Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR PERFORATING AND
STIMULATING OIL WELLS
TECHNICAL FIELD
The present invention relates in general to the
preparation of subterranean wells for the production of
fluids from underground reservoirs and, in particular, to
tools used in subterranean wells for casing perforation and
production stimulation.
BACKGROUND OF THE INVENTION
As the supply of highly-productive hydrocarbon
wells is exhausted, there is increasing interest in
producing hydrocarbon fluids from potentially productive
geological formations that contain sufficient volume of
such fluids, but have low permeability so that production
is slow or difficult. In order to economically produce
fluids from such formations, the formations must be
artificially "stimulated" to increase the permeability of
the production zone. Many methods have been invented for
artificially stimulating subterranean formations.
Generally, such methods are referred to as "well
fracturing". During well fracturing, pressurized fluids
are pumped through perforations in a well casing and into a
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production zone in order to break or fracture pores in the
zone to improve permeability so that the hydrocarbon fluids
can drain from the production zone into the casing. Those
pressurized fluids are often laden with abrasive
"proppants", such as sharp sand. In order to stimulate a
new well, it is first necessary to perforate a casing of
the well. This is generally accomplished using what is
known in the art as "perforating guns" such as taught, for
example, in United States Patent No. 4,598,776 which issued
on July 8, 1986 to Stout. After the casing is perforated,
a fracturing tool is lowered into the well and fluids
pressurized to 5,000-10,000 psi are pumped through the
perforations into the formation. The high pressures tend
to break up the formations to release trapped hydrocarbon
fluids. The proppants infiltrate the formation and prevent
collapse after the high stimulation pressure is released.
In order to increase the efficacy of the
stimulation process, "staged well stimulation" methods have
been developed. In staged stimulation, small sections of a
production zone are fractured in sequence by isolating
sections of the production zone or, if the production zone
is very small, isolating the entire production zone in
order to concentrate the area to which stimulation fluids
are delivered. This helps ensure that a production zone is
more evenly fractured. It is common practice today to
perforate all of the production zones through which a
casing extends. Tubing is then run into the well with
isolation packers to isolate a section of a zone to be
stimulated. Generally, about 10 feet (3.3 meters) of a
zone is isolated at a time using isolation packers and a
small fracturing treatment is applied to that section of
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the zone. Thereafter, the tubing is moved up and another
small fracturing treatment is performed. This process is
repeated until all of the production zones in the well have
been stimulated.
A disadvantage of the way in which the staged
stimulation process is performed is that if a condition
known as "screenout" occurs, the entire toolstring must be
pulled from the well in order to clean the apparatus and
recommence the stimulation. Screenout is a phenomenon that
occurs when abrasive proppants clog the injection tubing
during a fracturing process.
Furthermore, if all of the production zones) are
already perforated, and there are open perforations above
the upper isolation packer, fracturing fluids may migrate
upwards through a production zone outside the casing and
enter the casing above the upper isolation packer. This
can cause the casing to fill with high pressure fluid and
proppants. This has two disadvantages. First, the casing
above the upper isolation packer may fill with abrasive
proppant and trap the tool within the casing. Second, the
fracturing of one or more sections may be ineffective
because the fracturing fluids follow a path of least
resistance and the entire production zone is not uniformly
fractured.
Some of these problems with prior art methods of
staged well stimulation are overcome by inventions
described in United States Patent Nos. 5,865,252 and
6,116,343 which issued respectively on February 2, 1999 and
September 12, 2000 to Van Petegem et al. These patents
describe a method and apparatus for a one-trip production
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zone perforation and proppant fracturing operation carried
out using a workstring-supported perforation gun lowered
into a casing nipple located in the production zone.
Firing of the perforation gun creates spaced apart aligned
sets of perforations extending outwardly through a side
wall portion of the workstring, the nipple, cement
surrounding the nipple and into the production zone. After
firing, the gun falls into and is retained in an underlying
gun catcher portion of the workstring. Proppant slurry is
then pumped down the workstring and out through the side
wall perforations, and through the aligned perforations in
the nipple to stimulate the production zone. After
stimulation of the production zone, the workstring and the
spent perforation gun that it retains are pulled up out of
the casing.
While this method represents an advance over the
prior art, it still has several disadvantages. First, the
purpose of the invention is to enable a one-trip entry run
into the well to perform perforation as well as
stimulation. In order to make the one-trip into the well
profitable, a significant length of the nipple must be
perforated and stimulated in a single shot. Consequently,
if the perforated area is very large, the production zone
may not be evenly stimulated. In other words, this tool is
not suited for economical staged stimulation.
Furthermore, the perforating gun is designed for
single-shot operation. As described above, after the
perforating gun is fired, it is dropped into a gun-catching
section of the toolstring. Because of this, only one
contiguous region of a casing can be perforated each time
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the tool is run into the well. Consequently, the tool must
be run into the well at least once for each production zone
requiring stimulation. This is time-consuming and
contributes to the cost of production from the well. A
further disadvantage is the fact that a perforating gun
must be customized for each production zone. While this is
commonplace for wireline applications, it is more difficult
and time-consuming when the perforating gun must be
incorporated into a toolstring between well
perforation/stimulation operations.
There therefore exists a need for a method and
apparatus that permits selective perforation and
stimulation of staged sections of a production zone while
ensuring an even and complete distribution of fracturing
fluids within each stage of the production zone treated.
There is also a need for a method and apparatus
that permits a plurality of production zones, or stages in
a production zone to be successively perforated and
stimulated without withdrawing a toolstring from the well.
2 0 SUI~lARY OF THE INVENTION
It is therefore an object of the invention to
provide a method and apparatus for conducting subterranean
well casing perforation and production zone stimulation
using a process that saves time and reduces costs.
It is a further object of the invention is to
provide a well tool for perforating and stimulating
subterranean wells so that both a staged perforating
process and a staged stimulation process can be completed
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using the well tool in a one-trip insertion of the tool
into the well.
It is another object of the invention to provide a
well tool for selectively perforating and stimulating
subterranean wells so that the perforation of several
selected sections of a well casing and the stimulation of
several corresponding sections of one or more subterranean
production zones can be completed using the well tool in a
one-trip insertion of the tool into the well.
It is a further object of the invention to provide
a method for completing perforation and stimulation of
selected sections of a subterranean well using a well tool
adapted to perform staged perforation and fracturing in a
one-trip insertion of the tool into the well.
In general terms, these objects are achieved by
pumping production stimulation fluid through a novel
perforating gun to an injection nozzle connected to a lower
end of the gun.
In accordance with one aspect of the invention, an
apparatus is provided for perforating and stimulating
subterranean wells, such as oil wells, which comprises a
perforating gun for perforating a well casing, the
perforating gun being adapted for connection of a tubing
string and including a fluid passage to permit well
stimulation fluid to be pumped therethrough without
affecting functionality of components of the gun; and an
injection nozzle connected to a second end of the
perforating gun in fluid communication with the fluid
passage for injecting pressurized stimulation fluid into a
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production zone through perforations made through a casing
of the well by the perforating gun.
The charges of the perforating gun are preferably
adapted to be selectively detonated so that perforations in
selected sections of the well casing can be effected by a
single perforating gun. The injection nozzle preferably
includes an annular seal attached to a bottom end of the
nozzle for sealing an annulus between the apparatus and the
well casing to prevent pressurized stimulation fluid from
flowing into the well casing below the apparatus. In order
to balance fluid pressure on each side of the annular seal
when the apparatus is inserted into the well, it is
preferable to provide a fluid passage having a check valve
in the injection nozzle to permit fluid in the well below
the annular seal to flow through the injection nozzle when
the apparatus is inserted into the well, while inhibiting
the pressurized stimulation fluid from flowing through the
nozzle into the well casing below the apparatus.
In accordance with one embodiment of the invention,
the perforating gun includes a tubular sleeve and an inner
mandrel which define an annulus to accommodate charges
carried by the perforating gun. The injection nozzle
includes a sidewall, an axial passage in fluid
communication with the inner mandrel of the perforating
gun, and a plurality of radial passages for directing
pressurized well stimulation fluid into an annulus between
the apparatus and the well casing above the annular seal.
A check valve is located in the axial passage below the
radial passages. The apparatus preferably includes a
collar locator for detecting joint collars between sections
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of the well casing to permit a position of the apparatus to
be tracked as the apparatus is moved up or down through the
well casing. The apparatus also preferably includes a
temperature sensor and a pressure sensor for measuring the
downhole temperature and pressure during a well stimulation
process.
In accordance with another aspect of the invention,
there is provided a method for perforating and stimulating
subterranean cased wells, comprising steps of perforating a
selected section of the cased well by selectively firing
charges from a perforating gun; and stimulating the
selected section by pumping stimulation fluid down a fluid
passage through the perforating gun and through
perforations made by the selectively fired charges.
In accordance with yet another aspect of the
invention, there is provided a method for perforating and
stimulating subterranean cased wells, comprising steps of
(a) inserting a tool into a well, the tool including a
perforating gun that carries vertically spaced apart
charges that may be selectively fired for perforating the
well casing, the perforating gun being adapted for
connection to a tubing string and including a fluid passage
to permit well stimulation fluid to be pumped therethrough,
and an injection nozzle connected to an opposite end of the
perforating gun in fluid communication with the fluid
passage; (b) positioning the tool in the well so that a
selected number of the charges carried by the perforating
gun are positioned within a selected section of a
production zone to be stimulated; (c) detonating the
selected number of charges to perforate the well casing;
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(d) pulling the tool upward until the injection nozzle is
positioned in the selected section of the production zone;
and (e) pumping the well stimulation fluid through the
tubing string, the fluid passage and the nozzle so that the
well stimulation fluid is injected through the perforations
in the well casing into the selected section of the
production zone.
When more than one section of a cased well is to be
perforated and stimulated, the above steps a)-e) are begun
at a lowest section of the cased well and the well is
treated in a staged upward progression. During each stage
of the well treatment, the tool is first positioned to
locate a selected group of charges in an area of the casing
to be perforated. The selected group of charges are
detonated to perforate the well casing and steps d) and e)
are repeated until the stimulation of a production zone is
completed. Steps b) to e) are then repeated for other
production zones of the well. Remaining unused charges are
selectively detonated until the entire perforation and
stimulation of all selected sections of the well are
completed, or all of the charges are spent. Finally, the
tool is removed from the well.
The apparatus and method in accordance with the
invention provide an improved solution for well completion.
Perforation and stimulation of more than one section of the
subterranean well is enabled without withdrawing and
reinserting the toolstring between stimulation sessions for
various sections of the well. As a result, the time
required to complete a well is significantly reduced, and
costs are correspondingly reduced.
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Other features and advantages of the invention will
be better understood with reference to preferred
embodiments described below.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the nature of the
invention, the invention will be further described in
detail with reference to the accompanying drawings in
which:
FIG. 1 is a schematic longitudinal sectional view
of a well tool in accordance with one embodiment of the
invention, connected to a tubing string in a cased well;
FIG. 2 through FIG. 6 are schematic views of the
well tool shown in FIG. 1 in different stages of operation,
illustrating a method of using the tool to perforate and
. stimulate a plurality of selected sections of the cased
well.
DETAILED DESCRIPTION OF THE PREFERRED Et~ODIMENT
The invention provides a method and apparatus for
performing staged perforation and stimulation of a cased
well in a one-trip insertion of the apparatus into the
well. The apparatus is a tool that includes a selective-
shot perforating gun and a stimulation fluid injection
nozzle connected to the perforating gun. The perforating
gun is used to selectively perforate a section of the well
casing. After the casing is perforated, the tool is pulled
up until the injection nozzle is aligned with the
perforations and stimulation fluid is pumped down through
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the perforating gun and the injection nozzle into a
production zone through the perforations. The process is
then repeated for a next section of the well, until all
production zones are stimulated, or all the charges carried
by the perforating gun are fired. This staged process
ensures that all sections of a productive zone are more
evenly fractured, while considerably reducing the time and
cost of preparing a hydrocarbon well for production.
FIG. 1 schematically illustrates a well tool
assembly in accordance with the invention, generally
indicated by reference numeral 10. The well tool 10 is
connected to a tubing string 12 in a cased well 14. The
well tool assembly 10 includes a selective-shot perforating
gun 16 and an injection nozzle 18 connected in series. The
selective-shot perforating gun 16 includes a primer head 20
and nozzle adapter 22. An outer tubular sleeve 24 and an
inner mandrel 26 are respectively connected to the primer
head 20 and the nozzle adapter 22, and define an annulus 25
between the tubular sleeve 24 and the mandrel 26 for
accommodating a plurality of perforating charges 28. The
perforating charges 28 are axially spaced apart from one
another and may be selectively fired to perforate a well
casing 30. The charges 28 are supported in the annulus 25
and connected to respective primers and detonation
conductors (not shown), to permit the charges 28 to be
selectively detonated.
The primer head 20 includes a central bore 32 that
is sealingly connected to a top end of the inner mandrel.
The nozzle adapter 22 also includes a central bore 34 which
is sealingly connected to a bottom end of the inner
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mandrel 26 so that the inner mandrel 26 in conjunction with
the central bores 32 and 34 define a fluid passage that
extends through the perforating gun 16 to permit well
stimulation fluid to be pumped therethrough without
contacting the charges 28. A threaded connector 36 and a
seal ring 38 provide a high pressure fluid seal between the
primer head 20 and a tubing string 12. In this embodiment,
the connector 40 is a coil tubing hydraulic quick-connector
and the tubing string 12 is a coil tubing string of 23/8
inches. A threaded connector 42 is formed in the nozzle
adapter 22 to facilitate connection of the injection
nozzle 18.
A second passage 44 extends through the primer
head 20 to accommodate electrical conductors for detonating
the charges 28, and for conducting various sensor signals,
as will be explained below in more detail. The second
passage 44 is adapted for the connection of a wireline, or
a second small-diameter coil tubing (%" coil tubing, for
example) to accommodate the electrical conductors. A fluid
tight seal 48 between the wireline or coil tubing 46 and
the primer head 20 prevents fluid from infiltrating the
annulus 25 so that the charges 28 are kept dry.
In order to track the position of the well tool 10
while it is inserted into the well 14, a collar locator 50
is preferably provided on the well tool 10. The collar
locator 50 may be a mechanical type or an electronic type,
each of which are well known in the art. The collar
locator 50 may be incorporated into the well tool 10 at,
for example, the primer head 20, or any other suitable
location for detecting joint collars between sections of
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the well casing wall 30. The joint collars are counted to
determine the location of the well tool 10 in the cased
well 14, in a manner that is also well known in the art.
The injection nozzle 18 includes a side wall 52, a
top threaded connector 54 that compresses an O-ring seal 56
for providing a high-pressure seal between the perforating
gun 16 and the injection nozzle 18. The injection
nozzle 18 further includes a bottom thread connector 58 for
connection of a bull nose 60 for guiding the insertion of
the well tool 10 when it is inserted into the cased
well 14. An axial passage 62 extends through the injection
nozzle 18 in fluid communication with the inner mandrel 26,
and a center bore 64 of the bull nose 60. The axial
passage 62 includes a lower section having a smaller
diameter to form a valve seat 66 to receive a ball 68 of a
ball valve. Two valve stops 70 axially spaced from the
valve seat 66, extend through the sidewall 52 to limit
upward movement of the ball 68. The ball 68 blocks the
axial passage 62 to direct pressurized stimulation fluid
into the annulus between the well casing 30 and the well
tool 10. The radial passages 72 are axially spaced apart
form one another. The radial passages may be spaced apart
and oriented to correspond to the position and orientation
of a repetitive pattern used to position the charges 28 in
the perforating gun 10, although correspondence between the
position of the axial passages 72 and the charges 28 is not
required. The overall length of the injection nozzle is
preferably about 6-10 feet (2-3 meters), though other
lengths may be used. The length of the perforating gun is
preferably about 20 feet (6.3 meters), but longer or
shorter lengths may be used, depending on the number of
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charges that are required to perforate a given well casing,
for example.
An annular seal assembly 74 is connected to a lower
end of the injection nozzle 18 to seal the annulus between
the well tool 10 and the well casing 30 to inhibit
pressurized stimulation fluid from flowing into the well 14
below the well tool 10. The annular seal assembly 74
includes a cup member 76, an annular rubber element 78 and
a gauge ring 80. The annular seal assembly 74 is secured
between the bull nose 60 and an annular shoulder 82 of the
injection nozzle 18. The annular seal assembly 74 and the
bull nose 60 are described in more detail in the
Applicant's United States Patent No. 6,626,245 which issued
on September 30, 2003.
In order to control a well stimulation process, it
is desirable to know pressure and temperature at the
stimulation fluid injection site. Pressure and temperature
are routinely measured at the surface during a well
stimulation operation, but for a number of reasons well
understood in the art, surface measurements are often not
indicative of downhole conditions. Downhole measurements
permit a deduction of the porosity of a zone being
stimulated, for example, and likewise enable the early
detection of screenout, and the like. This permits
remedial action to be taken early, saving time and
materials. Consequently, the well tool 10 is preferably
equipped with a pressure sensor 84, such as a
pressure-sensing transducer, and temperature sensor 86,
such as a temperature-sensing transducer. The pressure
sensor 84 and the temperature sensor 86 respectively
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measure the instant pressure and temperature in the
stimulation zone during a stimulation process. The
pressure sensor 84 and the temperature sensor 86 may be
incorporated into the well tool at any convenient location.
For example at the nozzle adapter 22 of the perforating
gun 16.
A method for performing a staged perforation and
stimulation of selected sections of the cased well 14 using
the well tool 10 is described below with reference to
FIGS. 2-6.
FIG. 2 illustrates the well tool 10 being inserted
into the cased well 14. Fluid in the cased well 14 below
the well tool 10 is subject to pressure exerted by the
downward movement of the well tool 10 because the annular
seal assembly 74 blocks passage of the fluid around the
well tool 10. Consequently, a build-up of fluid pressure
in the center bore 64 of the bull nose 60 and the axial
passage 62 of the injection nozzle 18 force the ball 68 up
against the valve stops 70. The fluid flow illustrated by
arrows 88 bypasses the annular seal assembly 74 through the
axial passage 62 and radial passages 72 and fluid pressure
in the wellbore is equalized as the well tool 10 is
inserted into the cased well 14. At ground level,
Applicant's stage fracturing tool assembly (not shown) may
be used to provide an annulus valve (not shown) and
pressure isolation if required. Applicant's stage
fracturing tool assembly is described in United States
Patent No. 6,364,024 issued on April 2, 2002. A dual
string isolation tool (not shown) may also be used to run
the main coil tubing string 12 through a packoff on one
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side and the wireline or the second coil tubing string 46
simultaneously. Applicant's dual string isolation tool is
described in Applicant's United States Patent No. 6,145,596
issued on November 14, 2000. As the well tool 10 is
inserted into the well, the collar locator 50 (FIG. 1) is
used to count the joint collars of the well casing 30, to
provide an accurate indication of the location of the well
tool 10 in the cased well 14.
FIG. 3 illustrates the well tool 10 positioned in
the cased well 14 so that a plurality of charges 28 (only 3
are shown) of the perforating gun 16 are located within a
selected section of a production zone Z2. The fluid
pressure above and below the annular seal assembly 74 is
balanced and the ball 68 has returned to the valve seat 66
under its own weight, because the well tool 10 is no longer
moving. The charges 28 are selectively detonated, and the
explosive forces illustrated by arrows 90a create a first
group of perforations 92a in the well casing 30. The
number of charges detonated at each stage of a well
stimulation operation is dependent on a number of factors.
However, the length of the well casing 30 perforated at
each stage is preferably not greater than a length of the
injection nozzle 18. Preferably, the length of the well
casing 30 that is perforated is equivalent to the length of
the perforated portion of the injection nozzle 18, as shown
in FIG. 4.
After the well casing is perforated by the
selective firing of the perforating charges 28, as
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described above, the well tool 10 is pulled upwards in the
well until the injection nozzle 18 is aligned with the
perforations made in the well casing 30. In FIG. 4
illustrates the well tool 10 after it has been moved
upwards to locate the radial passages 72 of the injection
nozzle 18 in proximity with the perforations 92a. In order
to prevent fluid pressure from building up in the cased
well 14 above the annular seal assembly 74 when the well
tool 10 is moved upwards, an annulus valve at ground level,
if one is used, should be opened. The annulus valve is
closed before highly pressurized stimulation fluid is
pumped through the main coil tubing string 12 and the inner
mandrel 26 of the perforating gun 16 into the axial
passage 62 of the injection nozzle 18. The highly
pressurized stimulation fluid illustrated by arrows 94a
forces the ball 68 against the valve seat 66 and is ejected
through the radial passages 72 of the injection nozzle 18
and the perforations 92a in the well casing 30 into the
selected section of the production zone Z2. The
temperature sensor 84 and the pressure sensor 86 (FIG. 1)
provide real-time downhole pressure and temperature
readings to assist the well stimulation crew in tracking
and assessing the stimulation process. In the event of a
screenout, the well tool 10 and the annulus above it are
readily cleaned out without pulling the toolstring. This
may be accomplished by opening annulus valves (not shown)
at the surface and pumping gelled frac fluid down the coil
tubing 12. The gelled frac fluid displaces the clogged
proppants and forces them up through the annulus and out
through the annulus valves. After a stimulation process
has been completed normally, the annulus may be cleaned
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using the same procedure before the tool is moved to a new
selected position in the cased well 14.
In accordance with the invention, the cased well 14
is preferably perforated and stimulated in a staged
progression from a lowest point to be stimulated in the
cased well 14. In other words, selected sections of the
cased well 14 are perforated and stimulated in succession.
The reason for doing so is explained below in more detail.
The well tool 10 need not necessarily be moved to begin a
next stage of perforation and stimulation of the next
selected section of the oil well 14 if a next group of
charges 28 to be detonated are located within the next
selected section of the cased well 14, as shown in FIG. 5.
Otherwise, the well tool 10 is moved to position the next
group of charges in the selected section. As explained
above, the annulus valve at ground level is opened when the
well tool 10 is moved upwards in the cased well 14, and is
closed during well stimulation. The next group of
charges 28 are then detonated and the explosive forces
indicated by arrows 90b produce perforations 92b in the
well casing 30 and any cement surrounding the casing, as is
well understood by those skilled in the art.
FIG. 6 illustrates the next stage of stimulation in
which the well tool 10 is moved further upwards to position
the radial passages 72 into alignment with the
perforations 92b. Under the pressure induced by the
pressurized stimulation fluid illustrated by arrows 94b
that is pumped into the axial passage 62 of the injection
nozzle 18 through the inner mandrel wall 26 of the
perforating gun 16, the ball 68 is forced downwardly
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against the valve seat 66, as explained above, to inhibit
the pressurized stimulation fluid 94b from entering the
section of the cased well 14 below the annular seal
assembly 74 so that the stimulation pressure is maintained.
The pressurized stimulation fluid 94b is injected through
the radial passages 72 and the perforations 92b, into the
selected section of the production zone Z1. The annular
seal assembly 74, in combination with the ball 66, seals
the passages between the selected section and the other
lower sections so that the pressurized stimulation
fluid 94b is prevented from entering the perforations 92a
of the selected sections of the production zone Z2 that
were previously stimulated. Since perforation and
stimulation of the selected sections of the cased well 14
are performed in an upward progression, consistent
stimulation of all zones is ensured. Even i,f stimulation
fluids follow a path of least resistance to the lower
perforations 92a, the only result will be a filling of the
casing of the cased well 14 below the annular seal
assembly 74. As soon as the casing is filled, stimulation
pressure is exerted on the selected section being
stimulated, and the section is fractured, as desired.
Thus, the quality of the well stimulation is assured.
The process of perforation and stimulation is
repeated, as required, until all zones of the cased well 14
are stimulated or the charges 14 are all detonated. If the
charges are all detonated before stimulation of all
production zones is complete, the toolstring is pulled from
the well and the perforating gun is reloaded for another
run into the wellbore.
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The method and apparatus in accordance with the
invention therefore overcome all known disadvantages of the
prior art and enable a cased well 14 to be stimulated to
prepare for production more quickly, reliably and
efficiently than was achievable using prior art methods and
apparatus.
The foregoing description is intended to be
exemplary rather than limiting. The scope of the invention
is therefore intended to be limited solely by the scope of
the appended claims.
