Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DOWNHOLE DEGRADABLE STAGING TOOL
BACKGROUND
1. Technical Field
This disclosure relates generally to downhole tools and in particular to a
method and apparatus for selectively opening a passage through a downhole
casing.
2. Description of Related Art
In hydrocarbon production, wells frequently include a cemented in place liner
within the well bore. Such cement is located between the wellbore wall and
the liner after being pumped down the interior of the liner. Due to the long
lengths of such wellbores, it is frequently necessary to pass the cement
through the liner in stages at varying locations along the wellbore.
Accordingly, various methods have been developed to selectively open
passages through the liner to permit greater control of such cementing
operations.
In particular, one common method is to provide a selectably openable or
frangible port through the liner at each desired stage. When that stage is
desired to be opened to flow concrete therethrough, a ball or other blocking
body is dropped to below that stage to increase the pressure at that stage.
The increased pressure will cause the opening to rupture or a sleeve to
uncover the port whereupon the concrete flowing down the interior of the liner
is then passed through the port into the annulus between the liner and the
wellbore. After that stage has been completed, the ball may be milled out of
the liner.
Such methods commonly suffer from several difficulties. In particular, the
ball
for opening such a stage is typically located a distance downstream of the
opening through the liner. This permits a quantity of concrete to form above
the ball that is not moving and therefore subject to curing in place. The
pressure within this region may also increase substantially due to such
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accumulation resulting in some concrete flowing past the ball into the region
below the stage which is undesirable. Furthermore, it will be appreciated that
the process to mill or bore out the ball after the stage or multiple stages
have
been completed is time consuming and difficult.
SUMMARY OF THE DISCLOSURE
According to a first embodiment, there is disclosed an apparatus for
selectively
opening a passage through a downhole casing comprising an elongate
cylindrical housing defining a central bore therein and having at least one
passage extending therethrough between the central bore and an exterior of
the housing, the housing being configured to be located in line within a
downhole casing, a ball seat slidably located within the central bore so as to
be operable to selectively cover or uncover the at least one passage and a
sealing sleeve slidably located within the central bore between an open and
closed position wherein the sealing sleeve valve covers the at least one
passage in the closed position and uncovers the at least one passage in the
open position.
The apparatus may further comprise an intermediate sleeve adapted to cover
the at least one passage at a run in configuration. The open position of the
sealing sleeve may be upstream of the closed position.
The sealing sleeve may include at least one collet arm extending upwardly
therefrom, the at least one collet arm. The ball seat may be connected to and
operable to displace the release sleeve.
The sealing sleeve may include a shoulder adapted to receive a dart thereon.
Displacement of the sealing sleeve with the dart moves the sealing sleeve
from the open position to the closed position.
The ball seat may be slidable to uncover a transfer passage through the
sealing sleeve. The apparatus may further comprise a release sleeve
adapted to retain the sealing sleeve at an open position.
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The release sleeve includes a shoulder adapted to receive a dart thereon.
Displacement of the release sleeve disengages at least one collet arm on the
sealing sleeve permitting movement of the sealing sleeve from the open
position to the closed position. The ball, ball seat and dart may be formed of
a dissolvable material.
Other aspects and features of the present disclosure will become apparent to
those ordinarily skilled in the art upon review of the following description
of
specific embodiments in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings constitute part of the disclosure. Each drawing
illustrates exemplary aspects wherein similar characters of reference denote
corresponding parts in each view,
Figure 1 is an illustration of a hydrocarbon well having at least
one
degradable staging tool therein.
Figure 2 is a perspective view of a staging tool for use in the
hydrocarbon
well of Figure 1.
Figure 3 is a cross sectional view of the staging tool of Figure 2 at a
first or
run in configuration.
Figure 4 is a cross sectional view of the staging tool of Figure 2
with a ball
dropped onto the seat thereof.
Figure 6 is a cross sectional view of the staging tool of Figure 2
at a second
or open configuration.
Figure 6 is a cross sectional view of the staging tool of Figure 2
at a third or
closed configuration.
Figure 7 is a cross sectional view of a staging tool for use in the
hydrocarbon well of Figure 1 according to a further embodiment of
the present disclosure at a first or run in configuration.
Figure 8 is a cross sectional view of the staging tool of Figure 7
at a second
or open configuration.
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Figure 9 is a cross sectional view of the staging tool of Figure 7
with a ball
dropped onto the seat thereof.
Figure 10 is a cross sectional view of the staging tool of Figure 7
at a second
or open position.
Figure 11 is a cross sectional view of the staging tool of Figure 7 with a
dart
dropped on to the release sleeve.
Figure 12 is a cross sectional view of the staging tool of Figure 7
with the
dart and release sleeve shifted to a third configuration.
Figure 13 is a cross sectional view of the staging tool of Figure 7
at a closed
or fourth configuration.
Figure 14 is a cross sectional view of a staging tool for use in the
hydrocarbon well of Figure 1 according to a further embodiment of
the present disclosure at a first or run in configuration.
Figure 16 is a cross sectional view of the staging tool of Figure 14
at a
second position.
Figure 16 is a cross sectional view of the stating tool of Figure 14
at a third
position.
Figure 17 is a cross sectional view of the stating tool of Figure 14
at a fourth
or closed position.
Figure 18 is a cross sectional view of a dart according to a further
embodiment.
Figure 19 is a detailed cross sectional view of the dart of Figure 18
engaged
with a ball and ball seat.
Figure 20 is a detailed cross sectional view of a ball seat for use
in an
apparatus according to the present disclosure.
Figure 21 is a detailed perspective view of the ball seat of Figure
14
engaged with an deformed by the sliding sleeve.
DETAILED DESCRIPTION
Aspects of the present disclosure are now described with reference to
exemplary apparatuses, methods and systems. Referring to Figure 1, a
wellbore 10 is drilled into the ground to a production zone 16 by known
methods. The production zone 16 may contain a horizontally extending
hydrocarbon
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bearing rock formation or may span a plurality of hydrocarbon bearing rock
formations such that the wellbore 10 has a path designed to cross or intersect
each formation. As illustrated in Figure 1, the wellbore may include a
vertical
section 12 and a bottom or production section 14 which may be horizontal or
angularly oriented relative to the horizontal located within the production
zone
6. Optionally, a casing 18 may be located within the wellbore as are
commonly known. As utilized herein, all references to the wellbore in which
the present apparatus and tool are pumped down shall be taken to mean both
the wellbore formed in the surrounding rock as well as the passage formed by
the casing as located within the rock wellbore. In order to pass concrete or
other products into the annulus between the casing 18 and the wellbore, they
are frequently passed through the casing. As illustrated in Figure 1, an
exemplary apparatus for forming selectably openable passages through the
casing according to a first embodiment is generally indicated at 20 in line
within the casing 18.
With reference to Figures 3-6, the apparatus 20 comprises an outer casing 22
extending between first and second ends, 24 and 26, respectively and having
a central bore 28 extending therethrough between the first and second ends.
The outer casing 22 includes radial bores 30 extending therethrough and may
optionally include first and second end subs, 25 and 27, respectively at the
first and second ends for connection sections of casing 18. The casing
includes a shifting sleeve 40 having an associated ball seat 60 thereon which
is slidably displaceable within the central bore 28. The shifting sleeve 40
includes openings 46 therethrough adapted to be selectably aligned with the
radial bores 30 in the outer casing 22 when the shifting sleeve is displaced
to
a downward position. As illustrated, the shifting sleeve 40 may include a
wider portion 48 proximate to the second end 44 and a narrower portion 50
proximate to the first end. The narrow portion 50 is sized to be received
within an annular pocket 52 formed between the outer casing 22 and a
sealing sleeve 70 as will be more fully described below. The openings 46
may pass through the narrow portion 50 so as to prevent fouling or any
material entering such passages during run in operations. The shifting sleeve
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40 may be secured to the outer casing with one or more shear pings 54 sized
to be sheared at a predetermined pressure.
The apparatus further includes a ball seat 60 secured within the shifting
sleeve 40. The ball seat may be sealably secured within the shifting sleeve
40 and includes a ball seat surface 62 oriented towards the first end 24 of
the
apparatus. The ball seat surface 62 may have a radius selected to
correspond to a ball 66 intended to be dropped thereonto.
The apparatus 20 further includes a sealing sleeve 70 located concentrically
within the narrow portion 50 of the shifting sleeve 40 so as to contain the
narrow portion 50 between the sealing sleeve 70 and the outer casing 22.
The sealing sleeve 70 extends between first and second ends, 72 and 74,
respectively and may be secured to the outer casing with at least one shear
pin 77. The sealing sleeve 70 includes an annular shoulder 76 extending
radially into a passage through the sealing sleeve 70. The annular shoulder
may be formed of a separate ring secured to the interior bore of the sealing
sleeve or may optionally be co-formed therewith.
The apparatus 20 may be provided with a ball 66 as set out above having a
radius corresponding to the ball seat surface 62 and an optional closing dart
80. The closing dart 80 comprises a cylindrical body having a size selected to
fit within the central bore 28 and within the central passage of the sealing
sleeve 70. The dart 80 may optionally include a wiping seal 82 therearound
adapted to seal the gap between the dart and the central bore 28. The dart
80 extends between top and bottom ends, 84 and 86, respectively wherein
the bottom end includes a profile shaped to correspond to the profile of the
ball seat 60 with a ball 66 therein. In particular, the bottom end 86 includes
a
central spherical recess 88 having a radius similar to the radius of the ball
6
and a conical portion 90 therearound corresponding to the angle of top
surface of the ball seat. In such a manner, the dart will closely engage upon
the ball seat 60 and ball 6 at as further set out below to minimize locations
for
concrete or other materials to collect. The dart 80 includes an annular ridge
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92 extending therearound having a shoulder engaging surface 94 adapted to
be received upon and engage with the annular shoulder 76 of the sliding
sleeve.
In operation, the apparatus 20 may be located inline within a casing 18 at one
or more locations in the wellbore. As illustrated in Figure 3, at the first or
run-
in position, both the shifting sleeve 40 and the sealing sleeve are retracted
towards the first end 24 of the apparatus 20 and retained in such position by
shear pins 54 and 77. Furthermore, as illustrated in Figure 3, at the first
position, the openings 46 of the shifting sleeve 40 are misaligned with the
radial bores 28 such that the interior of the casing is isolated from the
annulus. When the openings is desired to be opened, such as to pass
concrete from the interior of the casing 18 into the annulus, a ball 66 is
dropped onto the ball seat thereby sealing the central bore 28 at that
location
as illustrated in Figure 4. Thereafter the pressure is increased within the
central bore above the pressure required to rupture the shear pins 54 thereby
permitting the shifting sleeve 40 to move in a direction towards the second
end 26 of the apparatus as illustrated in Figure 5. This first shear pressure
will vary depending upon the size and intended purpose of the apparatus but,
by way of non-limiting example, may be selected to be between 1000 and
5000 psi. At the downward or second position as illustrated in Figure 4, the
openings 46 are then aligned with the radial bores 30 so as to permit the flow
of concrete or other material from an the interior bore 28 into the annulus
between the casing 18 and the wellbore wall.
Once concrete pumping has completed, the dart 80 may be pumped down the
interior bore 28 until the shoulder engaging surface 94 of the dart 80 engaged
upon the annular shoulder 76 of the shifting sleeve. Thereafter, the pressure
within the central passage 28 may again be raised until a sufficient pressure
is
achieved to shear the shear pins 77 thereby releasing the sealing sleeve 70.
The sealing sleeve 70 and the dart 80 then shift towards the second end 26 of
the apparatus 20 until the spherical recess 88 surrounds the ball as
illustrated
in Figure 6. At such position, the close matching of the shape of the second
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end 86 of the dart 80 with the ball seat 60 and ball 66 ensures that a minimal
amount of concrete remains therebetweeen while any extra concrete is
pushed out through the openings 46 and 30. It will also be observed that at
the position illustrated in Figure 6, the sealing sleeve 70 covers the
openings
46 and 30 thereby sealing the central passage 28 from the well annulus.
Once the apparatus 20 has been resealed by the dart and sealing sleeve 70
the dart 80, wiper 82, ball seat 60 and ball 66 may be removed from the
central passage 28 by any known means. In particular, known techniques
such as milling may be utilized. It has been found to be particularly useful
to
form the ball 66, ball seat 60, dart 80 and wiper 82 from a dissolvable
material
so as to permit the pumping of a dissolving agent down the central passage
28 to dissolve the components thereby opening the central passage for
production or other operations. Such dissolvable materials may include
dissolvable magnesium alloys, dissolvable aluminium alloys, Parker A96 or
PGA poloymer by way of non-limiting example.
Turning now to Figures 7 through 13, an alternative embodiment of the
present apparatus is illustrated generally at 100. The apparatus comprises an
outer casing 102 extending between first and second ends, 104 and 106,
respectively and having a central bore 108 extending therethrough between
the first and second ends. The outer casing 102 may optionally include first
and second end subs, 110 and 112, respectively at the first and second ends
for connection sections of casing 102. The outer casing 102 includes radial
bores 114 extending therethrough which may optionally be covered by plugs
116 and/or a retaining band 118 therearound. As illustrated in Figure 7, the
interior wall of the outer casing 102 includes a release recess 120 at a
location towards the first end 104 from the radial bores 114 and a retaining
recess 122 towards the second end 106. The release recess may have an
angled entrance towards the radial bores 114 whereas the retaining recess
122 includes a radially oriented surface 124 the purpose of which will be more
fully described below.
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The casing includes a shifting sleeve 130 which is slidably displaceable
within
the central bore 108 having ball seat 150 slidably located within the shifting
sleeve 130. The shifting sleeve 130 includes openings 132 therethrough
adapted to be aligned with the radial bores 114 at the initial or run in
position
and selectably misaligned with the radial bores 114 in the outer casing 102
when the shifting sleeve is displaced to a downward position as will be more
fully described below. The shifting sleeve 130 extends between first and
second ends 134 and 136, respectively wherein the first end 134 includes at
least one flexible collet arm 138 extending longitudinally therefrom. Each
collet arm 138 includes a widened portion 140 at a distal end thereof
positioned to be received within the release recess 120 at the initial or run
in
position as illustrated in Figure 7.
The ball seat 150 is slidably located within the shifting sleeve 130. As
illustrated In Figure 7, the interior profile of the shifting sleeve may
include a
lip or raised portion 142 so as to prevent movement of the ball seat 150 out
of
the bottom of the shifting sleeve 130. The ball seat 150 includes a ball seat
surface 152 oriented towards the first end 104 of the apparatus with a
plurality
of bores 154 extending therethrough in radial alignment with the openings 132
in the shifting sleeve 130 such that the ball seat surface 152 surrounds the
bores 154. As set out above, the ball seat surface 154 may have a radius
selected to correspond to a ball 66 intended to be dropped thereonto. The
openings 154 may have a radius smaller than the openings 132 so as to
provide a lower flow rate when both openings are in operation. The interior
passage 108 of the apparatus further includes a release sleeve 160 extending
between first and second ends 162 and 164, respectively. The second end
164 includes an annular shoulder 166 which may be formed of a separate ring
secured to the interior bore of the sealing sleeve or may optionally be co-
formed therewith. The exterior surface 168 of the release sleeve 160 has a
diameter selected to bear against the widened portion 140 of the collet arms
138 so as to retain them within the release recesses 120. Proximate to the
first end 160 of the release sleeve, the exterior surface 168 includes a
radial
recess 170 such that when the recess sleeve is sidably displaced towards the
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second end 106, the widened portion 48 of the collet arms 138 will be
permitted to flex radially inward thereby releasing from the release recess
120
and permitting the shifting sleeve 130 to move towards the second end 106.
In operation, the apparatus 100 is located inline with the casing 18 and
inserted into the wellbore. At the run in position as illustrated in Figure 7,
the
shifting sleeve130, release sleeve 160 and ball seat 150 are all at a position
towards the first end 104 of the apparatus wherein the widened portions 140
of the collet arms 138 are retained within the release recess 120 by the
exterior surface 168 of the release sleeve 160. When an operator wishes to
open that particular zone, a ball (not shown) may be dropped through the
apparatus to seal off the casing after which the pressure may be increased
within the interior passage 108. The pressure is then increased to a
predetermined level selected to rupture or displace the retaining band 118
and/or plugs 116 and thereby open the radial bores 114 as illustrated in
Figure 8. Fluid from inside the interior passage 108 may then be permitted to
flow through the openings 154, 132 and 114 to the annulus of the wellbore.
The opening 154 may be selected to be smaller than the other passages so
as to provide a throttling to the fluid permitted to flow therethrough at this
position.
After opening the radial bores 114, a ball 66 may be dropped onto the ball
seat surface 154 as illustrated in Figure 9. This will then close the openings
154 as well as seal the interior passage 108 at that location. Further
pressure
increase within the interior passage 108 will then cause the ball 66 and ball
seat 150 to shift towards the second end 106 thereby uncovering the
openings 132 in the shifting sleeve 130 which are in alignment with the radial
bores 114 permitting a fluid such as concrete to flow therethrough. At such
position, the ball seat 150 will be only below the openings 132 so as to
uncover them thereby limiting any amount of concrete which could
accumulate and potentially leak past the ball 66.
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Once concrete pumping has completed, the dart 80 may be pumped down the
interior passage 108 until the shoulder engaging surface 94 of the dart 80
engage upon the annular shoulder 166 of the release sleeve 160. Thereafter,
the pressure within the central passage 108 may again be raised until a
sufficient pressure is achieved to displace the release sleeve 160 either by
overcoming the friction holding it in place or shearing shear pins, rings or
other frangible release devices provided to retain it at the initial position.
As
illustrated in Figure 12, once the release sleeve 160 is shifted towards the
second end, the exterior surface 168 is moved relative to the widened end
140 of the collet arms 138 so as to locate the widened portion 140 in the
radial recess 170 of the exterior surface as illustrated in Figure 12. At such
position, the exterior surface 168 no longer retains the widened portions 140
within the release recess 120 and the shifting sleeve 130 may also then be
displaced towards the second end 106 so as to misalign the openings 132
with the radial bores 114 thereby closing the apparatus as illustrated in
Figure
13. Optionally, the shifting sleeve 130, may include a snap ring 190 adapted
to engage within the retaining recess 122 so as to retain the shifting sleeve
in
the closed position. As illustrated in Figures 12 and 13, the dart may closely
match of the shape of the ball seat and the ball to ensure that a minimal
amount of concrete remains therebetweeen while any extra concrete is
pushed out through the openings 132 and 114.
As set out above, the apparatus 100 has been resealed by the dart and
shifting sleeve 130 the dart 80, wiper 82, ball seat 150 and ball 66 may be
removed from the central passage 28 by any known means. In particular,
known techniques such as milling may be utilized. Furthermore, as set out
above, the ball 66, ball seat 150, dart 80, wiper and optionally the annular
shoulder 166 may be formed of a dissolvable material so as to permit the
pumping of a dissolving agent down the central passage 108 to dissolve the
components thereby opening the central passage for production or other
operations.
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Turning now to Figures 14-16, a further embodiment is illustrated. In
particular, in some embodiments, the ball seat 200 may include an annular
cavity 202 around the outer surface thereof so as to provide a substantially
even thickness of a wall 204 for the ball seat 200. Such substantially even
ball seat wall 204 will enable the use of less material thereby allowing the
all
seat to dissolve quicker and more evenly. Furthermore, the shifting sleeve
220 may have a bottom portion 222 having a diameter offset to the inside of
the remainder thereof. The bottom portion may include annular teeth or
ridges 226 therearoud wherein the inside wall of the outer casing 102
similarly
includes interior annular teeth or ridges 204. An indexing ring 228 may
located therebetweeen having teeth corresponding to ridges 224 and 226. In
operation, each of the teeth or ridges may be angularly oriented to permit
movement toward the second end 106 while preventing movement towards
the top end 104 of the apparatus. Such angling will retain the shifting sleeve
220 from moving back towards the first end after being shifted downward as
set out below.
The bottom portion 222 of the sifting sleeve 220 is located around an interior
step in the outer casing 102 such that it is offset inwardly. Such an interior
radial offset creates a difference between the surface area of the shifting
sleeve located towards the first end 104 as compared to the end facing the
second end 106. Therefore under pressure within the central bore 108, the
shifting sleeve 220 will be displaced downward upon overcoming the rupture
pressure of one or more shear pins or the like. The interior surface of the
shifting sleeve also includes a tapered portion 230 oriented towards the ball
seat 200. The tapered portion 230 reduces the radius of the central passage
108 at that location so as to compress the ball seat 200 as it is forced
thereinto. Such compression will lock the ball seat 200 within the tapered
portion thereby preventing movement of the ball seat 200 back towards the
first end 104 as well as preventing rotation therebetweeen.
In operation, when the apparatus is desired to be opened, the pressure within
the central bore 108 may be increased to provide a pressure to the cavity 202
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and the openings 132 and the tapered portion as well as the bottom end of
the shifting sleeve. Due to the surface area differential between the top end
and the bottom end of the shifting sleeve 220, the shifting sleeve 220 will be
displaced towards the second and 106 upon reaching a sufficient pressure
thereby aligning the opening 132 with the radial bores 114 as illustrated in
Figure 16. As illustrated, bores 206 may extend between the central bore 108
and the cavity at any location which is at or below the ball seat surface 208.
In such position, fluids may then be passed from the central bore 108 to the
annulus around the apparatus through the bores 206, openings 132 and radial
bores 114. As set out above, when concrete is desired to be pumped through
that zone, a ball 66 may be dropped onto the ball seat surface 208 to seal the
passage at that location. Further pressure increase will then cause the ball
seat to be displaced towards the second end as illustrated in Figure 16
thereby uncovering the openings 132 and radial bores 114 to permit concrete
to pass therethrough. As illustrated in Figure 16, and in further detail in
Figure
21, the ball seat may include a tapered end foot portion 212. The end foot
portion 212 has a cylindrical outer surface and a tapering inner surface. As
illustrated in Figure 21, when the foot portion 212 encounters the tapered
portion 230 on the shifting sleeve 220, the foot portion will be compressed
inwardly and may optionally deform as illustrated. Such deformation and
compression will form a frictional engagement therebetvveeen thereby
preventing longitudinal and rotational movement therebetweeen. As set out
above, when concrete operations are completed, a dart 80 may be dropped
so as to engage an annular shoulder 166 on a release sleeve 160. Thereafter
movement of the release sleeve 160 permits inward movement of widened
portions 140 on the end of collet arms 138 thereby permitting the shifting
sleeve 220 and ball seat 200 to also be shifted to close the radial bores 114
as illustrated in Figure 17 at the final closed position.
The shifting sleeve 220 may also include one or more slots 1240 formed
longitudinally therearound. Bolts, pins or the like 242 may extend from the
outer casing 102 to be located therein. Such bolts will prevent rotation of
the
shifting sleeve 220 should milling me required to remove any components
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thereof. It will also be appreciated that other anti-rotation means may also
be
utilized such as longitudinal ridges, cross-sectional profiles or the like to
prevent rotation between the outer casing 102 and the shifting sleeve 220.
Optionally, as illustrated in Figure 18, the dart 80 may include at least one
deformable tip 89 extending longitudinally between the spherical recess 88
and the tapered portion 90. The deformable tip is selected to be substantially
similar to the outer surface of the ball 66 and a radisued entrance 214 to the
ball seat surface 208. In particular the tips 89 may have a substantial taper
so
as to be operable to be wedged between the ball sat 200 and the ball 66
thereby locking the dart 80 to the ball seat 200 and ball and preventing
rotation therebetweeen. Optionally, as illustrated in Figure 18, the dart 80
may include within an interior cavity, a quantity of a dissolving agent 96 as
are
known. In particular, the dissolving agent 96 may be a salt of an aqueous
dissolving fluid wherein the fluid is introduced to the fluid when the
dissolvable
components are desired to be dissolved. In particular, the salt 96 may be
contained within a container that is also dissolvable by the solvent for the
fluids. By way of non limiting example an aqueous based dissolving fluid may
be stored therein within a water soluble container such that the introduction
of
water to the well bore dissolves the container forming the dissolving fluid
for
dissolving the remaining components. A plug 98 may also be located in the
dart 80 to seal the salt 96 therein which may also be fluid soluble or
degradable. Optionally, the plug 98 may permit a selected amount of fluid to
leak therepast so as to provide a time delay to the dissolving.
Turning now to Figure 20, a further embodiment of a ball seat 250 is
illustrated. IN particular the ball seat 250 includes a cavity 252 therearound
as set out above and passages 256 extending between a ball seat surface
254 and the cavity. The ball seat 250 may furthermore include a coating layer
260 thereover that has a reduced dissolving rate than the material selected
for
the ball seat itself. In such a way the coating layer 260 may delay the
dissolving of the ball seat 250 until the coating layer has also been
dissolved.
Optionally, the coating layer 260 may be formed to be thinner or non-existent
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on one or more location of the ball seat 250, such as by way of non-limiting
example on the inside surface of the cavity as illustrated, so that any
dissolving of the ball seat will begin at that desired location. Optionally,
the
ball seat 250 or any other ball seat may include abbradable cover bodies 270
over one or more of the entrance, exit from or the ball seat surface itself.
The
cover bodies 270 may be formed of a material selected to be abraded by the
flow of fluid therepast such as drilling fluid which may include solid
particles
entrained therein and may be soft enough to be compressed or displaced by a
ball 66 dropped into place. Such cover bodies 270 will protect the ball seat
surface 254 itself from being thus abraded by being eroded in it's place.
While specific embodiments have been described and illustrated, such
embodiments should be considered illustrative only and not as limiting the
disclosure as construed in accordance with the accompanying claims.
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