CA1324573C

CA1324573C - Method and apparatus for placing a cement lining in a borehole

Info

Publication number: CA1324573C
Application number: CA000601120A
Authority: CA
Inventors: Rudolfus Petrus Antonius Robertus Van Kleef; Wilhelmus Christianus Maria Lohbeck
Original assignee: Shell Canada Ltd
Current assignee: Shell Canada Ltd
Priority date: 1988-06-14
Filing date: 1989-05-30
Publication date: 1993-11-23
Anticipated expiration: 2010-11-23
Also published as: NO302136B1; EP0346961A3; EP0346961A2; GB8814004D0; NO892412D0; DE68923585D1; DE68923585T2; NO892412L; EP0346961B1

Abstract

A B S T R A C T

METHOD AND APPARATUS FOR PLACING
A CEMENT LINING IN A BOREHOLE

A cement lining is placed in a borehole using a cementing apparatus including a centralized stinger which is suspended from a cement injection tubing. In use a cement slurry is introduced into the annular space surrounding the tubing and stinger and after subsequently filling the interior of the tubing and stinger with mud the apparatus is pulled thereby leaving in the borehole a cement plug with an open or soft core. The centralization of the stinger ensures that after hardening of the cement a cement layer of a constant thickness is created alongside the borehole wall.

Description

~32~573 METHOD AND APPARATUS FOR PLACING
A CEMENT LINING IN A BOREHOLE

The invention relates to a method and apparatus for placing a cement llning alon~side the wall of a borehole.
In the ar~ oE drllling wells in subsurface earth formations it is known to stabilize and relnforce the borehole , wall by a cement lining.
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 af temporary casing pipes i~ run into the well. Subsequently a cement slurry '1 i8 pumped into the annular space around the pipes using a weighted mud. As soon as the mud has reached the lower end of the casiny pipes the casing is pulled and guide plates mounted at ~he lower ~; end of the casing spread the cement slurry mix over the borehole wall. The mud is lefk in situ where its specific gravity enables it to act as shuttering whilst the cement hardens.
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 wlthln the borehole ' which results in an uneven thickness of the cement layer.
i ~, An object of the present invention is to remedy this ;, ~ drawbaak of the known process and to provide a method and an apparatus for placing a cement lining in a well such tha~ even in a deviated well a cement lining of a constant thickness is ,~ created.

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; Thus, accord~ng to one aspect, th~ invention provides a method of placing a cement lining alongside the wall of a borehole, the me~hod comprlsing~
- connec~ing a ~tinger to an end o~ a tublng, ~ - lowering the stin~er and tubing lnto the bor0hole until ; the stinger has reached the bottom of a section of the borehole which is to be llned, - introducln~ a cement slurry in an annular space surrounding the tubing and stinger and pumplng a slug of drilllng mud into the tubing and stinyer until the . mud haæ substantially reached the lower end o~ the ;, stinger and the cement ~lurry sub~tantially ~ills said annular Bpace over the length of the borehole sectlon which is to be l~ned~
- pulling the tubing and ~tinger in upward dlrection ', through the borehole ~hile centralizing the ou~le~ of j~ the stinger in ~he borehole using centralizer mean~
which are held in contact with the borehole wall and drag through the cement slurry alongside the borehole ~ wall, and ,.j .j - allowi~g the cement to set.
~ Accerdlng to another aspect, the lnvention pro~ide~ an ., apparatus for placing a ce~ent linlng alongside the wall of a borehole, the apparatus comprising a s~in~er connected ~o an end of a tubing and means for cen~ralizlng tha stinger in the .~ borehole, said centralizer means comprising a 6eries of , .

~32~73 2a- 63293-3111 centralizer elements which protrude in lateral direction away from the stinger such that ln use the elements are held in contact with the boxehole wall when the ~tinger and tubing are pulled in upward direction through the borehole.
Experlments have shown ~ha~ by proper ~lzing of the centrali~er means the tracks formed ln the cement ~lurry along~ide khe borehole wall by the sliding centralizer means fade away before hardening o~ the cement ælurry so that no irregularitie~
are left in th~ cemen~ layer.
Preferably the centralizer means co~prise a series of bow ~prlng centrali7er blades which axe mounted near the lower end o~ the s~inger. Utilization of bow sprlng cen~ralizer blades will generally be sufficient in gauge, vertical, holes and in wa~h-outs. However, if cementing operations are carried out in highly deviated holes i~ is preferred to provide the centralizer means in additlon with rlgid ribs which are mounted at equally distributed angular intervals around the stinger. In that case the rigid centralizer ribs give centralization in the gau~e and devlated hole sections whereas the bow spring centralizer blades ~tabilize the stinger ln the wash-outs.
It ls furthermore pre~erred to use ben~onitic mud ~o create a skable channel in the cament plug, because as 500n as it comes into contact wlth the cement the bentonite reacts with the cement. As a result a very vi~cous paste-like mud channel is le~t behind in the cement plug.

132~73 -2b- 63293 3111 After the cement has set ~he mud channel may be reamed or dressed to a desired size. The mud channel then acts as a guide to i,~
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1324~73 the bit. The bit may be of a special type, e.g. equipped with a nose section that can easily follow the mud channel.
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 ce~ent plug in a borehole.
However, in the absence oi 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 ce~ent lining of a constant thickness can be created.
The invention will now be explained in more detail with reference to the accompanying drawings ~n which - Fig. lA-lD show different stages of the well lining method according to the invention, and - ~ig. 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 ,~ 25 - Fig. 4 shows an embodiment of the apparatus where the stinger is co~nected to the tubing by a flexible hose~
Fig. LA 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.
The cement slurry is injected via a tubin~ 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 l by a bow spring centralizer 8, while the outlet 9 of ; the stinger is located just above the botto~ lO of the hole 1.

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~32~73 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.
The cement slurry 3 removes the scavenger slurry 12 from the annular space 7 and the cemen~ 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 , 20 and stinger 6 are pulled in upward direction as shown in Fig. lB.
,~ To compensate for the net tubing volume being pulled out of the ~ hole additional mud is allowed to flow into the tubing.
i5 ~ During the upward movement of the tubing 5 and stin~er 6 the stinger spots a mud channel 15 in the plug of cement slurry 3. The diameter of the mud ~hannel does not depend on the diameter of the outlet 9 of the stinger 6 but on the outer diameter of the tubing 5 j~ which is pulled through the collar.
1 After the outlet 9 of the stinger 6 is above the zone to be i treated as illustrated in Fig. lC the annular space 7 is opened and excess csment and heavy mud are circulated from the borehole.
Subsequently the cement is allowed to set.
',~ As illustrated in Fig. lD a drilling bit 17 is subsequently i 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 i~ 35 to centralize the bit in the borehole. The bit 17 may be an Ji~
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5 ~324573 underreamer bit or an eccentric bit and may be equipped with a nose (not shown) that can easily follow the mud channel 15.
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.
Alternatively said next section may have the same or larger l diameter than the previously treated sections and it may be ¦ 10 provided with a conventional steel casing or it may be left uncased, or be lined in the same manner as described before.
As shown in Fig. 2 the cement in;ection apparatus may be provided with a stinger 20 which is suspended from a coiled tubing 21 and equipped with a mixing sub 22.
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 ce~ent are ~ixed within the sub 22 in response to pulling of the tubing 21 and stinger 20 after in~ecting a cement slurry 25 ; into the annular space of a well interval which is to be treated.The cement which enters the top 26 is mixed with mud which flows downwardIy 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/cemqnt mixture in the centre 27 of the borehole 28 3~ forms a soft core which can be easily drilled out.
To ensure stabile centralization of the apparatus in the borehole 28 a series of rigid ribs 29 and a series of bow spring ' 30 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 tbe 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 ~,~
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132~73 blades having a larger stiffness than the other stabilizer blades 30. Alternatively the stabilizer means may consist of a steel pin scraper or oi a series of circumferentially spaced expandable arms which are held in contact with the borehole wall by spring action.
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 ll) out the soft core a cement lining with a regular thickness is left alongside the borehole wall 28.
, Fig. 3 shows an embodiment of the apparatus according to the I 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 ~i of the tubing 34 until the stinger 33 is in the extended position.
Fig. 4 shows an embodiment of the apparatus accordin~ 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 i`lexible hose 45. The flexible hose 45 avoids that 3~ ~ 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.
The cement slurry may contain various additives to adapt its physical properties to well operations. La~ex, polymers and epoxies may be added to the slurry to optimize the elastic properties of ~j 30 the cement and polypropylene or other fibres may be added to the .j~ slurry to i~prove the impact resistance of the cement and to plug off loss zones. Furthermore the wear resistance of the ~e~ent can .~ be impro~ed by adding wear resistant granules whereas the friction coefficient of the cement can be decreased by adding graphite, for example.
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132~73 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.
It is furthermore preferred to use bentonitic mud to create S the channel in the cement plug because, as soon as it comes into contact with the hydraulic cement, the bsntonite 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.
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.
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 J~ cake can be washed away before placing the cement. However a disa~vanta~e of suspending the stinger from a pipe string is that pullin~ of t~e stinger has to be interrupted to break the pipe ll connections. These interruptions may cause balloons in the mud iJ~ channel because of pressure relaxation.
To avoid creating of balloons in the mud channel it is generally preferred to suspe~d the stinger from a small diameter ~l~ 25 coiled tubing which can be pulled at a constant speed throughout ~ the interval to be treated.
`~ In view oi the large annular space around such a small diameter coiled tubing it may be necessary to attach steel reinforced r~bber wipers tu the outer surface of the stinger and/or -,~ 30 tubing. These w~pers act as a flow restriction in the annular space resulting locally in high an~ular 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.

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It will further be understood that after in;ecting 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 ~he cement plug in order to create the mud channel or soft core thereln.
Finally, it will be understood that instead of in~ecting 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 whPn 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 `l 15 completely filled with mud, whereupon the tubing and stinger are' retrieved from the borehole in the manner described with reference -.
to Figure lB.
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 tha invention.

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Claims

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.