Note: Descriptions are shown in the official language in which they were submitted.
~32~573
- 1 - 6 3 2 9 3 - 3 1 1 1
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.
132~7~
-2- 63293-3111
; 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,~
~ .
, ~
~,
,1
;
., ~.
:
,~
,., ~
` ;;'`~
1324~73
- 3 -
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.
:
~32~73
- 4 -
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~
` i ~
~,~
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
~,~
. .
. ,
:
~;~'i `
,~
132~73
-- 6
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.
; :
`I
132~73
- 7 -
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.
:
:,~
::~
: i\
132~73
~ - 8
;
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.
~?
: `
.1:
,~ ,
: '.~
,~
