Note: Descriptions are shown in the official language in which they were submitted.
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MULTIPLE BALL - BALL SEAT FOR HYDRAULIC FRACTURING
WITH REDUCED PUMPING PRESSURE
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Patent Application No.
13/091,988 filed
on April 21, 2011, which in turn is entitled to the benefit of, and claims
priority to
U.S. Provisional Patent Application Serial No. 61/327,509, filed on April 23,
2010.
This application also claims priority to U.S. Provisional Application Serial
No. 61/360,796, filed on July 1, 2010.
BACKGROUND OF INVENTION
Field of the Invention
[0002] Embodiments disclosed herein generally relate to a downhole
isolation tool.
More specifically, embodiments disclosed herein relate to a downhole isolation
tool
having a ball seat mandrel having two or more ball seats. Additionally,
embodiments
disclosed herein relate to a downhole isolation system having two or more
downhole
isolation tools. Further, embodiments disclosed herein relate to methods of
running a
downhole isolation system into a well and isolating zones of a well with a
downhole
isolation system.
ck nnd Art
[0003] In drilling, completing, or Backgroundwells,
it often becomes necessary to isolate
particular zones within the well. In some applications, downhole isolation
tools are
lowered into a well to isolate a portion of the well from another portion. The
downhole
tool typically includes a sleeve coupled to a ball seat. A ball may be dropped
from the
surface and seated in the ball seat to seal or isolate a portion of the well
below the tool
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from a portion of the well above the tool. More than one downhole isolation
tool may be
run into the well, such that multiple zones of the well are isolated.
[0004] The downhole isolation tool may be run in conjunction with other
downhole tools,
including, for example, packers, frac (or fracturing) plugs, bridge plugs,
etc. The
downhole isolation tool and other downhole tools may be removed by drilling
through
the tool and circulating fluid to the surface to remove the drilled debris.
[0005] The downhole isolation tool may be set by wireline, coil tubing, or
a conventional
drill string. The tool may be run in open holes, cased holes, or other
downhole
completion systems. The ball seat disposed in the downhole isolation tool is
configured
to receive a ball to isolate zones of a wellbore and allow production of
fluids from zones
below the downhole isolation tool. The ball is seated in the seat when a
pressure
differential is applied across the seat from above. For example, as fluids are
pumped
from the surface downhole into a formation to fracture the formation, the ball
is seated in
a ball seat to maintain the fluid, and therefore, provide fracturing of the
formation in the
zone above the downhole isolation tool. In other words, the seated ball may
prevent fluid
from flowing into the zone isolated below the downhole isolation tool.
Fracturing of the
formation allows enhanced flow of formation fluids into the wellbore. The ball
may be
dropped from the surface or may be disposed inside the downhole isolation tool
and run
downhole within the tool.
[0006] At high temperatures and pressures, i.e., above approximately 300 F
and above
approximately 10,000 psi, the commonly available materials for downhole balls
may not
be reliable. Furthermore, as shown in Figures 1A and 1B, a conventional ball
seat 36
includes a tapered or funnel seating surface 40. The ball 38 makes contact
with the
seating surface 40 and forms an initial seal. Based on the geometries of the
seating
surface 40 and ball 38, there is a large radial distance between the inside
diameter of the
seating surface 40 and the outside diameter of the ball 38. Thus, the bearing
area
between the seating surface 40 and the ball 38 is small. As the ball 38 is
loaded to
successively higher loads, the ball 38 may be subjected to high compressive
loads that
exceed its material property limits, thereby causing the ball 38 to fail. Even
if the ball 38
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deforms, the ball 38 cannot deform enough to contact the tapered seating
surface 40, and
therefore the bearing surface 40 of the ball seat 36 for the ball 38 remains
small. An
increase in ambient temperature can also increase the likelihood of extruding
the ball 38
through the seat 36 due to decreased properties of the material. The
mechanical
properties of the ball 38 material may decrease, e.g., compressive stress
limits and
elasticity, which can lead to an increased likelihood of the ball cracking or
extruding
through the ball seat 36. Thus, in high temperature and high pressure
environments,
conventional downhole isolation tool, i.e., balls 38 and ball seats 36 within
the downhole
isolation tool, may leak or fail.
[0007] In open hole fracturing systems that use such balls and ball drop
devices as means
to isolate distinct zones for hydraulic fracturing treatment, different sized
balls are used
for each isolation zone. Specifically, in a wellbore where multiple zones are
isolated, a
series of balls are used to isolate each zone. A ball of a first size seals a
first seat in a first
zone and a ball of a second size seals a second seat in a second zone. The
lowermost
zone uses the smallest ball of the series of balls and the uppermost zone uses
the largest
ball of the series of balls. The smallest sized ball is typically 3/4 inch to
1 inch in
diameter. The corresponding ball seat and corresponding throughbore must have
a
diameter smaller than the ball to receive and support the ball. Typical
hydraulic
fracturing fluid rates are between 20 BPM (barrels per minute) and 40 BPM. The
pressure drop through a restriction, i.e., the ball seat and corresponding
axial
throughbore, as small as 3/4 inch is substantial. Such a pressure drop
increases the total
pump horsepower needed on location to complete an isolation job.
[0008] Accordingly, there exists a need for a downhole isolation tool that
effectively
seals or isolates the zones above and below the plug in high temperature and
high
pressure environments and provides sufficient through flow through the system.
SUMMARY OF INVENTION
[0009] In one aspect, embodiments disclosed herein relate to a downhole
isolation tool
including a sub, a sleeve disposed in the sub, and a ball seat mandrel coupled
to the
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sleeve, the ball seat mandrel having at least two ball seats axially aligned
with at least
two throughbores disposed within the ball seat mandrel.
[0010] In another aspect, embodiments disclosed herein relate to a
downhole isolation
system, the system including a first downhole isolation tool including a first
sub, a first
sleeve disposed in the first sub, and a first ball seat mandrel coupled to the
first sleeve,
the first ball seat mandrel having at least two ball seats axially aligned
with at least two
throughbores disposed within the first ball seat mandrel, and a second
downhole isolation
tool including a second sub, a second sleeve disposed in the second sub, and a
second ball
seat mandrel coupled to the second sleeve, the second ball seat mandrel having
at least
two ball seats axially aligned with at least two throughbores disposed within
the second
ball seat mandrel.
[0011] In yet another aspect, embodiments disclosed herein relate to a
method of
isolating a well, the method including running a downhole isolation system
into a well,
wherein the downhole isolation system includes a first downhole isolation
tool, the first
downhole isolation tool including a first sub, a first sleeve disposed in the
sub, and a first
ball seat mandrel coupled to the first sleeve, the first ball seat mandrel
having at least two
ball seats of a first size axially aligned with at least two throughbores
disposed within the
first ball seat mandrel, dropping at least two balls of a first size into the
well, and seating
the at least two balls of the first size in the at least two ball seats of the
first ball seat
mandrel.
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[0011a] According to another aspect, there is provided a downhole
isolation tool
comprising: a sub; a ball seat mandrel disposed in the sub, the ball seat
mandrel comprising: at
least two ball seats each having a corresponding through bore disposed within
the ball seat
mandrel, wherein at least one of the at least two ball seats comprises a
seating surface having
an arcuate profile with a radius of curvature that is substantially equal to a
radius of curvature
of a profile of a drop ball.
[0011b] According to another aspect, there is provided a downhole
isolation system, the
system comprising: a first downhole isolation tool comprising: a first sub; a
first sleeve
disposed in the first sub; and a first ball seat mandrel coupled to the first
sleeve, the first ball
seat mandrel comprising: at least two ball seats axially aligned with at least
two throughbores
disposed within the first ball seat mandrel, wherein at least one of the at
least two ball seats
comprises a seating surface having an arcuate profile with a radius of
curvature that is
substantially equal to a radius of curvature of a profile of a drop ball; and
a second downhole
isolation tool comprising: a second sub; a second sleeve disposed in the
second sub; and a
second ball seat mandrel coupled to the second sleeve, the second ball seat
mandrel
comprising: at least two ball seats axially aligned with at least two
throughbores disposed
within the second ball seat mandrel.
10011c] According to another aspect, there is provided a method of
isolating a well, the
method comprising: running a downhole isolation system into a well, wherein
the downhole
isolation system comprises a first downhole isolation tool, the first downhole
isolation tool
comprising: a first sub; a first sleeve disposed in the sub; and a first ball
seat mandrel coupled
to the first sleeve, the first ball seat mandrel comprising: at least two ball
seats of a first size
axially aligned with at least two throughbores disposed within the first ball
seat mandrel;
dropping at least two balls of a first size into the well; and seating the at
least two balls of the
first size in the at least two ball seats of the first ball seat mandrel,
wherein the at least two
ball seats each comprise a seating surface having an arcuate profile with a
radius of curvature
that is substantially equal to a radius of curvature of a profile of the
balls.
[0012] Other aspects and advantages of the invention will be apparent
from the
following description and the appended claims.
4a
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BRIEF DESCRIPTION OF DRAWINGS
[0013] Figure lA shows a cross-sectional view of a conventional ball
seat
and ball disposed in the ball seat.
[0014] Figure 1B is a detailed view of the conventional ball seat and
ball of
Figure 1A.
[0015] Figures 2A and 2B show cross-sectional views of a downhole
isolation tool in accordance with embodiments disclosed herein.
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[0016] Figures 3A and 3B show a perspective view and a cross-sectional
view,
respectively, of a ball seat mandrel for a downhole isolation tool in
accordance with
embodiments disclosed herein.
[0017] Figures 4A and 4B show a perspective view and a cross-sectional
view,
respectively, of a ball seat mandrel for a downhole isolation tool in
accordance with
embodiments disclosed herein.
[0018] Figure 5A shows a cross-sectional view of a ball seat in accordance
with
embodiments disclosed herein.
[0019] Figure 5B shows a detailed view of Figure 5A.
[0020] Figure 6A shows a cross-sectional view of a ball seat in accordance
with
embodiments disclosed herein.
[0021] Figure 6B shows a detailed view of Figure 6A.
[0022] Figure 7 shows a cross-sectional view of a ball seat mandrel for a
downhole
isolation tool in accordance with embodiments disclosed herein.
[0023] Figures 8A and 8B show a perspective view and a top view,
respectively, of a ball
seat mandrel for a downhole isolation tool in accordance with embodiments
disclosed
herein.
DETAILED DESCRIPTION
[0024] Embodiments disclosed herein generally relate to a downhole
isolation tool.
More specifically, embodiments disclosed herein relate to a downhole isolation
tool
having a ball seat mandrel having two or more ball seats. Additionally,
embodiments
disclosed herein relate to a downhole isolation system having two or more
downhole
isolation tools. Further, embodiments disclosed herein relate to methods of
running a
downhole isolation system into a well and isolating zones of a well with a
downhole
isolation system.
[0025] Figures 2A and 2B show a downhole isolation tool 200 in accordance
with
embodiments disclosed herein. Tool 200 includes a sub 202 that may be coupled
to a
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drillstring, production string, coiled tubing, or other downhole components.
The sub 202
may be a single tubular component or may include two or more components. For
example, as shown in Figures 2A and 2B, sub 202 may include an upper housing
204 and
a lower housing 206. The upper housing 204 and the lower housing 206 may be
threadedly coupled to one another or coupled by any other means known in the
art, e.g.,
welding, press fit, and coupling with mechanical fasteners. For example, one
or more set
screws 222 may couple the lower housing 206 to the upper housing 204. One or
more
ports 221 are disposed in the sub 202 to allow fluid communication between the
bore of
the sub 202 and an annular space (not shown) formed between the sub 202 and
the well
(not shown).
[0026] Tool 200 further includes a sleeve 208 disposed within the sub 202.
The sleeve
208 is configured to slide axially downward within the sub 202 when a
predetermined
pressure is applied from above the tool 200, as will be described in more
detail below.
Sleeve 208 is initially coupled to the sub 202 proximate a first or upper end
of a main
cavity 210 of the sub 202. A shearing device 212 couples the sleeve 208 to an
inner
surface of the sub 202. In one embodiment, the shearing device 212 may include
one or
more shear pins or shear screws configured to retain the sleeve 208 in an
initial position
until a predetermined pressure is applied from above the tool 200.
[0027] Tool 200 further includes a ball seat mandrel 218 coupled to the
sleeve 208. In
one embodiment, the ball seat mandrel 218 may be disposed within the sleeve
208
proximate an upper end 220 of the sleeve 208. However, in other embodiments,
the ball
seat mandrel 218 may be disposed proximate the center or lower end 214 of the
sleeve
208. The ball seat mandrel 218 may be coupled to the sleeve by any means known
in the
art. For example, in one embodiment, ball seat mandrel 218 may be threadedly
engaged
with the sleeve 208. In another embodiment, the ball seat mandrel 218 may be
welded to
the ball seat mandrel 218.
[0028] Referring now to Figures 3A and 3B, a perspective view and a cross-
sectional
view, respectively, of a ball seat mandrel 218 in accordance with embodiments
disclosed
herein are shown. As shown, in one embodiment, ball seat mandrel 218 may
include two
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ball seats 224A, 224B formed in an upper face 226 of the ball seat mandrel
218. Each
ball seat 224A, 224B is axially aligned with one of two throughbores 228A,
228B
extending through the ball seat mandrel 218. The diameters of ball seats 224A,
224B and
corresponding throughbores 228A, 228B are sized so as to maximize the fluid
flow area
through the ball seat mandrel 218.
[0029] The upper face 226 of the ball seat mandrel 218 is contoured so as
to ensure
proper seating of a dropped ball (not shown) in each of the seats 224A, 224B.
Additionally, the contour of the upper face 226 may be configured to enhance
the
hydrodynamics of the ball seat mandrel 218, i.e., to help direct flow through
the
throughbores 228A, 228B, reduce friction of fluid flowing through the seats
224A, 224B
and the throughbores 228A, 228B, and reduce wear of the upper face 226 and the
ball
seat mandrel 218 in general.
[0030] While Figures 3A and 3B show a ball seat mandrel 218 having two
ball seats
224A, 224B and two corresponding throughbores 228A, 228B, one of ordinary
skill in
the art will appreciate that three, four, or more ball seats 224 may be formed
in the upper
face 226 of the ball seat mandrel 218. Figures 4A and 4B show a perspective
view and a
cross-sectional view, respectively, of a ball seat mandrel 318 having four
ball seats 324A,
324B, 324C, 324D in accordance with embodiments of the present disclosure. As
shown,
each ball seat 324A, 324B, 324C, 324D is axially aligned with one of four
throughbores
(only two are shown in this view) 328A, 328B extending through the ball seat
mandrel
318. The diameters of ball seats 324A, 324B, 324C, 324D and corresponding
throughbores 328A, 328B are sized so as to maximize the fluid flow area
through the ball
seat mandrel 318.
[0031] The upper face 326 of the ball seat mandrel 318 is contoured so as
to ensure
proper seating of a dropped ball (not shown) in each of the seats 324A, 324B,
324C,
324D. Additionally, the contour of the upper face 326 may be configured to
enhance the
hydrodynamics of the ball seat mandrel 318, i.e., to help direct flow through
the
throughbores 328A, 328B, reduce friction of fluid flowing through the seats
324A, 324B,
324C, 324D and the throughbores 328A, 328B, and reduce wear of the upper face
326
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and the ball seat mandrel 318 in general. For example, as shown in Figures 4A
and 4B,
the upper face 326 of the ball seat mandrel 318 may be contoured such that a
central
portion 330 of the upper face 326 is higher than a circumferential portion 332
proximate
each of the four ball seats 324A, 324B, 324C, 324D. This elevated or raised
central
portion 330 of the upper face 326 prevents a ball (not shown) from settling or
seating
against the surface of the upper face 326 instead of seating within one of the
ball seats
324A, 324B, 324C, 324D. Portions of the upper face 326 between one or more
ball seats
may similarly be raised so as to ensure proper seating of a ball within the
ball seats 324A,
324B, 324C, 324D. As fluid flows down the well with balls (not shown)
contained
within the fluid flow, the contour of the upper face 326, in addition to the
fluid pressure,
help seat each of the balls (not shown) in each one of the ball seats 324A,
324B, 324C,
324D.
[0032] One or more ball seats 224A-B, 324A-D of the embodiments
described with
respect to Figures 3A, 3B, 4A, and 4B may include a seating surface 4015
having an
arcuate profile, as shown in Figures 5A and 5B, and as disclosed in
U.S Application Serial No. 61/327,509. As shown, the profile of the
seating surface 4015 corresponds to the profile of a ball 4009
dropped into the well and seated in the ball seat 224, 324. In particular, the
profile of the
seating surface 4015 is curved. The arcuate profile may be spherical or
elliptical. Thus,
the radius of curvature of the arcuate profile may be constant or variable.
The radius of
curvature of the seating surface 4015 may be approximately equal to the radius
of
curvature of the ball 4009. Thus, in one embodiment, the seating surface 4015
provides
an inverted dome-like seat with a bore therethrough configured to receive the
ball 4009.
[0033] In one embodiment, the seat 224A-B, 324A-D may include a first
section 4017
and a second section 4019, as shown in Figure 5A. The first section 4017 is
disposed
axially above the second section 4019. In this embodiment, the first section
4017 may
include a tapered profile, such that a conical surface is formed. The second
section 4019
may include a profile that corresponds to the profile of the ball 4009. As the
ball 4009 is
dropped or as it moves downward within the downhole isolation tool when a
differential
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pressure is applied from above the tool, the first section 4017 may help
center or guide
the ball 4009 into the seat and into contact with the second section 4019.
[0034] As shown in Figures 6A and 6B, the seat 224A-B, 324A-D of a
downhole
isolation tool in accordance with embodiments disclosed herein, may include a
seating
surface 5015 having a profile. As shown, the profile of the seating surface
5015
substantially corresponds to the profile of the ball 5009. In particular, the
profile of the
seating surface 5015 includes a plurality of discrete sections 5015a, 5015b,
5015c, 5015d
that collectively form a continuous profile to correspond to the profile of
the ball 5009.
In some embodiments, the profile of the seating surface 5015 may include 2, 3,
4, 5, or
more discrete sections. The discrete sections may be linear or arcuate. For
example, in
one embodiment, each discrete section has a radius of curvature different from
each other
discrete section. Alternatively, each discrete section may have the same
radius of
curvature, but the radius of curvature of each discrete section is smaller
than the radius of
curvature of the ball 5009. In another example, each discrete section may be
linear and
may include an angle with respect to the central axis of the mandrel 5007 or
ball seat
224A-B, 324A-D different from the angle of each other discrete section. An
average of
the overall profile of the seating surface 5015 provides a profile that
substantially
corresponds to the profile of the ball 5009.
[0035] In one embodiment, the seat 224A-B, 324A-D may include a first
section 5017
and a second section 5019, as shown in Figure 6A. The first section 5017 is
disposed
axially above the second section 5019. In this embodiment, the first section
5017 may
include a tapered profile, such that a conical surface is formed. The second
section 5019
may include a profile that substantially corresponds to the profile of the
ball 5009. As the
ball 5009 is dropped or as it moves downward within the downhole isolation
tool when a
differential pressure is applied from above the tool, the first section 5017
may help center
or guide the ball 5009 into the seat and into contact with the second section
5019.
[0036] Referring to Figures 5A-B and 6A-B, the geometry (i.e., profile) of
the seat 224A-
B, 324A-D provides sufficient contact between the ball 4009, 5009 and the seat
224A-B,
324A-D to effect a seal. An increasing load on the ball due to the
differential pressure
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may deform the ball 4009, 5009 slightly into the ball seat 224A-B, 324A-D,
thereby
enhancing the seal. Because the radial clearance between the outside diameter
of the ball
4009, 5009, and the seat 224A-B, 324A-D is small, in some embodiments, the
ball 4009,
5009 may only need to deform a small amount to provide full contact with the
seating
surface 4015, 5015 of the ball seat 224A-B, 324A-D.
[0037] The profile of the seating surface 4015, 5015 as described above
allows for a
larger contact surface between the seated ball 4009, 5009, and the seating
surface 4015,
5015. This contact surface provides additional bearing area for the ball 4009,
5009,
thereby preventing failure of the ball material due to compressive stresses
that exceed the
maximum allowable compressive stress of the material. If the differential
pressure is
increased, the ball 4009, 5009 may deform and contact the ball seat 224A-B,
324A-D as
described above for additional bearing support by the seat 224A-B, 324A-D. Due
to the
small radial clearance between the ball 4009, 5009 and the seating profile
4015, 5015, the
deformation of the ball 4009, 5009 may be minimal.
[0038] Referring back to Figures 3A and 3B, ball seat mandrel 218 may also
include a
notch, groove, or other opening configured to be engaged with an assembly
tool.
Specifically, one or more notches 334 may be formed in the upper face 226 of
the ball
seat mandrel 218 to allow an assembly tool to engage the ball seat mandrel 218
and
assemble the ball seat mandrel 218 in the sleeve 208 (Figures 2A and 2B). For
example,
in one embodiment, an assembly tool (not shown) may engage the notch 334 and
be
rotated to engage threads on an outer surface of the ball seat mandrel 218 and
threads on
an inner surface of the sleeve 208. One of ordinary skill in the art will
appreciate that
various assembly tools may be used and various means for coupling the ball
seat mandrel
218 to the sleeve 208 may be used as known in the art.
[0039] Referring now to Figure 7, a cross-sectional view of a ball seat
mandrel 518 is
shown in accordance with embodiments disclosed herein. As shown, the ball seat
mandrel 518 includes at least two ball seats 524A, 524B disposed on a
contoured upper
face 526. In this embodiment, a lower end 515 of the ball seat mandrel 518
includes a
cavity 536. Cavity 536 is formed within the lower end 514 of the ball seat
mandrel 518
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so as to provide a cylindrical lower section of the ball seat mandrel 518
having an outer
diameter D1 and an inner diameter D2. Thus, a ball sat mandrel 518 formed in
accordance with the embodiment shown in Figure 6 may include two or more
throughbores (Figure 6 shows one of these throughbores 528A) having an axial
length
less than a throughbore formed in accordance with embodiments shown in Figures
3 and
4. Such a cavity 536 may reduce the total volume of material to be drilled up
once the
fracturing treatment or other job has been completed. As such, the time it
takes to
remove the downhole isolation tool may be reduced.
[0040] In some wells, multiple zones may need to be isolated in a well. In
such an
application, multiple downhole isolation tools may be run into the well to
isolate each
section of the well. Specifically, a system of multiple downhole isolation
tools may be
run into the well so as to provide fracturing of each isolated section and to
allow
production of fluids from each of the zones. In one embodiment, two or more
downhole
isolation tools may be run into the well. Because the tools are run in series,
i.e., one
downhole isolation tool is disposed axially downward of a second downhole
isolation
tool, a series of different sized balls may be used to seat or seal within
each tool.
Specifically, smaller balls are used to seat against a first downhole
isolation tool than the
balls used to seat against a downhole isolation tool positioned axially above
the first
downhole isolation tool. Different sized balls are used such that the balls
used to seat
against the first downhole isolation tool (i.e., the lower tool) are small
enough to safely
pass through the downhole isolation tools disposed above the first downhole
isolation
tool as the balls are run within a fluid downhole to be seated. Similarly,
once production
of fluids from below is resumed, the balls need to be small enough to safely
pass upward
through downhole isolation tools positioned above the tool with the seated
ball to allow
the balls to be removed from the system with the production fluid.
[0041] Accordingly, in one embodiment, a downhole isolation system may
include two
or more downhole isolation tools in accordance with the present disclosure.
Specifically,
a first downhole isolation tool may be similar to that described above with
respect to
Figures 2A, 2B, 4A and 4B. The first downhole isolation tool, i.e., the
lowermost
downhole isolation tool, is configured to receive and seat the smallest ball
of a series of
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balls to be used with downhole isolation system. Thus, in this example, the
first
downhole isolation tool may include a ball seat mandrel 318 that includes four
ball seats
324A, 324B, 324C, 324D and four corresponding throughbores (only two shown in
this
view) 328A, 328B, as shown and described with respect to Figures 4A and 4B.
The four
ball seats may be equally spaced about the inner perimeter of the ball seat
mandrel 318
and may maximize the fluid flow area through the ball seat mandrel 318 when a
ball is
not seated in one or more of the ball seats 324A, 324B, 324C, 324D.
[0042] A second downhole isolation tool may be run above the first
downhole isolation
tool. The second downhole isolation tool is configured to allow passage of the
dropped
balls to the first downhole isolation tool or from the first downhole
isolation tool to the
surface during production of fluids from lower zones. Thus, the second
downhole
isolation tool is configured to receive and seat a ball having a size (i.e.,
diameter) larger
than the balls used to seat against the first downhole isolation tool. As
such, in one
embodiment, the second downhole isolation tool, as shown in Figures 2A and 2B,
may be
used having a ball seat mandrel 218 as shown in Figures 3A and 3B.
Specifically, the
second downhole isolation tool may include a ball seat mandrel 218 having two
ball seats
224A, 224B axially aligned with two corresponding throughbores 228A, 228B. The
ball
seats 224A, 224B may be equally spaced about the inner perimeter of the ball
seat
mandrel 318 and may maximize the fluid flow area through the ball seat mandrel
218
when a ball is not seated in one or more of the ball seats 224A, 224B. Thus,
the size (i.e.,
diameter) of each ball seat 224A, 224B of the second downhole isolation tool
is larger
than the size (i.e., diameter) of each ball seat 324A, 324B, 324C, 324D of the
first
downhole tool.
[0043] In other embodiments, additional downhole isolation tools may be
run with the
first and second downhole isolation tools described above, such that each
lower
positioned downhole isolation tool is configured to receive and seat a smaller
ball than
the downhole isolation tools positioned above. In one example, a third
downhole
isolation tool having a ball seat mandrel 718 having three ball seats 724A,
724B, 724C
and three axially aligned corresponding throughbores (not shown), as shown in
Figures
8A and 8B, may be positioned above the first downhole isolation tool and below
the
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second dowhole isolation tool. As such, each ball seat 724A, 724B, 724C of the
third
downhole isolation valve is larger than each ball seat 324A, 324B, 324C, 324D
of the
first downhole isolation tool, but smaller than each ball seat 224A, 224B of
the second
downhole isolation tool. While in this example, the number of ball seats
decreases from
the lowermost tool to the uppermost tool, one of ordinary skill in the art
will appreciate
that the number of ball seats of each downhole isolation tool may be the same,
but the
size (i.e., diameter) of the ball seats increases from the lowermost downhole
tool to the
uppermost downhole tool. In still other embodiments, downhole isolation tools
having at
least two ball seats as described herein may be run with downhole isolation
tools having
only one ball seat and one corresponding throughbore. In such a system, the
downhole
isolation tool having one ball seat may include a ball seat mandrel with a
contoured upper
face as described herein, and the size of the ball seat may be sized based on
the axial
position of the downhole isolation tool with one seat with respect to other
downhole
isolation tools with two or more ball seats when run in hole.
[0044] A method of running a downhole isolation system as described herein
and a
method of isolating a well with a downhole isolation system as described
herein is now
discussed. A method of isolating a well in accordance with embodiments
disclosed
herein includes running a downhole isolation system into a well, the downhole
isolation
system including a first downhole isolation tool. The first downhole isolation
tool
includes a first sub, a first sleeve disposed in the sub, and a first ball
seat mandrel coupled
to the first sleeve, the first ball seat mandrel including at least two ball
seats of a first size
axially aligned with at least two throughbores disposed within the first ball
seat mandrel.
When the zones above and below the downhole isolation tool need to be
isolated, e.g., so
hydraulic fracturing of the zone above the downhole isolation tool may be
performed, at
least two balls of a first size are dropped into the well. The balls may be
placed in a fluid
that is pumped down through the string into the well. When the balls reach the
first
downhole isolation tool, each ball moves into a ball seat of the isolation
tool. The
contour of the face of the ball seat mandrel, as well as the pressure of the
fluid flow, help
position the balls in the ball seats. Pressure from above the first downhole
isolation tool,
i.e., fluid pressure, against the seated balls effects a seal across the
inside diameter of the
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downhole isolation tool, thereby isolating the zone(s) below the tool from the
zone(s)
above the tool. Once such seal is effected, other processes may be performed,
for
example, hydraulic fracturing of the formation or cased well, as discussed
above.
[0045] Additional zones may be isolated in a downhole isolation system
having two or
more downhole isolation tools. In this embodiment, a second downhole isolation
tool is
run into the well above the first downhole isolation tool. The second downhole
isolation
tool includes a second sub, a second sleeve disposed in the sub, and a second
ball seat
mandrel coupled to the second sleeve. The second ball seat mandrel includes at
least two
ball seats of a second size axially aligned with at least two throughbores
disposed within
the second ball seat mandrel. When the zones above and below the second
downhole
isolation tool need to be isolated, e.g., so hydraulic fracturing of the zone
above the
downhole isolation tool may be performed, at least two balls of a second size
are dropped
into the well. The balls may be placed in a fluid that is pumped down through
the string
into the well. When the balls reach the second downhole isolation tool, each
ball moves
into a ball seat of the second downhole isolation tool. The contour of the
face of the ball
seat mandrel, as well as the pressure of the fluid flow, help position the
balls in the ball
seats. Pressure from above the first downhole isolation tool, i.e., fluid
pressure, against
the seated balls effects a seal across the inside diameter of the downhole
isolation tool,
thereby isolating the zone(s) below the tool from the zone(s) above the tool.
Once such
seal is effected, other processes may be performed, for example, hydraulic
fracturing of
the formation or cased well, as discussed above.
[0046] Balls of varying sizes may be used to seat in and seal different
downhole isolation
tools of a downhole isolation system. Balls of a first size are dropped to
seat against the
first downhole isolation tool. The ball of a first size are smaller than the
balls of a second
size, which are dropped to seat against the second downhole isolation tool
positioned
axially above the first downhole isolation tool. The balls of a first size are
small enough
to fit safely through (i.e., without plugging or sealing) the ball seats of
the second
downhole isolation tool, but small enough to seat against the ball seats of
the first
downhole isolation tool and to effect a seal. The balls of a second size are
larger than the
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ball seats of the second downhole isolation tool, so as to seat against and
seal the second
downhole isolation tool.
[0047] Once the additional processes have been completed, production of
lower zones
may be initiated or resumed. Referring back to Figures 2A and 2B, production
of lower
zones may be initiated or resumed by removing the seal effected by the balls
seated in the
ball seat. To do this, a pressure differential across the ball seat mandrel
218 is applied by
increasing the fluid pressure acting on the upper face 226 of the ball seat
mandrel 218
having balls (not shown) seated within each ball seat (not shown). The
pressure above
the ball seat mandrel 218 is increased above a predetermined value that
corresponds to a
maximum rating of shearing device 212 that couples the sleeve 208 to the sub
202. Once
the predetermined value is exceeded, the shearing device 212 is sheared,
thereby allowing
the sleeve 208 to move axially downward until a lower end 214 of the sleeve
208 contacts
an internal shoulder 216 in the sub 202. Because the ball seat mandrel 218 is
coupled to
the sleeve 208, the ball seat mandrel 218 moves axially downward with the
sleeve 208.
The sleeve 208 moves axially downward a distance sufficient to open one or
more ports
221 of the sub 202. Once the ports 221 are open, i.e., the sleeve 208 has
moved
downward and no longer blocks the ports 221, fluid flow from above the
downhole
isolation tool may flow into the annulus (not shown) formed between the
outside
diameter of the sub 202 and the well, casing, or other downhole tools.
Production of
fluids from zones below the downhole isolation tool will lift the balls seated
in the ball
seats and carry the balls to the surface. Because the ball seats and
corresponding
throughbores of higher positioned downhole isolation tools have larger
diameters than the
balls dropped for lower downhole isolation tools, as discussed above, the
balls may be
carried by a produced fluid up through other downhole isolation tools and
returned to the
surface.
[0048] Embodiments described herein advantageously provide downhole
isolation tools
having large equivalent throughbores by using multiple ball seats and multiple
balls to
effect a seal across each downhole isolation tool. A downhole isolation system
in
accordance with the present disclosure advantageously allows for multiple
distinct zones
to be isolated, fractured, and produced, but reduces the amount of pumping
horsepower
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needed. Specifically, because the fluid flow area through each downhole
isolation system
is maximized with the use of multiple ball seats, the pressure drop across a
ball seat of a
downhole isolation tool in accordance with embodiments disclosed herein may be
as low
as 600 psi, or lower, as compared to the 1000 psi differential of conventional
ball seats.
Thus, a lower pumping horsepower is required to isolate the tool and shift the
sleeve of
the tool to open ports to the annulus. Decreasing the required pumping
horsepower may
advantageously reduce the over all cost of a fracturing job.
[0049] Additionally, some embodiments may advantageously provide a ball
seat mandrel
having a cavity disposed within a lower end of the mandrel. Such cavity may
provide
easier drilling of the ball seat mandrel to remove the ball seat mandrel from
the well. As
such, embodiments disclosed herein may provide a shorter drill time for
removal of a ball
seat mandrel.
[0050] While the invention has been described with respect to a limited
number of
embodiments, those skilled in the art, having benefit of this disclosure, will
appreciate
that other embodiments can be devised which do not depart from the scope of
the
invention as disclosed herein. Accordingly, the scope of the invention should
be limited
only by the attached claims.
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