Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02539511 2006-03-14
METHOD AND APPARATUS FOR CEMENTING A WELL USING
CONCENTRIC TUBING OR DRILL PIPE
This application claims the benefit of U.S. Provisional Application No.
60/594,130, filed March 14, 2005.
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for cementing
individual
and multi- zones in a vertical, directional or horizontal wellbore, using
concentric
tubing or drill pipe.
BACKGROUND OF INVENTION
Cementing is used in the oil and gas industry to seal off fluids and
unconsolidated materials from entering the well bore, for packing off unwanted
zones, as a loss circulation material and to abandon wells.
Current cementing technology requires a casing or finer to be placed in the
well
bore and a cement slurry is then pumped downhole and back up into the space
or annulus between the casing or liner and the wall of the well bore. However,
each succeeding casing or liner placed in the wellbore has an outside diameter
significantly reduced in size when compared to the casing or liner previously
installed. Thus, each time casing is run in a wellbore the diameter of the
wellbore is reduced by the size of that casing.
Further, where operations require the cementing of casing such as the setting
of
surface or production casing, there is the need to use the following
equipment: a
casing shoe, float equipment and cement plug. This equipment can only be
removed by drilling them out. This takes up valuable drilling time and can
prove
difficult when using an air hammer drilling system.
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The present invention allows the cementing operation to be completed without
the need to run casing or have cement returned to surface thereby allowing
larger diameter wellbores to be drilled into zones of interest. Further, time
and
money are saved on not having to run intermediate casing strings, and cement
does not have to be pumped to surface. Finally, the present invention allows
cementing to be completed without having to drill out equipment such as cement
plugs, float equipment and casing shoes.
SUMMARY OF THE INVENTION
An apparatus for cementing a well for use with concentric drill string, e.g.,
concentric drill pipe, concentric coiled tubing and the like, comprising an
isolation
cementing tool and a cementing flow control means and a method of using such
cementing apparatus is disclosed. Concentric drill string comprises an inner
string and an outer string, forming an annular conduit therebetween.
The isolation cementing tool has a center tube and an outer casing forming an
annular conduit therebetween, said isolation cementing tool further having an
expandable packer means surrounding at feast partially the outer casing. The
tool is adapted to connect to a bottom of a piece of concentric drill string
in such
a fashion as to be in fluid communication with the concentric drill string.
The cementing flow control means also has a center tube and an outer casing
forming an annular conduit therebetween and is either directly connected to
the
bottom of the isolation cementing tool or separated from the isolation
cementing
tool by additional shorter pieces of concentric drill string such that the
cementing
flow control means is also is in fluid communication with both the isolation
tool
and the concentric drill string.
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The cementing flow control means further has a plurality of check valves,
either
positioned in the center tube or within the annular conduit between the inner
tube
and the outer casing or both to prevent cement from flowing upwardly
therethrough when cement is being pumped through either the inner conduits or
annular conduits. In one embodiment where the cementing flow control means
comprises check valves in the annular conduit thereof, which is used when
cement is pumped through the inner conduits, the inner diameter of the center
tube of the cementing flow control means is reduced at or near the bottom end
thereof to prevent a cement plug from exiting therefrom as will be explained
in
more detail below.
In operation, when the packer means of the isolation cementing tool is in the
expanded position, the isolation cementing tool is in the "closed position"
and
when the packer means is in the contracted position the isolation cementing
tool
is in the "open position". When in the contracted or open position, fluids are
free
to flow from the outer annulus between the concentric drill string and the
formation walls. In a preferred embodiment, the expansion of the packer means
is controlled by an electric current for quicker opening and closing of the
isolation
cementing tool.
When running the cementing apparatus in the hole, the isolation cementing tool
is in the open position, i.e., the packer means is contracted. When the tool
is in
the open position it does not restrict or reduce the radius of the annulus
between
the outside wall of the drill string and the wellbore, as the outside diameter
of the
tool is preferably equal to, less than or slightly larger than the outside
diameter of
the concentric drill string.
When cementing is required, the isolation cementing tool, which is now
positioned directly above the zone to be cemented, is put in the closed
position,
i.e., the packer means is expanded to abut the adjacent wellbore walls. Thus,
the portion of the well bore below the isolation cementing tool is shut off or
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isolated from the portion of the well bore above the tool as the expanded
packer
means will not allow fluids to flow passed it.
In one embodiment of the method, cement is first pumped down the inner string
of the concentric drill string, through the center tube of the isolation
cementing
tool and ultimately through the center tube of the cementing flow control
means
and into the formation. A cement plug of a type well known in the art is then
inserted into the inner string and "chased" with a fluid such as water, gas,
air,
etc., which fluid is also pumped through the inner string, etc. The insertion
of the
plug and the subsequent pumping of fluid force the cement out through the
bottom of the cementing flow control means and into the formation. In this
embodiment, cementing flow control means has a plurality of check valves of a
type well known in the art in the annular conduit thereof. Cement is prevented
from flowing up the annular conduit of the concentric drill string as a result
of the
I5 upward pressure exerted on the check valves in the annular conduit of the
cementing flow control means, closing same.
The cement plug, which preferably has a diameter slightly less than the inner
diameter of the center tube of the cementing flow control means, will
ultimately
hit the portion of the center tube where the inner diameter of the center tube
is
reduced. The plug becomes wedged at that point such that no further fluid can
be pumped therethrough. The fluid contained in the inner conduit of the inner
string of the concentric drill string can then be easily removed by reverse
circulating compressed fluid such as water, air, gas and the like through the
annular conduit of the concentric drill string thereby forcing the plug to be
dislodged and travel up the inner conduit of the inner string of the
concentric drill
string to the surface. Further, any fluid present in the inner string will
also be
forced up to the surface.
The invention has one or more of the following advantages over current
cementing methods;
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- Cementing back to surface isn't required;
- Less damage to producing formations from the lower hydrostatic cement
weight;
- Less cement in the formation for fracture treatment to deal with;
- No need to run a string of casing, cementing shoe and float equipment
- Much less cement is needed;
The main portion of the well can be drilled with the same hole diameter;
- Chemicals to strength and accelerate the curing of the cement can be added
through the concentric drill string, preferably through the annular conduit;
- Loss circulation material can be added through the concentric drill string,
preferably through the annular conduit;
- Single or multiple zones can be cemented off to prevent fluid invasion or
unconsolidated materials from plugging the well bore;
- Significant cost savings on cementing time, actual cement and casing costs;
- When water is used as the "chasing" fluid, water is conserved by collecting
it
through the concentric drill string, preferably out through the inner string,
to
the surface prior to removing the concentric drill string;
- Lower productive zones can be completed with larger diameter casing, liners
or left open hole;
- Compressed air can be blown down the concentric drill string, either through
the annular conduit or through the inner string to greatly reduce the curing
time of the cement;
- No casing shoe, float equipment and cement plug to drill out; and
- Well abandonment programs can be done much quicker and cheaper;
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 is a schematic of an embodiment of an isolation cementing tool of the
present invention.
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Figure 2 is a cross-sectional view on and enlarged scale of the isolation
cementing tool shown in Figure 1.
Figure 3a and 3b is a schematic of the isolation cementing tool of Figure 1 in
the
open position and closed position, respectively.
Figure 4 is a cross-sectional view of an embodiment of a cementing flow
control
means with cement flowing down the center tube and check valves in the closed
position.
Figure 5 is a cross-sectional view of an embodiment of a cementing apparatus
of
the invention assembled on concentric drill string.
Figure 6 is a cross-sectional view of an embodiment of a cementing flow
control
means, in the displacement position, showing compressed fluid being delivered
down the annular conduit for pushing the cement plug and water up the center
tube to the surface.
Figure 7 is a schematic of the surface and downhole equipment involved in
cementing a wellbore where casing has been run in the wellbore.
Figure 8 is a cross-sectional view of an embodiment of a cementing apparatus
of
the invention when using concentric coiled tubing.
Figure 9 is a cross-sectional view showing the connection of a single wall
high-
pressure cement pumping hose to concentric drill string.
Figure 10 is a cross-sectional view showing the connection of a double wall
high-
pressure cement pumping hose to concentric drill string.
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Figure 11 is a schematic illustration of a concentric coiled tubing unit
pumping
cement down a wellbore.
DETAILED DESCRIPTION
The cementing apparatus and method will be described with reference to the
following preferred embodiments.
Figure 1 schematically illustrates an embodiment of an isolation cementing
tool
30 and means for attaching the tool between two pieces of concentric drill
string
45 and 47. Preferably, isolation cementing tool 30 is attached to concentric
drill
string 45 and 47 such that the isolation cementing tool 30 is position the
proper
distance from the bottom of the wellbore to allow the desired zone to be
properly
cemented.
Concentric drill string 45 and 47 both comprise an inner string 57 and an
outer
string 59, forming an annular conduit 16 therebetween. Concentric drill string
45
and 47 are designed such that at one end of the concentric drill string is a
threaded pin end and at the other is a threaded box end. Thus, pieces of the
concentric drill string can be connected end to end by screwing the thread pin
end of the new piece of concentric drill string to be added into the box end
of the
drill string below. It is understood that concentric drill string could also
be a
continuous length of concentric coiled tubing having an inner coiled tube and
an
outer coiled tube in which case isolation cementing tool would be operably
attached to the end thereof by coupling means known in the art for coupling
downhole tools to coiled tubing.
As can be seen in Figure 1, concentric drill string 45 has threaded pin end 31
at
its bottom end and concentric drill string 47 has threaded box end 35 at its
top
end. Isolation cementing tool 30 is adapted to be inserted between concentric
drill string 45 and 47 by means of threaded box end 37 and threaded pin end
33.
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Thus, threaded pin end 31 of concentric drill string 45 screws into threaded
box
end 37 and threaded pin end 33 screws into threaded box end 35 of concentric
drill string 47. It is anticipated that in some instances concentric drill
string 47
may only be a short piece of concentric drill string, e.g., it may not be a
complete
length of concentric drill pipe as is known in the art. Isolation cementing
tool 30
further comprises a packer means 39 surrounding the isolation cementing tool,
the operation of which will be described in more detail below.
With reference to Figure 2, isolation cementing tool 30 further comprises a
center
tube 34, an outer casing 32, an annular conduit 36 between the center tube and
outer casing, an inner conduit 38, and a packer means 39 surrounding said
outer
casing 32. When isolation cementing tool 30 is inserted between concentric
drill
string 45 and 47, the center tube 34 of the isolation cementing tool 30 is in
fluid
communication, i.e., via inner conduit 38, with the inner string 57 of the
concentric drill string 45 and 47 and the annular conduit 36 of the isolation
cementing tool 30 is in fluid communication with the annular conduit 16 of the
concentric drill string 45 and 47.
In one embodiment, packer means 39 can be expanded or contracted by means
of an electric current flow path. In another embodiment, the packer means
comprises an inflatable ring, which can be inflated by pumping various types
of
fluid into and out of the ring, as is known in the art.
Figures 3a and 3b schematically illustrate the isolation cementing tool
attached to
the concentric drill string in the open and closed position, respectively,
during
cementing operations. When packer means 39 is contracted or deflated as
shown in Figure 3a, the tool is in the open position and fluids can flow
freely
through the wellbore annulus 43 formed between the outer wall of the outer
string
59 of the concentric drill string and formation wall 41. When packer means 39
is
expanded or inflated as shown in Figure 3b, the packer means is forced against
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formation wall 41 thereby closing oft annulus 43 to fluid movement above and
below packer means 39.
Figure 4 is a cross-sectional view of one embodiment of a cementing flow
control
means 10, which forms part of the cementing apparatus of the invention.
Cementing flow control means 10 comprises a center tube 4 and an outer casing
2, forming an annular conduit 7 therebetween. A plurality of check valve means
3 are shown situated within the annular conduit 7 in this particular
embodiment.
It is understood that in an embodiment where it is desirable that cement is
pumped down the annular conduit of the concentric drill sting, cementing flow
control means would comprise a plurality of check valve means in the inner
conduit 57 of the center tube 4. Check valve means useful in downhole tools
are
well known in the art to allow fluid or gas to flow through tools in only one
direction.
It can be seen in Figure 4 that the inner diameter of the center tube 4 is
reduced
at the bottom end thereof. Thus, the inner wall diameter at the bottom end is
less
than the diameter of the rest of the center tube 4. This area of reduced
diameter
is often referred to in the art as a stinger and is designated element 99.
Figure 4
shows cement 9 being pumped through the inner conduit 1 of center tube 4.
Annular conduit 7 is closed off both at the bottom and the top of the
cementing
flow control means by two sets of check valves 3, thereby preventing cement 9
from flowing up annular conduit 7. Cementing flow control means 10 further
comprises threaded box end 52 so that this end can be attached to either the
threaded pin end of a piece of concentric drill string or directly attached to
the
threaded pin end 33 of isolation cementing tool.
When the cementing flow control means 10 is properly connected, it is
understood that center tube 4 of the cementing flow control means 10 is in
fluid
communication with the inner string of concentric drill string and the center
tube
of the isolation cementing tool, and that annular conduit 7 of the cementing
flow
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control means 10 is in fluid communication with the annular conduit of
concentric
drill string and the annular conduit of the isolation cementing tool.
Figure 5 illustrates an embodiment of an assembled cementing apparatus of the
present invention. Isolation cementing tool 30 is shown operably attached to
concentric drill string 45 by means of threads as described above. It can be
seen
that annular conduit 16 of concentric drill string 45 is in fluid
communication with
annular conduit 36 of isolation cementing tool 30 and that inner conduit 19 of
concentric drill string 45 is in fluid communication with inner conduit 38 of
isolation cementing tool 30. In this embodiment, cementing flow control means
10 is directly attached to the end of isolation cementing tool 30 by thread
means.
It can be seen that annular conduit 7 of cementing flow control means 10 is in
fluid communication with annular conduit 36 of the isolation cementing tool 30
and that inner conduit 1 of cementing flow control means 10 is in fluid
communication with inner conduit 38 of isolation cementing tool 30.
In operation, in one embodiment of the invention, cement 9 is first pumped
through inner conduit 19 of inner string 57, then through inner conduit 38 of
isolation cementing tool 30, and finally through inner conduit 1 of cementing
flow
control means 10. Isolation cementing tool is in the closed position. Cement
plug 5, which has on outside diameter slightly smaller than the inner diameter
of
the inner string 57, the inner diameter of center tube 34 and the largest
inner
diameter of center tube 4, but larger than the diameter of stinger 99, is
inserted
through inner string 57 and "chased" with a fluid such as air, gas or water
79.
The pumping pressure of fluid 79 pushes cement plug 5 down to the bottom of
the cementing flow control means 10 and thereby forces cement 9 out into the
wellbore. The pressure ultimately forces cement 9 past formation wall 41 and
into the formation itself causing that part of the formation to become sealed
off.
Some of the cement 9 will be forced up outer annulus 43 but flow will be
stopped
when cement 9 reaches the bottom of inflated/expanded packer means 39 (the
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closed position). Cement 9 is further prevented from returning up annular
conduit 7, and subsequently up annular conduit 16 of concentric drill string
45 to
the surface, by means of check valves 3 sealing off the annular conduit 7 as a
result of the upward pressure which results when pumping down cement 9 and
fluid 79 through the various inner conduits.
Cement plug 5, which follows cement 9, is eventually chased down by fluid 79
to
the bottom of cementing flow control means 10. As previously mentioned, the
inner diameter of center tube 4 is reduced near its bottom forming stinger 99
so
that travel of cement plug 5 is stopped. At this point, essentially all of the
cement
9 will have been forced out of the various inner conduits and into the
formation,
leaving most of fluid 79 still contained in the inner conduit of inner string
57 of the
concentric drill string 45.
At this point, chemicals or other additives to strengthen or speed up the cure
time
for the cement 9 can be pumped down annular conduit 16 by applying downward
pressure to both sets of check valves 3 thereby opening them to allow the
chemicals or additives to reach the cement 9. Further, fluid 79, which has
essentially been contained in the various inner conduits, can now be removed
by
reverse circulating air or other fluid down annular conduit 16 of concentric
drill
string 45, through the annular conduit 36 of isolation cementing tool 30 and
through annular conduit 7 of cementing flow control means 10 and up through
the center of the center tube 4 of cementing flow control means 10, the center
tube 34 of the isolation cementing tool 30 and the inner string 57 of
concentric
drill spring 45, thereby forcing out the fluid 79 which can then be collected
at the
surface of the well.
Figure 6 is a cross-sectional view of cementing flow control means 10 during
the
displacement/removal of fluid 79 operation. Essentially, both cement plug 5
and
fluid 79 are displaced as a result of a fluid such as compressed air 13 being
pumped down the various annular conduits and reverse circulated up through the
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various inner conduits. When the pressure of air 13 is exerted on check valves
3, these check valves are forced in the open position as shown in Figure 6.
Thus, air 13 eventually displaces both cement plug 5 and fluid 79 up through
the
center of the concentric drill string to the surface. This both eliminates the
necessity to drill out the cement plug and conserves fluids such as water.
Once cementing is completed and the cement plug and excess water removed,
isolation cementing tool 30 is placed back in the open position, i.e., packer
means is deflated as shown in Figure 2, and the concentric drill string is
tripped
out of the well bore. In a preferred embodiment, isolation cementing tool 30
is
put in the open position and concentric drill string 47 is pulled up above the
cement in the well bore. Isolation cementing tool 30 is then placed in the
closed
position and compressed air or other gas is pumped down annular conduits 16,
36 and 7 to reduce the amount of time for the cement to cure so drilling or
other
operations may resume.
Figure 7 illustrates another embodiment of the invention wherein casing is
first
placed in the wellbore. Figure 7 shows the surface equipment required to pump
cement down a well bore. Casing 69 is run in the wellbore between formation
walls 41, thereby forming annulus 129 between the casing 69 and formation wall
41. Cement 9 is pumped from surface by cement pump 101 down inner tube 57
of concentric drill string 45, followed by cement plug 5 and fluid 79.
Isolation
cementing tool 30 is in the closed position, i.e., packer means 39 is in the
expanded position and abuts against the inside wall of casing 69. When
isolation
cementing tool 30 is in the closed position, this allows both the pumping
pressure
and the hydrostatic weight of the cement 9 and fluid 79 to push cement 9 up
annulus 129 to the surface of the weiibore. As previously explained, cementing
flow control means 10 prevents cement 9 from entering annular conduit 16 of
the
concentric drill string 45.
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Once the cement 9 returns to surface, displacement operations as shown in
Figure 6 and described above commence. Thus, no casing shoe, float
equipment and cement plug are left in the well bore that have to be drilled
out
when drilling operations resume.
Figure 8 is a cross-sectional view of another embodiment of a cementing
apparatus of the present invention using concentric coil tubing 12. Concentric
coiled tubing 12 comprises inner tube 14, which provides inner conduit 20 for
pumping cement and water, and outer tube 18, wherein outer tube 14 and inner
tube 12 form an annular conduit 17 therebetween for pumping air/fluid during
the
displacement operations.
Isolation cementing tool 30 is attached to concentric coiled tubing 12 by
coupling
or connecting means 21, as known in the art for connecting downhole tools to
coiled tubing, to be in fluid communication with concentric coiled tubing 12
as
previously described with drill pipe. A length of concentric coiled tubing 112
is
routinely attached at the end of isolation cementing tool 30 via another
connecting means 23 known in the art. The length of concentric coiled tubing
112 is determined based on the length of the zone to be cemented.
25
Cementing flow control means 10 is attached to the free end of concentric
coiled
tubing 112 by connecting means 62 as known in the art. As previously
mentioned, the inner diameter of the center tube 4 of the cementing flow
control
means 10 is reduced, forming stinger 99. As mentioned, this reduction in the
inside diameter of the center tube prevents cement plug 5 from passing through
the center tube and into the wellbore.
Electrical cable 22 provides electric current to operate the isolation
cementing
tool 30 by expanding and contracting packer means 39. Other means of
operating isolation cementing tool 30 could include fiber optic cables, radio
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frequency, electric magnetic or small diameter capillary tubes which transmit
hydraulic or pneumatic pressure.
Figure 9 is a cross-sectional view showing the connection of a single wall
high-
pressure cement pumping hose 26, which can be used to pump cement 9
through the inner conduit 19. Cement pumping hose 26 is connected to inner
string 57 of concentric drill string 45 by connecting means 63. Preferably,
the
annular conduit 7 is sealed off prior to pumping cement 9 down the inner
string
57 by means of donut spacer 31. It is understood that donut spacer 31 could be
a separate element or could be an integral part of high-pressure cement
pumping
hose 26. Cement 9 is pumped down the inner string 57 through inner conduit 19
to the desired depth. Cementing flow control means 10 (not shown) prevents
cement 9 from flowing back up the annular conduit 7.
Figure 10 is a cross-sectional view of another embodiment of the invention
showing the connection of a double-walled high-pressure cement pumping hose
126 to concentric drill string 45 which can be used for both delivering cement
and
chasing fluid and also during the displacement of chasing fluid operation. Pin
end 31 of double-walled high-pressure cement pumping hose 29 connects to box
end 35 of concentric drill string 45 by means of threads as previously
described.
During displacement operations, fluid such as air 13 is reverse circulated
through
annular conduit 7 of concentric drill string 45 and up through inner conduit
1,
which pushes cement plug 5 and chasing fluid 79 back to surface.
In an embodiment as shown in Figure 11, truck 101 comprises a cement tank
114 for holding cement, a fluid tank 112 for holding fluid such as water,
which is
used to chase the cement and cement plug to the bottom of the cementing
apparatus, and an air compressor 118 for delivering compressed air for
displacing the chasing fluid. Truck 101 further comprises a pumping mechanism
(not shown) and a valve manifold system (not shown) both of which are
connected to cement tank, water tank and air compressor by double-wall cement
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pumping hose 29. Valve manifold system operates to switch between pumping
cement, pumping fluid such as water and pumping air.
Double-wall cement pumping hose 29 connects to concentric coiled tubing 12,
which is wrapped around reel 116 on coifed tubing truck 107. In operation,
cement 9 is pumped via pumping mechanism through hose 29 and ultimately
through either the annular conduit or inner conduit of the concentric drill
string 12.
Cement 9 is pushed to the bottom of wellbore 115 by adding cement plug (not
shown) and pumping fluid such as water from fluid tank 112. This forces cement
9 to be squeezed through formation wall 41 into a fluid zone of the formation
required to be sealed off with cement. Once cement 9 has cured or set, it then
prevents formation fluid 111 from entering wellbore 115.
Surface blowout preventor (BOP) 109 provides a surface seal for concentric
coiled tubing 12 so that pumping pressure can deliver cement 9, water or air
down to the wellbore 115 to prevent formation fluid 111 from entering wellbore
115. It can be seen from Figure 11 that surface casing 69 has previously been
cemented in place and the rest of welibore 115 has been drilled open hole.
Once cementing has been completed, isolation cementing tool 30 is placed in
the
open position by contracting packer means 39 and concentric coiled tubing 12
and cementing apparatus are pulled up several feet from the top of the cement
deposited in the wellbore 115. Then, isolation cementing tool 30 is put back
in
the closed position by expanding packers means 39 at which point displacement
operations are commenced.
Compressed air 13 is now pumped through annular conduit i 7 to first assist in
drying cement 9 and then to push cement plug and chasing fluid back to the
surface where it is returned to fluid tank 112. It is understood that, in
addition to
compressed air, other chemicals, loss circulation materials and other fluids
can
also be pumped through annular conduit 17.
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In an embodiment of the invention (not shown) where cement and chasing fluid
is
delivered through the annular conduit of concentric drill string, it is the
radius of
the annular conduit of the cementing fluid control means which is reduced,
thereby forming an annular conduit stinger. In this embodiment, a donut shaped
cement plug, which has a radius small enough to freely slide through the
various
annular conduits but large enough that it can not get passed the annular
conduit
stinger of this embodiment of the cementing fluid control means is used. Fluid
such as water is then pumped through the annular conduits to chase down the
donut shaped cement plug until it gets lodged in the stinger. Compressed fluid
such as compressed air is then pumped through the various inner conduits to
force the donut shaped cement plug and chasing fluid back to the surface. In
this
embodiment, the cementing fluid control means comprises a plurality of check
valves situated in the inner tube rather than the outer conduit.
In an embodiment where reverse circulating cementing operations are taking
place within zones containing hydrocarbons, a surface and downhole well
control
system is added for safety reasons to prevent the flow of hydrocarbons to the
surface. Examples of cementing operations where well control may be needed
are during abandonment of a wellbore, when trying to stop a loss circulation
problem, and when squeezing a wet zone or a depleted zone in a multi - zone
well. During these reverse circulation cementing operations, the well could
"kick" and without the well control downhole and at surface a blow out
situation
could arise. Examples of surface flow control means and downhole flow control
means that can be used with concentric drill string are given in U.S. Patent
No.
6,854,534 and U.S. Patent No. 6,892,829, both of which are incorporated herein
by reference.
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