Note: Descriptions are shown in the official language in which they were submitted.
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MOLDED FOAM PLUGS, PLUG SYSTEMS
AND METHODS OF USING SAME
BACKGROUND
The present invention relates generally to subterranean well construction, and
more
particularly to plugs, plug systems, and methods for using these plugs and
systems in
subterranean wells.
Cementing operations may be conducted in a subterranean formation for many
reasons. For instance, after (or, in some cases, during) the drilling of a
well bore within a
subterranean formation, pipe strings such as casings and liners are often
cemented in the well
bore. This usually occurs by pumping a cement composition into an annular
space between
the interior wall of the well bore and the exterior surface of the pipe string
disposed therein.
Generally, the cement composition is pumped down into the well bore through
the pipe
string, and up into the annular space. Prior to the placement of the cement
composition into
the well bore, the well bore is usually full of fluid, e.g., a drilling mud or
fluid. Oftentimes,
an apparatus known as a cementing plug may be employed and placed in the fluid
ahead of
the cement composition to separate the cement composition from the drilling
fluid as the
cement slurry is placed in the well bore. The cementing plug also wipes
drilling fluid from
the inner surface of the pipe string as it travels through the pipe string,
thereby preventing
contamination of the cement slurry by the drilling fluid as it is pumped
downhole. Once
placed in the annular space, the cement composition is permitted to set
therein, thereby
forming an annular sheath of hardened substantially impermeable cement therein
that
substantially supports and positions the pipe string in the well bore and
bonds the exterior
surface of the pipe string to the interior wall of the well bore.
In some circumstances, a pipe string will be placed within the well bore by a
process
comprising the attachment of the pipe string to a tool (often referred to as a
"casing hanger
and nm-in tool" or a "work string") which may be manipulated within the well
bore to
suspend the pipe string in a desired location. In addition to the pipe string,
a sub-surface
release cementing plug system comprising a plurality of cementing plugs may
also be
attached to the casing hanger and run-in tool. Such cementing plugs may be
selectively
released from the run-in tool at desired times during the cementing process.
Additionally, a
check valve, typically called a float valve, will be installed near the bottom
of the pipe string.
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The float valve may permit the flow of fluids through the bottom of the pipe
string into the
annulus, but not the reverse. A cementing plug will not pass through the float
valve.
When a first cementing plug (often called a "bottom plug") is deployed from a
sub-
surface release cementing plug system and arrives at the float valve, fluid
flow through the
float valve is stopped. Continued pumping results in a pressure increase in
the fluids in the
pipe string, which indicates that the leading edge of the cement composition
has reached the
float valve. Operations personnel then increase the pump pressure to rupture a
rupturable
member, within the bottom plug. Said rupturable member may be in the form of a
pressure
sensitive disc, rupturable elastomeric diaphragm, or detachable plug (stopper)
portion which
may or may not remain contained within the bottom plug. After the rupturable
member has
failed, the cement composition flows through the bottom plug, float valve and
into the
annulus. When the top plug contacts the bottom plug which had previously
contacted the
float valve, fluid flow is again interrupted, and the resulting pressure
increase indicates that
all of the cement composition has passed through the float valve. It is
important that all of
the desired cement composition be pumped into the annulus from the pipe
string. If not, the
cement remaining in the pipe string will have to be drilled out before any
further activities
can take place. Furthermore, the annulus might not be properly filled with
cement, and
undesirable formation-fluid migration or failure of the pipe string may
result. On the other
hand, if the cement is overdisplaced, a lower portion of the annulus might not
be properly
filled with cement, and undesirable formation-fluid migration or failure of
the pipe string
could result. Overdisplacement of the cement is considered a worse problem
than
underdisplacement, as it can be more difficult to correct.
Conventional cementing plugs are formed with wiper fins on their exterior
surface,
which function to wipe the pipe string as they travel downhole. Conventional
cementing
plugs used to wipe large diameter casing strings (18-5/8 and larger) are by
their very nature
expensive to make, both heavy and bulky to handle, and require additional time
to drill out
due to the sheer volume of drillable materials to be removed. Under some
conditions it may
be advantageous to the well operator to run casing strings consisting of two
or more pipe
sizes, with the larger pipe size being at the shallowest depth and
progressively tapering to the
minimum pipe size. These casing configurations are typically known as "tapered
strings" and
require specially designed cementing plugs to wipe the different pipe
diameters involved.
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Figure 1 illustrates a typical cementing plug used to wipe a tapered string
consisting
of two different pipe sizes. Especially when three more pipe sizes are to be
wiped, the plugs
become overly long to fit in conventional plug containers and due to the long,
flexible wiper
segments used to wipe the larger pipe sizes the plugs may be generally
unbalanced, or
unstable, such that they may not clean the pipe wall and separate fluids with
the desired
efficiency. As can be seen from this drawing, the cementing plug has a complex
design. In
particular, it is formed of two or more distinct body sections that have been
joined together
end to end. The front body section contains a set of wiper fins that project
radially outward a
first diameter. The rear body section contains a separate set of wiper fins
that project radially
outward a second diameter, which is greater than the first diameter. The wiper
fins of the
front body section operate to wipe the section of the pipe string having the
smallest diameter,
which is the section of the pipe string that penetrates deepest into the well
bore. The wiper
fins of the rear body section operate to wipe the sections) of the pipe string
having the
greatest diameter(s), which is the section of the pipe string that penetrates
into the shallow
section of the well bore. Both body sections are machined so that they can be
threadably
engaged. Accordingly, conventional cementing plugs are fairly complex devices
that are
relatively time-consuming and thus expensive to manufacture, difficult to use,
and are more
costly to drill out due to the increased plug length and/or material content.
In addition, cementing plugs may be required to pass through internal
restrictions
designed into special tools which may be incorporated into the pipe string,
such as the seats
in a plug operated multiple stage cementing device. The specially designed
cementing plugs
required to pass through these types of internal restrictions must both
effectively wipe the
casing ID and pass through the internal restrictions with minimal pressure
increase to avoid
prematurely activating the tool. In these instances, it is generally
impossible to place the
special devices in tapered strings unless the device is located in the largest
pipe size due to
the increased pressure that would otherwise be required to force the mass of
the larger wiper
segments through the restrictions.
Thus, there is a need for a new type of cementing plug capable of wiping
multiple
pipe diameters andlor easily passing through small internal restrictions that
is more efficient,
less costly, more user friendly, and be at least partially comprised of easily
removed
materials.
SUMMARY
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The present invention relates generally to subterranean well construction, and
more
particularly, to cementing plugs, plug systems, and methods for using these
plugs and
systems in subterranean wells.
In one embodiment, the present invention is directed to an improved plug for
separating successively introduced fluids into a wellbore, in particular a
casing string. The
plug comprises a cylindrically-shaped inner mandrel that has a hollow inner
passage through
which fluids may pass and an outer elastomeric foam sleeve secured thereto.
The outer
elastomeric foam sleeve may be generally cylindrically shaped or contain a
series of ribs or
similar geometric shapes forming its outer longitudinal profile. The plug has
a nose profile
formed at the lower end of the inner mandrel and an internal upset formed at
the upper end
having a bore larger than the m of the inner mandrel at that end. A recess is
also formed
within the nose profile. In one embodiment, a high pressure disc is secured
within the recess
formed in the upper end. In another embodiment, a rupturable member is secured
within the
recess formed in the upper end.
In another embodiment, the elastomeric foam outer body may be attached to a
solid
inner mandrel comprising a nose section designed to perform a specific
function, such as seat
and/or seal into a special profile such as a first stage baffle/baffle adapter
typically used in
multiple stage cementing operations using plug operated multiple stage
cementing devices.
As described below, the inner mandrel may take many different forms.
In another embodiment, the present invention is directed to a plug system for
separating fluids successively introduced into a passage. The plug system
comprises an
assembly of at least two plugs of the type described immediately above. In one
embodiment,
two plugs linearly aligned in a top and bottom configuration are provided. The
plugs are
placed into the casing string with the nose portions facing down. The top plug
is formed with
the high pressure dislc and the bottom plug is formed with the rupturable
member, which
mates with the nose portion of the top plug. The plug system according to the
present
invention may further comprise a float valve, which is designed to be
installed inside the
casing string at the end disposed at the bottom of the well bore. The recess
in the nose of the
inner mandrel of the bottom plug is fitted with a flat face seal ring that is
adapted to mate
with, and seal against, the top face of the float valve.
In yet another embodiment, the present invention is directed to a method of
separating
fluids successively introduced into a subterranean well bore. The method
comprises the steps
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of suspending an assembly comprising a plurality of plugs within a casing
string, wherein at
least one of the plugs comprises an inner mandrel and an outer elastomeric
foam sleeve
secured thereto; introducing a first fluid into the well bore through the
casing string;
introducing a second fluid into the well bore behind the first fluid such that
an interface
between the two fluids is formed; and deploying at least one plug within the
casing string at
the interface of the first and second fluids. The assembly preferably includes
a top and
bottom plug of the type described immediately above.
In one embodiment, the first fluid is a drilling fluid, the second fluid is a
cement
slurry and the bottom plug wipes the inside of the casing string clean of the
drilling fluid as it
travels down the casing string. The cement slurry is pumped downhole until the
bottom plug
lands against the uppermost float valve, the rupturable member fails at which
point the
cement slurry is displaced through the bottom plug, float valve(s), casing
shoe and into the
annulus formed between the casing string and the inside of the well bore. The
top plug is
deployed behind the cement slurry and wipes the inside of the casing string
clean of the
cement slurry as it travels downhole. Once the top plug engages the bottom
plug at the
bottom of the well bore the only cement left inside the casing is between the
upper float valve
and the casing shoe. The top plug prevents the cement slurry from being
overdisplaced.
After the cement slurry has cured, the top and bottom plugs and the float
valve may be drilled
out of the casing string, if required.
The features and advantages of the present invention will be readily apparent
to those
skilled in the art upon a reading of the description of the preferred
embodiments, which
follows.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the present disclosure and advantages thereof
may
be acquired by referring to the following description taken in conjunction
with the
accompanying drawing, wherein:
Figure 1 is an elongational cross-sectional view of a prior art cementing
plug.
Figure 2 is an elongational cross-sectional view of a top plug of a plug
system in
accordance with the present invention.
Figure 3 an elongational cross-sectional view of a bottom plug of a plug
system in
accordance with the present invention.
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Figure 4 illustrates a plug in accordance with the present invention as it
travels
downhole through the inside of a telescoping casing string.
Figure 5 illustrates the deformation of the outer foam sleeve of the plug of
Figure 4 as
it passes into a restriction in the casing string.
Figure 6 illustrates the engagement of the bottom plug of a two plug system
with a
float valve in accordance with the present invention.
Figure 7 illustrates engagement of the top plug with the bottom plug of the
two plug
system shown in Figure 6.
Figure 8 is an elongational cross-sectional view of an alternate embodiment of
a plug
in accordance with the present invention.
Figure 9 is another embodiment of a plug in accordance with the present
invention
similar to that of Figure 8, except that its outer sleeve is formed with ribs.
Figure 10 is an elongational cross-sectional view of another alternate
embodiment of a
plug in accordance with the present invention.
Figure 11 is another embodiment of a plug in accordance with the present
invention
similar to that of Figure 10, except that its outer sleeve is formed with
ribs.
Figure 12 is an elongational cross-sectional view of an alternate embodiment
of a plug
system in accordance with the present invention.
Figure 13 is another embodiment of a plug system in accordance with the
present
invention similar to that of Figure 12, except that the outer sleeves of the
plugs are formed
with ribs.
Figure 14 is an elongational cross-sectional view of an yet another alternate
embodiment of a plug system in accordance with the present invention.
Figure 15 is another embodiment of a plug system in accordance with the
present
invention similar to that of Figure 14, except that the outer sleeves of the
plugs are formed
with ribs.
DESCRIPTION
The details of the present invention will now be described with reference to
the
figures. Turning to Figure 2, a plug in accordance with the present invention
is shown
generally by reference numeral 10. The plug 10 is formed of two basic
components, an inner
mandrel 12 and an outer sleeve 14.
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The inner mandrel 12 is preferably generally cylindrical in shape and formed
of a
suitable drillable material such as, but not limited to, aluminum, high
strength plastic, glass
fiber composite, and other materials having similar properties. The inner
mandrel 12
preferably has a hollow cylindrically-shaped interior region through which
drilling muds and
fluids, cementing slurries and the like may flow uninhibited. The inner
mandrel 12 could
also be made of a solid elastomeric material. The inner mandrel 12 also has a
nose portion 16
formed at one of its ends. Preferably, the combined length of the inner
mandrel 12 and nose
portion 16 has a minimum length equal to the largest pipe OD that the plug is
designed to
wipe. This length-to-pipe diameter relationship is necessary to prevent the
plugs from
inverting in the largest pipe diameter. The foam outer body may or may not
encompass the
entire mandrel length.
The nose 16 may be disc-shaped. In one preferred certain embodiment, the nose
16 is
removably attached to the inner mandrel 12 so that different noses may be
attached for
different applications. There is a recess 18 formed within the lower end of
the inner mandrel
12. An elastomeric flat face seal ring 19 is disposed within the recess 18,
which is adapted to
mate with the top end of another plug andlor the face of a float valve or
other similar device.
In an alternate embodiment, the entire nose 16 may be formed of rubber.
The inner mandrel 12 also has a recess 20 formed in the upper end. The recess
20 is
adapted to receive either a high pressure disc 22, as shown in Figure 2, or a
rupturable
member such as a rupture disc 24, as shown in Figure 3. As those of ordinary
skill in the art
will recognize, there are many other ways in which the discs 22 and 24 can be
attached to the
upper end of the inner mandrel 12. As but one example, they could be formed
into a cap that
slips over and seals on the end of the mandrel.. As those of ordinary skill in
the art will
further recognize, the overall size of the plug 10 and thus the diameter of
the inner mandrel
12 will be a function of the diameter of the pipe into which it will be
inserted as well as the
particular application that the plug will be used in.
The high pressure disc 22 is designed to withstand high pressures and
preferably will
resist failing when exposed to pressures up to approximately 9,000 psi. Of
course, higher
pressure devices may be used in its place. In one preferred application, the
high pressure disc
22 is used to bridge the top plug mandrel m with a high pressure bridge. The
entire top plug
stops fluid flow upon landing upon a suitable seat. The high pressure disc 22
can either be
cemented into the recess 20 using a two-part epoxy or other similar adhesive
known in the
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art; integrally formed into the body of the inner mandrel 12; or otherwise
secured to the inner
mandrel by means known in the art.
The rupture disc 24 is designed to fail at a predetermined pressure, e.g.;
approximately 750 psi and above. Of course, the rupture disc 24 can be
designed to fail at
any desired pressure. The rupture disc 24 can either be cemented into the
recess 20 using a
two-part epoxy or other similar adhesive known in the art; integrally formed
into the body of
the inner mandrel 12; or otherwise secured to the inner mandrel by means known
in the art.
The outer sleeve 14 is preferably generally cylindrical or rib shaped and
formed of a
compressible elastomeric foam. It is preferably formed of an open cell
polyurethane foam.
However, as those of ordinary skill in the art will appreciate, other foamable
elastomers may
be used instead. The outer sleeve 14 is coaxially disposed around the outer
surface of the
inner mandrel 12. The outer sleeve 14 is preferably molded to the inner
mandrel 12 by any
suitable molding process, such as injection molding, cold pour molding, or
other similar
known process.
Figures 8 - 11 illustrate additional embodiments of the plug 10 in accordance
with the
present invention. Figure 8 illustrates an alternate embodiment of the plug
(210), wherein the
imier mandrel 212 is formed of a solid material, i.e., it is not formed with
an internal flow
passage. Furthermore, in this embodiment the nose 216 is integrally formed
with the inner
mandrel 212. The outer sleeve_214 is similar to outer sleeve 14 in that it is
preferably formed
of a foam or similar material and generally cylindrical in shape. Figure 9
illustrates an
alternate embodiment of the plug 210' shown in Figure 8. The only difference
in this
embodiment is that the outer sleeve 214' contains a series of ribs 221.
Figure 10 illustrates an alternate embodiment of the plug (310), wherein the
inner
mandrel 312 is formed of a tubular member having a nose 316 integrally formed
within its
lower end and a recess 320 formed within its upper end. A high pressure disc
or rupturable
member such as a rupture disc (not shown) can be inserted within the recess
320, as described
above. The outer sleeve 314 is similar to outer sleeve 14 in that it is
preferably formed of a
foam or similar material and generally cylindrical in shape. Figure 11
illustrates an alternate
embodiment of the plug 310' shown in Figure 10. The only difference in this
embodiment is
that the outer sleeve 314' contains a series of ribs 321.
Referring now to Figures 4 and 5, the flow of the plug 10 inside of a casing
string 50
is illustrated. In particular, Figure 4 shows plug 10 traveling through the
large diameter
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section 52 of the casing string 50. As can be seen in that figure, the outer
foam sleeve 14 is
sized so as to come into contact with the inside surface of the large diameter
section 52 of the
casing string 50. Indeed, the outer foam sleeve 14 of plug 10 wipes the inside
surface of the
large diameter section 52 of the casing string 50 clean as it travels through
that section of the
casing string. Figure 5 illustrates how the outer foam sleeve 14 of plug 10
conforms to the
inner surface of the casing string 50 as it encounters a restriction 54, which
is where the
inside of the casing string narrows in its diameter. The compressible nature
of the foam
forming the outer sleeve 14 enables the plug 10 to conform to the changing
diameter within
the casing string 50. Complex devices of the type shown in Figure 1, which
employ multi-
diameter fins, were previously needed to wipe clean the inner surface of a
multi-diameter
casing string.
A simple plug system in accordance with the present invention is shown
generally in
Figure 6 by reference numeral 100. It comprises plug 110, which is a plug of
the type shown
in Figure 3, i. e., a plug that has a rupture disc 24 at the upper end.
However, as those of
ordinary skill in the art will recognize, any of the above described
embodiments of the plug
may be employed. The plug system 100 further comprises a float valve 112,
which is
mounted within the casing string 50 proximate the bottom of the well bore. The
nose 16 of
the plug 110 is designed to mate with the top face of the float valve 112.
The float valve 112 comprises a collar valve housing encased in cement 114 and
a
linearly moveable plunger 116. The plunger 116 includes a central shaft, which
has a disc
attached to it at one end and a partially ball-shaped member attached to it at
the other end. It
also includes a spring, which biases the plunger 116 into a position where the
ball-shaped
member seats itself into a partially spherically-shaped recess formed within
the collar. The
plug 110 is designed to seat against the top face of the float valve 112. The
float valve 112 is
a one way check valve, and therefore prevents fluid below the valve from
flowing up hole.
Fluid flows down hole through the float valve 112 as follows. Once the
downward
fluid pressure exceeds the threshold of the rupture disc of the plug 110, the
fluid flows
through the hollow region of the plug and acts on the plunger 116 and ball-
shaped member.
The fluid pressure then pushes the plunger 116 downward, which in turn unseats
the partially
ball-shaped member from the partially spherical recess, which in turn allows
the fluid to flow
through the float valve 112. As those of ordinary skill in the art will
appreciate, the exact
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design of the float valve 112 is not critical to the present invention. Other
equivalently
functioning devices may be employed in its place.
The plug system 100 may include a second plug 118, which is formed with a high
pressure disc at its upper end. The plug 118 is designed to engage and mate
with plug 110.
More particularly, the upper end of plug 110 mates with the nose portion of
plug 118. For
ease of reference, plug 110 will be referred to as the bottom plug and plug
118 will be
referred to as the top plug. As those of ordinary skill in the art will
appreciate, plug system
100 may include any number of plugs. At least two plugs are generally
required, however,
for most cementing applications.
A method of separating fluids successively introduced into a subterranean well
bore
according to the present invention will now be described. A plug assembly
containing a top
and bottom plug are run in a casing string to the desired depth and suspended
therein. A first
fluid, generally a drilling mud used to drill the hole, is introduced into the
well bore through
casing string 50. Next, a second fluid, generally comprising a cement slurry,
is introduced
into the well bore behind the first fluid such that an interface between the
two fluids is
formed. Simultaneously, the bottom plug 110 is released from the assembly and
deployed
within the casing string at the interface of the first and second fluids. As
the bottom plug 110
travels downhole between the first and second fluids, it wipes the inside of
the casing string
50 removing it of any residue left behind by the first fluid. Once the bottom
plug 110 reaches
the bottom of the casing string 50, it seats into the float valve 112 in the
manner described
above.
The slurry is continuously pumped into the casing string 50 until the desired
amount
is reached. The volume of slurry required is a function of drilled hole size,
casing OD and
the amount of fillup that is desired in the annulus. It is quite common for
the bottom plug
110 still to be traveling down the pipe when the desired volume of cement is
pumped and the
top plug 118 released. Both plugs with the cement (and spacer fluid if pumped
on top of the
bottom plug) trapped in between are displaced by a third fluid, which can be,
but does not
have to be, the same as the first fluid used to circulate the hole initially,
until the bottom plug
110 lands and the rupture disk 24 (or other rupturable member) fails.
Displacement continues
until such time as the top plug 118 lands on top of the bottom plug 110 (as
shown in Figure
7), or float valve 112 in a top plug only application) wherein a pressure
increase signals that
the top plug has landed and final displacement is finished.
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As the top plug 118 travels downhole behind the second fluid, it also wipes
the inside
of the casing string 50 removing it of any residue left behind by the second
fluid. Once the
top plug 118 reaches the bottom of the casing string 50, it seats into the
bottom plug 110 in
the manner described above and shown in Figure 7. The high pressure disc 22 of
the top plug
118 is designed to preclude any further displacement of the second fluid into
the well bore.
Indeed, the timing of the release of the bottom and top plugs 110, 118,
respectively, as well
as the monitoring of the precise amount of the second fluid being pumped into
the casing
string 50 are important in avoiding under and overdisplacement of the second
fluid. This is
particularly important wherein the second fluid is a cementing slurry used to
cement the
casing string to the inside of the well bore.
Figure 12 shows two alternate plugs, top plug 410 and bottom plug 510, which
together form alternate plug system 400. Top plug 410 is formed with a solid
inner mandrel
412, which has a threaded lower end. A taper-shaped nose 416 attaches to the
threaded lower
end of the inner mandrel 412. However, as those of ordinary skill in the art
will appreciate,
the nose 416 can talce any desired form and may be integrally formed with the
inner mandrel
412. The outer sleeve 414 is similar to outer sleeve 14 in that it is
generally cylindrically
shaped and formed of a foam or other similar material.
The bottom plug 510 is formed with a generally tubular-shaped inner mandrel
512,
which has a hollow interior for channeling fluids. The bottom plug 510 has a
funnel-shaped
tapered upper end adapted for engagement with the taper-shaped nose 416 of the
top plug
410. The outer sleeve 514 is identical to outer sleeve 414 of the top plug
410. A nose 516 is
integrally formed at the lower end of inner mandrel 512. A flow stopper plug
524 is
temporarily secured to the nose 516 with one or more shear pins or other
securing means.
The flow stopper plug 524 is formed of a high strength thermoplastic or other
similar
material. As those of ordinary skill in the art will appreciate, the nose 516
may be removable.
Furthermore, as those of ordinary slcill in the art will appreciate, the
release pressure of the
flow stopper plug 524 may be adjustable.
Figure 13 illustrates an alternate embodiment of the plugs 410' and 510' shown
in
Figure 12. The only difference in this embodiment is that the outer sleeves
414' and 514'
contain a series of ribs 421 and 521, respectively.
Figure 14 illustrates another embodiment of a plug system (600). Plug system
600
includes a top plug 610 and a bottom plug 710. The top plug 610 includes a
generally
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tubular-shaped inner mandrel 612 having a taper-shaped nose 616 integrally
formed at its
lower end. The inner mandrel 612 is simply open at its upper end. As a
consequence of its
tubular design the inner mandrel has an internal flow channel, through which
fluids may
travel. Holes are formed in the nose 616 for inserting pins, which attach a
flow stopper plug
670 to the internal flow channel of the plug 610. The flow stopper plug 670 is
preferably
formed of a high strength thermoplastic material and is designed to stop the
flow of fluids
through the internal flow channel. The number and strength of the pins holding
the flow
stopper plug 670 in place within the internal flow channel are selected so
that the flow
stopper plug 670 is forced out of the plug 610 at the desired pressure, which
is dependent
upon the particular application. A pair of elastomeric rings 672 are also
provided to seal the
flow stopper plug 670 to the inside wall of the internal flow chaimel, so that
no fluid is
allowed to seep past the flow stopper plug 670. The outer sleeve 614 is
similar to outer
sleeve 14 in that it is generally cylindrically shaped and formed of a foam or
other similar
material.
Bottom plug 710 includes an inner mandrel 712, which is generally tubular in
shape
having an internal flow channel, through which fluids may pass. The bottom
plug 710 further
includes a nose 716, which is generally taper-shaped and integrally formed
with the inner
mandrel 712 at its lower end. Holes are formed in the nose 716 for inserting
pins, which
attach a flow stopper plug 770 to the internal flow channel of the plug 710.
The flow stopper
plug 770 is preferably formed of a high strength thermoplastic material and is
designed to
stop the flow of fluids through the internal flow channel. The number and
strength of the
pins holding the flow stopper plug 770 in place within the internal flow
channel are selected
so that the flow stopper plug 770 is forced out of the plug 710 at the desired
pressure, which
is dependent upon the particular application. An elastomeric ring 772 is also
provided to seal
the flow stopper plug 770 to the inside wall of the internal flow channel, so
that no fluid is
allowed to seep past the flow stopper plug 770. The bottom plug 710 has a
funnel-shaped
tapered upper end 774 adapted for engagement with the taper-shaped nose 616 of
the top plug
610. The outer sleeve 714 is similar to outer sleeve 14 in that it is
generally cylindrically
shaped and formed of a foam or other similar material.
Figure 1 S illustrates an alternate embodiment of the plugs 610 and 710 shown
in
Figure 12. The only difference in this embodiment is that the outer sleeves
614' and 714' of
plugs 610' and 710' comprise a plurality of ribs 621 and 721, respectively.
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13
The function, operation and advantages of the plug systems 600 and 600'
illustrated in
Figures 14 and 15 over conventional plug systems will now be described. As can
be seen in
Figures 14 and 15, the bottom plug 710, 710' is equipped with a sealing nose,
which
optionally may incorporate a latch down feature, and which may optionally be
formed as an
integral part of the bottom plug mandrel. The bottom plug 710, 710' is capable
of being
pumped down into a baffle profile (not shown) located above the upper most
float valve
assembly. The nose 716, 716' seals against the minimum ID of the baffle
profile and resists
further downward movement. Both of the profiles of the bottom plug nose 716,
716' and the
baffle (not shown) are designed such that they will be capable of sustaining
high pressure
loads applied from above at high temperatures. Upon landing in the baffle
profile, increased
pressure will shear the rupturable members) retaining the plug 770, 770' in
the nose of the
bottom plug 710, 710', thereby allowing circulation to continue. A catching
device located
above the top float valve (not shown) would be provided to catch the nose plug
to keep the
plug from interfering with the float valves.
After displacing the top plug 610, 610', with its tapered nose piece 616,
616', to a
shut-off against the funnel shaped seat profile built into the top of the
bottom plug mandrel
712, 712', further application of pressure will shear the plug pinned in the
ID of the top plug
mandrel 612, 612'. As the major ID's of both the top and bottom plug mandrels
will be the
same, the plug 670, 670' in the top plug 610, 610' will, after shearing free,
continue to pass, in
sealing engagement, through the major ID of the bottom plug mandrel 712, 712'
until it is
restrained by the internal restriction indicated near the lower end of the
bottom plug nose 716,
716'. At this point, the only components that will be subjected to high
pressure testing are the
baffle profile, the bottom plug nose 716, 716', and the nose plug 616, 616'
from the top plug
610, 610', no further loading will be imposed on either the top or bottom plug
mandrels, 612,
612 ; 712, 712', respectively. The only strength requirements for the bottom
plug mandrel
712, 712', including the plug seat built into its top, is that it be strong
enough to resist the
forces imposed by pressuring up against the top plug 610, 610' to shear out
its nose 616, 616'.
The top plug mandrel 612, 612' has no special strength requirements other than
the nose 616,
616' be capable of supporting the loads imposed in shearing out its nose plug
670, 670'. The
nose plug 770, 770' shear pinned inside the bottom plug mandrel 712, 712' may
be pinned to
shear at a pressure valve that makes sense in view of job conditions. The plug
670, 670' shear
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14
pinned in the nose 616, 616' of the top plug 610, 610' also may be pinned to
an appropriate
value as the nose plug will remain pressure balanced until such time as the
top plug is landed.
As those of ordinary skill in the art should recognize, the inner mandrels
612, 612' and
712, 712' of the plugs 610, 610' and 710, 710' need only be strong enough to
resist the
forces/loads imposed to land the plugs 610, 610' and 710, 710' and release the
nose plugs 670,
670' from the top plugs 610, 610', as discussed in the preceding paragraph.
While the use of the cementing plugs of the present invention in sub-surface
release
applications has been described, other embodiments of the present invention
may
advantageously employ these cementing plugs as conventional surface-release
plugs and or
as subsurface released plugs as well.
Therefore, the present invention is well-adapted to carry out the objects and
attain the
ends and advantages mentioned as well as those which are inherent therein.
While the
invention has been depicted, described, and is defined by reference to
exemplary
embodiments of the invention, such a reference does not imply a limitation on
the invention,
and no such limitation is to be inferred. The invention is capable of
considerable
modification, alteration, and equivalents in form and function, as will occur
to those
ordinarily skilled in the pertinent arts and having the benefit of this
disclosure. The depicted
and described embodiments of the invention are exemplary only, and are not
exhaustive of
the scope of the invention. Consequently, the invention is intended to be
limited only by the
spirit and scope of the appended claims, giving full cognizance to equivalents
in all respects.
