore
pressure, then the cooler wellbore fluids are drawn into the gun
housing. The rapid acceleration through perforation openings in the
gun housing breaks the fluid up into droplets and results in rapid
cooling of the gas. Hence, rapid loss of pressure in the gun that
results in rapid wellbore fluid drainage causes a drop in the
wellbore pressure. The drop in wellbore pressure creates the
underbalance condition in the desired wellbore interval.
[0042] The next section 402C in the tool string 400 includes a
control module 412 and a gun and propellant module 414. The gun and
propellant module 414 can be similar to the gun and propellant
module 406 (containing shaped charges that can extend perforations
into surrounding formation) or the gun and propellant module 414
can include smaller shaped charges that are designed to blow
openings through the housing of the module 414 but do not have
sufficient energy to extend perforations into surrounding
formation.
[0043] The next section 402D of the tool string 400 includes a
control module 416 and a gun module 418. The gun module 418 can be
similar to the gun module 410. The other section 402E includes a
control module 420 and a gun and propellant module 422, which also
includes both shaped charges and propellant elements. Note that
sections 402A, 402C, and 402E when activated causes the creation of
overbalance conditions in wellbore intervals proximal respective
sections 402A, 402B, and 402C. Each of the sections 402B and 402D
is able to cause creation of an underbalance conditions in wellbore
intervals proximal the sections.
[0044] The order of the modules illustrated in FIG. 4 is provided
for the purpose of example. In other implementations, other orders
of the modules can be employed. Also, the order in which the
modules are activated can also be controlled by the well operator.
Activation of each section 402 is controlled by a respective
control module. In some implementations, each of the control
modules can include a timer that, when activated, causes a delay of
some preset period before activation of the section.
[0045] FIG. 5 is a timing diagram illustrating a sequence of
transient pressure conditions generated by activation of different
modules of a tool string (such as tool string 400 of FIG. 4 or tool
string 100 of FIG. 1) in the wellbore interval. According to FIG.
5, a perforating gun is first fired (which initially causes a
relatively small transient overbalance condition 450 to be
generated in the wellbore interval). The pressure then drops back
to the normal pressure of the wellbore, which due to existence of
the perforations in the surrounding formation is at the formation
pressure.
[0046] Next, if a propellant has been initiated, then a larger
overbalance condition 452 (having higher pressure than overbalance
condition 450) is generated. After burning of the propellant, the
pressure drops back down to the normal wellbore pressure. Next, a
perforating gun that includes a module for creating a transient
underbalance condition is activated, which causes a transient
underbalance condition 454 to be generated. The module can be a
hollow carrier that contains low pressure gas that when opened
(such as by firing of shaped charges) causes surrounding pressure
to drop (as discussed above). After activation of this module, the
wellbore pressure returns to close to the normal wellbore pressure.
Next, in response to initiation of another propellant, a transient
overbalance condition 456 is created in the wellbore interval.
Thus, in FIG. 5, the sequence of overbalance and underbalance
conditions is as follows: first overbalance, second overbalance,
underbalance, and third overbalance.
[0047] FIG. 6 shows another sequence of overbalance and
underbalance conditions. After the first initiation of a
perforating gun that is associated with an underbalance pressure
creating module, a transient underbalance condition 460 is created.
Next, after the wellbore interval has returned to the normal
wellbore pressure, a propellant is activated to create an
overbalance condition 462. Subsequently, additional underbalance
conditions 464 and 468 and overbalance conditions 466 and 470 are
created.
[0048] FIG. 7 shows yet another sequence of underbalance conditions
and overbalance conditions. Note that FIGS. 5-7 show some example
sequences. Many other sequences of underbalance and overbalance
conditions are possible.
[0049] The intervals among the various pressure conditions
illustrated in FIGS. 5-7 can be on the order of milliseconds,
seconds, or even minutes apart if timers are provided in tools
according to some embodiments. If timers are not provided, then the
intervals among the various pressure conditions in FIGS. 5-7 can be
on the order of microseconds.
[0050] FIG. 8 illustrates a tool for creating an underbalance
condition, in accordance with an embodiment. Note that the tool of
FIG. 8 can be used as part of the tool string illustrated in FIG.
1. The FIG. 8 tool includes an atmospheric container 510A used in
conjunction with a perforating gun 530. In the embodiment of FIG.
8, the container 510A (which can be expendable in one
implementation) is divided into two portions, a first portion above
the perforating gun 530 and a second portion below the perforating
gun 530. The container 510A contains a low-pressure gas (e.g., air,
nitrogen, etc.) or other compressible fluid.
[0051] The container 510A includes various openings 516A that are
adapted to be opened by an explosive force, such as an explosive
force due to initiation of a detonating cord 520A or detonation of
explosives connected to the detonating cord 520A. The detonating
cord is also connected to shaped charges 532 in the perforating gun
530. In one embodiment, as illustrated, the perforating gun 530 can
be a strip gun, in which capsule shaped charges are mounted on a
carrier 534. Such a perforating gun 530 is also referred to as a
capsule perforating gun. In alternative embodiments, the shaped
charges 532 may be non-capsule shaped charges that are contained in
a sealed container.
[0052] The openings 516A, in alternative embodiments, can include a
valve or other element that can be opened to enable communication
with the inside of the container 510A. Once opened, the openings
516A cause a fluid surge into the inner chamber of the atmospheric
container 510A.
[0053] The fluid surge can be performed relatively soon after
perforating. For example, the fluid surge can be performed within
about one minute after perforating. In other embodiments, the
pressure surge can be performed within (less than or equal to)
about 10 seconds, one second, or 100 milliseconds, or 10
milliseconds, as examples, after perforating. The timing delay can
be set by use of a timer in the tool.
[0054] Referring to FIG. 9, yet another embodiment for creating an
underbalance condition during a perforating operation is
illustrated. A perforating gun 700 includes a gun housing 702 and a
carrier line 704, which can be a slickline, a wireline, or coiled
tubing. In one embodiment, the perforating gun 700 is a hollow
carrier gun having shaped charges 714 inside a chamber 718 of a
sealed housing 716. In the arrangement of FIG. 9, the perforating
gun 702 is lowered-through a tubing 706. A packer (not shown) can
be provided around the tubing 706 to isolate an interval 712 in
which the perforating gun 700 is to be shot (referred to as the
"perforating interval 712"). A pressure P.sub.W is present in the
perforating interval 712.
[0055] During detonation of the shaped charges 714, perforating
ports 720 are formed in the housing 702 as a result of perforating
jets produced by the shaped charges 714. During detonation of the
shaped charges 714, hot gas fills the internal chamber 718 of the
gun 716. If the resultant detonation gas pressure, P.sub.G, is less
than the wellbore pressure, P.sub.W, by a given amount, then the
cooler wellbore fluids will be drawn into the chamber 718 of the
gun 702. The rapid acceleration of well fluids through the
perforation ports 720 will break the fluid up into droplets, which
results in rapid cooling of the gas within the chamber 718. The
resultant rapid gun pressure loss and even more rapid wellbore
fluid drainage into the chamber 718 causes the wellbore pressure
P.sub.W to be reduced. Depending on the absolute pressures, this
pressure drop can be sufficient to generate a relatively large
underbalance condition (e.g., greater than 2000 psi), even in a
well that starts with a substantial overbalance (e.g., about 500
psi). The underbalance condition is dependent upon the level of the
detonation gas pressure P.sub.G, as compared to the wellbore
pressure, P.sub.W.
[0056] When a perforating gun is fired, the detonation gas is
substantially hotter than the wellbore fluid. If cold wellbore
fluids that are drawn into the gun produce rapid cooling of the hot
gas, then the gas volume will shrink relatively rapidly, which
reduces the pressure to encourage even more wellbore fluids to be
drawn into the gun. The gas cooling can occur over a period of a
few milliseconds, in one example. Draining wellbore liquids (which
have small compressibility) out of the perforating interval 712 can
drop the wellbore pressure, P.sub.W, by a relatively large amount
(several thousands of psi).
[0057] In accordance with some embodiments, various parameters are
controlled to achieve the desired difference in values between the
two pressures P.sub.W and P.sub.G. For example, the level of the
detonation gas pressure, P.sub.G, can be adjusted by the explosive
loading or by adjusting the volume of the chamber 718 or adjusting
the area of opening(s) into the chamber 718. The level of wellbore
pressure, P.sub.W, can be adjusted by pumping up the entire well or
an isolated section of the well, or by dynamically increasing the
wellbore pressure on a local level.
[0058] FIG. 10 illustrates an embodiment of a tool 600 (useable in
the tool string of FIG. 1) that can be used to generate an
overbalance pressure condition for the purpose of stimulating a
wellbore interval. The tool 600 includes a propellant 602 and a
pressure chamber 604. The pressure chamber 604 is used to collect
gas byproducts created by initiation of the propellant 602. The
tool 600 further includes a rupture element 606 (e.g., rupture
disk) at one end of the pressure chamber 604. The tool 600 also
incudes a vent sub 608 attached to the pressure chamber 604. The
vent sub 608 includes multiple openings 610.
[0059] In operation, upon initiation of the propellant 602,
high-pressure gas is collected in the pressure chamber 604. When
the pressure in the pressure chamber 604 reaches a sufficiently
high level, the rupture element 606 is ruptured. Upon rupture of
the rupture element 606, the gas pressure in the pressure chamber
604 is released through the openings 610 of the vent sub 608.
[0060] The rupture element 606 is designed to rupture at a
predetermined pressure, such as when 1/2, 3/4, or some other
fraction of the propellant 602 is consumed. The rupture pressure
can be varied by changing the number of rupture disks used in the
rupture element 606. By employing the tool 600 according to some
embodiments, the pressure pulse that is applied to the surrounding
formation can be controlled. This control can also be achieved by
varying the volume of the pressure chamber 604, and/or by varying
the area of the openings 610 in the vent sub 608. A reservoir of
high-pressure gas is thus provided by the pressure chamber 604 and
released in a controlled manner to the surrounding formation
through the vent sub 608. In this manner, by controlling the
release of high-pressure gas, damage to the surrounding formation
due to unpredictable high pressure applied against the
formation.
[0061] 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 such modifications and
variations as fall within the true spirit and scope of the
invention.
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