[0001] The invention relates to a method and apparatus for placing a cement lining alongside
the wall of a borehole.
[0002] In the art of drilling wells in subsurface earth formations it is known to stabilize
and reinforce the borehole wall by a cement lining.
[0003] A borehole stabilization technique of this type is known from U.S.S.R. Inventor's
Certificate No. 723102. In accordance with the known technique a shuttering in the
form of temporary casing pipes is run into the well. Subsequently a cement slurry
is pumped into the annular space around the pipes using a weighted mud. As soon as
the mud has reached the lower end of the casing pipes the casing is pulled and guide
plates mounted at the lower end of the casing spread the cement slurry mix over the
borehole wall. The mud is left in situ where its specific gravity enables it to act
as shuttering whilst the cement hardens.
[0004] A drawback of the known process is that in particular in non-vertical wells the weight
of the casing and guide plates may cause an eccentric position of the plates within
the borehole which results in an uneven thickness of the cement layer.
[0005] An object of the present invention is to remedy this drawback of the known process
and to provide a method and an apparatus for placing a cement lining in a well such
that even in a deviated well a cement lining of a constant thickness is created.
[0006] The method according to the invention comprises:
- connecting a stinger to an end of a tubing,
- lowering the stinger and tubing into the borehole until the stinger has reached
the bottom of a section of the borehole which is to be lined,
- introducing a cement slurry in an annular space surrounding the tubing and stinger
and pumping a slug of drilling mud into the tubing and stinger until the mud has substantially
reached the lower end of the stinger and the cement slurry substantially fills said
annular space over the length of the borehole section which is to be lined,
- pulling the tubing and stinger in upward direction through the borehole while centralizing
the outlet of the stinger in the borehole using centralizer means which are held in
contact with the borehole wall and drag through the cement slurry alongside the borehole
wall, and
- allowing the cement to set.
[0007] Experiments have shown that by proper sizing of the centralizer means the tracks
formed in the cement slurry alongside the borehole wall by the sliding centralizer
means fade away before hardening of the cement slurry so that no irregularities are
left in the cement layer.
[0008] Preferably the centralizer means comprise a series of bow spring centralizer blades
which are mounted near the lower end of the stinger. Utilization of bow spring centralizer
blades will generally be sufficient in gauge, vertical, holes and in wash-outs. However,
if cementing operations are carried out in highly deviated holes it is preferred to
provide the centralizer means in addition with rigid ribs which are mounted at equally
distributed angular intervals around the stinger. In that case the rigid centralizer
ribs give centralization in the gauge and deviated hole sections whereas the bow spring
centralizer blades stabilize the stinger in the wash-outs.
[0009] It is furthermore preferred to use bentonitic mud to create a stable channel in the
cement plug, because as soon as it comes into contact with the cement the bentonite
reacts with the cement. As a result a very viscous paste-like mud channel is left
behind in the cement plug.
[0010] After the cement has set the mud channel may be reamed or dressed to a desired size.
The mud channel then acts as a guide to the bit. The bit may be of a special type,
e.g. equipped with a nose section that can easily follow the mud channel.
[0011] If desired instead of creating a mud channel in the cement plug a soft centred cement
plug may be created as well. To accomplish setting of a soft centred cement plug the
stinger may be equipped with a cement mixing sub in which mud is mixed with cement,
thus creating in the cement plug a soft core which is easily drillable. It is observed
that it is known from US patent 3,774,683 to drill a core in a solid cement plug in
a borehole. However, in the absence of a mud channel or a soft centre in the cement
plug placed in accordance with the known technique drilling of a centralized bore
in the cement plug is difficult to accomplish. Thus it can be seen that a principal
advantage of the borehole lining technique according to the invention over the prior
art techniques is that always a cement lining of a constant thickness can be created.
[0012] The invention will now be explained in more detail with reference to the accompanying
drawings in which
- Fig. 1A-1D show different stages of the well lining method according to the invention,
and
- Fig. 2 shows a suitable embodiment of the well lining apparatus according to the
invention,
- Fig. 3 shows an embodiment of the apparatus where the stinger is connected in a
telescoping manner to the tubing, and
- Fig. 4 shows an embodiment of the apparatus where the stinger is connected to the
tubing by a flexible hose.
[0013] Fig. 1A shows a borehole 1 of which an upper section has been lined with a cement
coating 2 whereas a cement slurry 3 is being introduced by injection into a lower
section of the borehole.
[0014] The cement slurry is injected via a tubing 5 and a stinger 6 into an annular space
7 surrounding the tubing 5 and stinger 6. The bottom-end of the stinger 6 is held
in a centralized position in the borehole 1 by a bow spring centralizer 8, while the
outlet 9 of the stinger is located just above the bottom 10 of the hole 1.
[0015] Before injecting the cement slurry the borehole has been cleaned, for example to
remove any mud cake from the borehole wall, by circulating a scavenger slurry 12 at
high velocity through the annular space 7. If the annulus has a large width expandable
rubber wipers (not shown) may be secured to the outer surface of the stinger so as
to provide a flow restriction in the annulus and to scrape off the mud cake when the
stinger is run through the section to be treated.
[0016] The cement slurry 3 removes the scavenger slurry 12 from the annular space 7 and
the cement slurry 3 is displaced from the interior of the stinger by a mud 13 of the
same density as the cement slurry. The volume of the injected cement slurry 3 is selected
such that the cement slurry fills the annular space 7 over the borehole section to
be treated, whereas the volume of the heavy mud 13 injected behind the cement slurry
is sufficient to establish the required mud core over the length of the borehole section
to be treated. Subsequently the upper end of the annular space is closed by a collar
known as a hydril or lubricator, so as to create a fixed cement column in the annular
space 7, whereupon the tubing 5 and stinger 6 are pulled in upward direction as shown
in Fig. 1B. To compensate for the net tubing volume being pulled out of the hole additional
mud is allowed to flow into the tubing.
[0017] During the upward movement of the tubing 5 and stinger 6 the stinger spots a mud
channel 15 in the plug of cement slurry 3. The diameter of the mud channel does not
depend on the diameter of the outlet 9 of the stinger 6 but on the outer diameter
of the tubing 5 which is pulled through the collar.
[0018] After the outlet 9 of the stinger 6 is above the zone to be treated as illustrated
in Fig. 1C the annular space 7 is opened and excess cement and heavy mud are circulated
from the borehole. Subsequently the cement is allowed to set.
[0019] As illustrated in Fig. 1D a drilling bit 17 is subsequently used to ream or dress
the channel 15 to a bore of a desired diameter. The mud channel 15 acts as a guide
to the drilling bit 17 to centralize the bit in the borehole. The bit 17 may be an
underreamer bit or an eccentric bit and may be equipped with a nose (not shown) that
can easily follow the mud channel 15.
[0020] The same bit 17 may be used to drill a next section of the borehole after the hole
bottom 10 has been reached. Said next section may have a smaller diameter than the
previously treated sections and may be provided with a cement lining which is placed
using the same procedure as described above.
[0021] Alternatively said next section may have the same or larger diameter than the previously
treated sections and it may be provided with a conventional steel casing or it may
be left uncased, or be lined in the same manner as described before.
[0022] As shown in Fig. 2 the cement injection apparatus may be provided with a stinger
20 which is suspended from a coiled tubing 21 and equipped with a mixing sub 22.
[0023] The mixing sub 22 comprises a tubular element which is mounted co-axially around
the lower end of the stinger 20 and a conical mixing device 24 which is mounted within
said sub 22 below said end of the stinger 20 such that it points towards the stinger.
In use mud and cement are mixed within the sub 22 in response to pulling of the tubing
21 and stinger 20 after injecting a cement slurry 25 into the annular space of a well
interval which is to be treated.
[0024] The cement which enters the top 26 is mixed with mud which flows downwardly through
the stinger 20 in response to pulling of the tubing so that below the sub 22 a mixture
of mud and cement is created in the centre 27 of the borehole 28. After hardening
of the cement the mud/cement mixture in the centre 27 of the borehole 28 forms a soft
core which can be easily drilled out.
[0025] To ensure stabile centralization of the apparatus in the borehole 28 a series of
rigid ribs 29 and a series of bow spring centralizer blades 30 are mounted at equally
spaced angular intervals on the tubular outer surface of the mixing sub 22. The bow
spring centralizer blades 30 serve to centralize the apparatus in gauge, vertical,
hole sections whereas the rigid ribs 29 serve to centralize the apparatus in highly
deviated hole sections. If desired the rigid ribs 29 may be replaced by bow spring
stabilizer blades having a larger stiffness than the other stabilizer blades 30. Alternatively
the stabilizer means may consist of a steel pin scraper or of a series of circumferentially
spaced expandable arms which are held in contact with the borehole wall by spring
action.
[0026] In the above manner proper centralization of the assembly is accomplished both in
vertical and deviated boreholes or in holes with a varying diameter or irregular shape.
Proper centralization of the assembly within the borehole ensures that the soft core
is always placed in the centre of the borehole so that after drilling out the soft
core a cement lining with a regular thickness is left alongside the borehole wall
28.
[0027] Fig. 3 shows an embodiment of the apparatus according to the invention where the
stinger 33 is mounted in a telescoping manner inside a tubing 34. During lowering
the assembly through the borehole the stinger 33 is in the illustrated contracted
position. However, if after injecting the cement slurry into the annular space the
tubing 34 is pulled, friction between the borehole wall and stabilizer blades 35 will
cause the stinger 33 to be pulled out of the tubing 34 until the stinger 33 is in
the extended position.
[0028] Fig. 4 shows an embodiment of the apparatus according to the invention where the
stinger 41 is provided with two stabilizer assemblies 42 and 43, respectively. The
stinger is connected to the tubing 44 by a flexible hose 45. The flexible hose 45
avoids that an eccentric position of the lower end 46 of the tubing in a deviated
well section 47 would result in an eccentric position of the stinger 41.
[0029] The cement slurry may contain various additives to adapt its physical properties
to well operations. Latex, polymers and epoxies may be added to the slurry to optimize
the elastic properties of the cement and polypropylene or other fibres may be added
to the slurry to improve the impact resistance of the cement and to plug off loss
zones. Furthermore the wear resistance of the cement can be improved by adding wear
resistant granules whereas the friction coefficient of the cement can be decreased
by adding graphite, for example.
[0030] The cement may contain Portland cement but alternatively it may consist of an epoxy,
polymeric or any other resin. The cement composition may furthermore vary over the
length of the borehole.
[0031] It is furthermore preferred to use bentonitic mud to create the channel in the cement
plug because, as soon as it comes into contact with the hydraulic cement, the bentonite
reacts with the cement. As a result a very viscous, paste-like, mud channel is left
in the cement plug. The mud may furthermore contain additives which thicken the mud
when it comes into contact with hydraulic cement or which act as an accelerator for
the setting of cement.
[0032] It is important that a good bonding is obtained between the cement and the borehole
wall. Hence it is generally necessary to wash away any mud cake or debris from the
borehole wall before injecting the cement.
[0033] If the stinger is suspended from a large diameter pipe string a narrow annular space
is created in which a high fluid velocity can be created. As a result of said high
fluid velocity the mud cake can be washed away before placing the cement. However
a disadvantage of suspending the stinger from a pipe string is that pulling of the
stinger has to be interrupted to break the pipe connections. These interruptions may
cause balloons in the mud channel because of pressure relaxation.
[0034] To avoid creating of balloons in the mud channel it is generally preferred to suspend
the stinger from a small diameter coiled tubing which can be pulled at a constant
speed throughout the interval to be treated.
[0035] In view of the large annular space around such a small diameter coiled tubing it
may be necessary to attach steel reinforced rubber wipers to the outer surface of
the stinger and/or tubing. These wipers act as a flow restriction in the annular space
resulting locally in high annular fluid velocities. The wipers furthermore scrape
off the filter cake from the borehole wall when the stinger is run through the borehole
section to be treated. Mud may be circulated while running the stinger down.
[0036] It will further be understood that after injecting the cement slurry into the annular
space around the tubing and stinger and before hardening of the cement the tubing
may be moved up and down again through the borehole section to be treated before eventually
filling the tubing and stinger with heavy mud and pulling them through the cement
plug in order to create the mud channel or soft core therein.
[0037] Finally, it will be understood that instead of injecting the cement slurry via the
stinger into the well, the slurry may also be introduced prior to lowering of the
stinger into the well. In that case the slurry completely fills a lower section of
the borehole when the stinger is lowered into the hole. Once the stinger has reached
the bottom of the hole drilling mud is injected into the interior of the tubing and
stinger until this interior is completely filled with mud, whereupon the tubing and
stinger are retrieved from the borehole in the manner described with reference to
Figure 1B.
[0038] Many other variations and modifications may be made in the apparatus and techniques
hereinbefore described, both by those having experience in this technology. Accordingly,
it should be clearly understood that the apparatus and method depicted in the accompanying
drawings are illustrative only and are not intended as limitations on the scope of
the invention.
1. A method of placing a cement lining alongside the wall of a borehole, the method
comprising:
- connecting a stinger to an end of a tubing,
- lowering the stinger and tubing into the borehole until the stinger has reached
the bottom of a section of the borehole which is to be lined,
- introducing a cement slurry in an annular space surrounding the tubing and stinger
and pumping a slug of drilling mud into the tubing and stinger until the mud has substantially
reached the lower end of the stinger and the cement slurry substantially fills said
annular space over the length of the borehole section which is to be lined,
- pulling the tubing and stinger in upward direction through the borehole while centralizing
the stinger in the borehole using centralizer means which are held in contact with
the borehole wall and drag through the cement slurry alongside the borehole wall,
and
- allowing the cement to set.
2. The method of claim 1 wherein the stinger is centralized in the borehole by centralizer
means comprising a series of bow spring centralizer blades which are mounted near
the lower end of the stinger.
3. The method of claim 2 wherein the centralizer means are further equipped with rigid
ribs which are mounted at equally distributed angular intervals around the stinger.
4. The method of claim 1 wherein behind the cement slurry a slug of bentonitic mud
is pumped into the tubing and stinger, said mud having a density which is substantially
equal to the density of the cement slurry.
5. The method of claim 1 wherein after the step of pumping the mud into the tubing
and stinger the annular space is closed either at the wellhead or at the top of the
borehole section which is to be lined.
6. The method of claim 1 further comprising providing the stinger with a mixing sub
consisting of a tubular element having an internal diameter which is larger than the
outer diameter of the tubing, and arranging within said sub a mixing device, and mixing
within said sub cement and mud to a soft cement core in response to pulling the tubing
and stinger in upward direction through the borehole.
7. The method of claim 1 wherein prior to lowering the tubing and stinger into the
borehole a cement slurry is introduced into the borehole such that the slurry completely
fills a lower section of the borehole, whereupon after lowering the tubing and stinger
to the borehole bottom drilling mud is injected into the interior of the tubing and
stinger until the mud has reached the lower end of the stinger.
8. The method of any one of claims 1 to 7 wherein after hardening of the cement a
drilling device is lowered into the borehole and actuated to drill an open space of
a desired width in the centre of the cement plug.
9. An apparatus for placing a cement lining alongside the wall of a borehole, the
apparatus comprising a stinger connected to an end of a tubing and means for centralizing
the stinger in the borehole, said centralizer means comprising a series of centralizer
elements which protrude in lateral direction away from the stinger such that in use
the elements are held in contact with the borehole wall when the stinger and tubing
are pulled in upward direction through the borehole.
10. The apparatus of claim 9 wherein said series of centralizer elements include a
plurality of bow spring centralizer blades which are mounted at equally distributed
angular intervals around the stinger.
11. The apparatus of claim 10 wherein said series of centralizer elements further
include a plurality of rigid ribs which are mounted at equally distributed angular
intervals around the stinger.
12. The apparatus of claim 9 further comprising a collar which is slidably secured
around the tubing for closing the upper end of the annular space during the step of
pulling the tubing and stinger.
13. The apparatus of claim 9 wherein the stinger is equipped with a mixing sub, said
sub comprising a tubular element which is mounted co-axially around the lower end
of the stinger, and a conical mixing device mounted within the sub below and pointing
towards said end of the stinger for mixing cement and mud to a soft cement core in
response to pulling the tubing and stinger in upward direction through the borehole.
14. The apparatus of claim 13 wherein said centralizer means are mounted on the outer
surface of said mixing sub.
15. The apparatus of claim 9 wherein the stinger is connected to the tubing by a flexible
hose.
16. The apparatus of claim 15 wherein the stinger is secured in a telescoping manner
to the lower end of the tubing.
17. The apparatus of claim 9 wherein said tubing consists of a flexible hose.