U.S. patent application number 09/328729 was filed with the patent office on 2002-05-23 for cable connection to sensors in a well.
Invention is credited to CHOUZENOUX, CHRISTIAN JEAN MARCEL, MONTILLET, STEPHANIE MARIE-ODILE, RAMOS, ROGERIO TADEU, WIJNBERG, WILLEM A..
Application Number | 20020060070 09/328729 |
Document ID | / |
Family ID | 23282173 |
Filed Date | 2002-05-23 |
United States Patent
Application |
20020060070 |
Kind Code |
A1 |
RAMOS, ROGERIO TADEU ; et
al. |
May 23, 2002 |
CABLE CONNECTION TO SENSORS IN A WELL
Abstract
A description of a cable for connection to sensors permanently
downhole is provided, the cable comprising a plurality of elongate
conductors capable of operative connection to sensors, a sheath
surrounding the elongate conductors and holding the conductors so
as to extend substantially parallel to an elongate axis, wherein
the sheath has a cross-section, perpendicular to the direction of
the elongate axis, which has a major dimension and a minor
dimension. The cross-section of the cable is thus flattened and can
be in the shape of an ellipse, a crescent or comprise a circle with
wing-like portions attached on opposite sides of the circle. The
sheath is made from a resilient material, so as to provide a robust
outer surface of the cable which ensures the cable can be placed
downhole without breaking. A number of the elongate conductors are
grouped together and inter-weaved in a helical arrangement, so as
to reduce electrical cross-talk between the conductors.
Strengthening cords are included in the sheath which are hollow to
allow passage of fiber optic cables within the wire cords. A method
of cementing a well is also provided, comprising forming a
borehole, placing elongate tubing within the borehole to form an
annulus in the borehole, and placing within the annulus a cable
with a cross-section which has a major and a minor dimension, such
that the minor dimension extends along a radius of the borehole,
and passing cement, or thixotropic fluid, downhole to secure the
cable in the annulus.
Inventors: |
RAMOS, ROGERIO TADEU;
(BETHEL, CT) ; WIJNBERG, WILLEM A.; (HOUSTON,
TX) ; CHOUZENOUX, CHRISTIAN JEAN MARCEL; (SAINT
CLOUD, FR) ; MONTILLET, STEPHANIE MARIE-ODILE;
(CHATENAY, FR) |
Correspondence
Address: |
INTELLECTUAL PROPERTY LAW DEPT
SCHLUMBERGER-DOLL RESEARCH
OLD QUARRY RD
RIDGEFIELD
CT
06877
|
Family ID: |
23282173 |
Appl. No.: |
09/328729 |
Filed: |
June 9, 1999 |
Current U.S.
Class: |
166/285 ;
166/65.1 |
Current CPC
Class: |
H01B 7/046 20130101;
H01B 7/0869 20130101 |
Class at
Publication: |
166/285 ;
166/65.1 |
International
Class: |
E21B 033/13 |
Claims
We claim:
1. A cable for connection to sensors permanently downhole, the
cable comprising one or more elongate conductors capable of
operative connection to sensors, a sheath surrounding the elongate
conductors and holding the conductors so as to extend substantially
parallel to an elongate axis, wherein the sheath has a
cross-section, perpendicular to the direction of the elongate axis,
which has a major dimension and a minor dimension.
2. A cable accordingly to claim 1, wherein the cross-section of the
sheath is substantially in the shape of an ellipse.
3. A cable according to claim 1, wherein the shape of the
cross-section comprises a circle with wing-like portions attached
on opposite sides of the circle.
4. A cable according to claim 1, wherein the cross-section of the
sheath is substantially in the shape of a crescent.
5. A cable according to claim 1, wherein the sheath comprises a
resilient material, so as to provide a robust outer surface of the
cable.
6. A cable according to claim 5, wherein the resilient material is
a thermoset material to allow for ease of welding of electrodes to
the cable.
7. A cable according to claim 1, wherein the conductors are each
made from a solid conductive material.
8. A cable according to claim 7, wherein the conductors are plated
with a protective material to provide protection against corrosive
liquids and gases.
9. A cable according to claim 7, wherein each conductor is
insulated with a polymer material so as to electrically isolate the
conductor from other conductors carried within the sheath.
10. A cable according to claim 1, wherein a number of elongate
conductors are grouped together and inter-weaved in a helical
arrangement.
11. A cable according to claim 1, wherein further comprises one or
more strengthening elements, spaced from the conductors.
12. A cable according to claim 11, where each strengthening element
comprises a hollow wire cord.
13. A cable according to claim 12, wherein the hollow wire cord
allows passage of a fibre optic cable within the wire cord.
14. A method of cementing a well, comprising forming a borehole,
placing elongate tubing within the borehole to form an annulus in
the borehole, and placing within the annulus a cable with a
cross-section which has a major and a minor dimension, such that
the minor dimension extends along a radius of the borehole, and
passing cement, or thixotropic fluid, downhole to secure the cable
in the annulus.
15. A method according to claim 14, wherein the cable is placed so
as to adjoin the elongate tubing.
16. A method according to claim 14, further comprising securing the
cable to the elongate tubing before the tubing is placed
downhole
17. A well comprising a borehole, elongate tubing placed within the
borehole so as to form an annulus extending along the length of the
borehole, and a cable placed within the annulus, the cable having a
cross-section, perpendicular to the length of the borehole, which
has a major and a minor dimension.
18. A well according to claim 17, wherein the cable adjoins the
elongate tubing, such that the minor dimension of the cross-section
runs along part of the borehole radius.
19. A well according to claim 18, wherein the cable is secured
within the annulus by introducing cement, or thixotropic fluid,
into the annulus.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a cable for connection to sensors
permanently downhole within a well, to a method of placing such a
cable downhole, and to a well with such a cable permanently in
position.
BACKGROUND OF THE INVENTION
[0002] Cables used within wells to provide power downhole are
typically circular in cross-section, although it is known to use
power cables with a non-circular cross-section downhole. These
power cables are placed within production tubing to reach, for
example, a motor or pump and are large gauge insulated copper
conductors bound together with a pre-formed/interlocking steel
tape. The power cable is not placed permanently downhole, generally
being replaced when the motor or pump to which it supplies power is
removed from the well for repair or maintenance.
SUMMARY OF THE INVENTION
[0003] It is an aim of the present invention to provide an improved
cable for use downhole.
[0004] In accordance with one aspect of the present invention,
there is provided a cable for connection to sensors permanently
downhole, the cable comprising a plurality of elongate conductors
capable of operative connection to sensors, a sheath surrounding
the elongate conductors and holding the conductors so as to extend
substantially parallel to an elongate axis, wherein the sheath has
a cross-section, perpendicular to the direction of the elongate
axis, which has a major dimension and a minor dimension. The cable
thus has a substantially elongate, or flattened, cross-sectional
shape.
[0005] Such a cable is particularly advantageous in permanent
monitoring of wells producing oil where sensing of parameters
downhole is required throughout the life of a well.
[0006] The cross-section of the sheath may be substantially in the
shape of an ellipse, which simplifies manufacture of the cable.
However other types of cross-section are also suitable, and thus
the sheath may have a cross-section where the major dimension and
minor dimension are provided by a shape comprising a circle with
wing-like portions attached on opposite sides of the circle.
Alternatively the cross-section may be substantially in the shape
of a crescent.
[0007] The sheath preferably comprises a resilient material, so as
to provide a robust outer surface of the cable which prevents the
cable breaking when being installed downhole.
[0008] The resilient material may be a thermoset material, such as
nitrile rubber, to allow for ease of welding of electrodes to the
cable.
[0009] The conductors are preferably each made from a solid
conductive material, such as copper, so as to provide maximum
conductivity in minimum cross-sectional area. As the cable is
intended primarily for use downhole, the conductors may desirably
be plated with a protective material, such as nickel, to provide
protection against corrosive liquids and gases.
[0010] Additionally, the conductors may also include optical
fibres.
[0011] Preferably each conductor is insulated with a polymer
material, such as ethylene propylene copolymer, so as to
electrically isolate the conductor from other conductors carried
within the sheath.
[0012] Further reduction in electrical interaction between the
conductors may be achieved by a number of elongate conductors being
grouped together. Each group has the conductors inter-weaved in a
helical arrangement so as to reduce electrical cross-talk amongst
the different conductors within the group.
[0013] Typically the cable will include four groups of conductors,
each group consisting of four conductors. However the number of
groups used, and the number of conductors in those groups, will
depend on the number of conductors used in the cable.
[0014] The cable may also comprise a plurality of strengthening
elements, spaced from the conductors, so as to improve robustness
and rigidity of the cable. Typically each strengthening element is
a wire cord or rope of greater diameter than each group of
conductors, and generally a first wire rope is placed near one end
of the major dimension, and a second wire rope placed near the
opposite end of the major dimension. The wire ropes provide crush
resistance should the cable be subjected to force perpendicular to
its elongate axis, and also stiffen the cable and provide axial
strength. Cable stiffness is of particular advantage when feeding
the cable downhole and cementing the cable in place.
[0015] The wire cord may comprise a number of separate strands and
may be hollow to allow passage of a fibre optic cable within the
wire cord. This is of particular use where optical signals are to
be transmitted along the length of the borehole as the hollow wire
cord provides both a conduit for the fibre optic cable and also a
protective shield for the fibre optic cable.
[0016] The invention also lies in a method of cementing a well,
comprising forming a borehole, placing elongate tubing within the
borehole to form an annulus in the borehole, and placing within the
annulus a cable with a cross-section which has a major and a minor
dimension, such that the minor dimension extends along a radius of
the borehole, and passing cement, or thixotropic fluid, downhole to
secure the cable in the annulus.
[0017] The cable may have the preferred features as set out
above.
[0018] The cable preferably adjoins the elongate tubing, such that
the major dimension of the cross-section extends generally in an
arc within the annulus.
[0019] As the minor dimension of the cross-section runs partially
along a radius of the borehole, the distance from an outside wall
of the borehole to the cable is maximised. This reduces the
likelihood of mud not being displaced from the region between the
cable and the outer wall of the borehole when cementing occurs.
[0020] The method may further comprise securing the cable to the
elongate tubing, or sections of tubing before the tubing is placed
downhole. The cable may be secured by clamps designed to withstand
pressure downhole.
[0021] In accordance with a further aspect of the present
invention, there is provided a well comprising a borehole, elongate
tubing placed within the borehole so as to form an annulus
extending along the length of the borehole, and a cable placed
within the annulus, the cable having a cross-section, perpendicular
to the length of the borehole, which has a major and a minor
dimension.
[0022] The cable may have the preferred features as set out
above.
[0023] The cable preferably adjoins the elongate tubing, such that
the minor dimension of the cross-section runs along part of the
borehole radius.
[0024] Desirably the cable is secured within the annulus by
introducing cement, or thixotropic fluid, into the annulus.
[0025] The substantially elongate cross-section of the cable
ensures that by appropriate placing of the cable, the distance
between the cable and the outer wall of the borehole is maximised.
This improves the likelihood of successful cementing of cable into
the borehole.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The invention will now be described by way of example, and
with reference to the accompanying drawings in which:
[0027] FIG. 1 shows a schematic diagram of a well with a cable
placed within a well borehole in accordance with the various
aspects of the present invention;
[0028] FIG. 2 shows a cross-section through a preferred embodiment
of a cable in accordance with the present invention;
[0029] FIG. 3 shows a sectional view along line III-III of FIG.
1;
[0030] FIG. 4 shows an equivalent sectional view to that depicted
in FIG. 3 for two further embodiments of a cable according to the
present invention;
[0031] FIGS. 5 and 6 show schematic diagrams illustrating how a
borehole is cemented;
[0032] FIG. 7 shows a sectional view along line VII-VII of FIG. 8
where an annulus between casing and a wall of a borehole is of
variable width; and
[0033] FIG. 8 shows a schematic diagram illustrating cementing for
an annulus of variable width.
DETAILED DESCRIPTION OF THE INVENTION
[0034] A schematic diagram of a well 10 is shown in FIG. 1, where a
borehole 12 has been drilled down to a stratum 14 from which oil or
another substance is to be produced. Casing 16, through which oil
flows to reach surface 20, is shown positioned within the circular
cross-section borehole 12. In many cases, the oil flows through one
or more production tubings that are provided within casing 16.
Running alongside the production tubing, or casing, is a cable 22
which is permanently positioned within the borehole by cement 24
that has been injected into the casing to displace mud left within
the borehole 12 after the drilling process. The cable 22 is
connected to surface electronics 26 on surface 20, and a number of
sensors 28 are in contact with the cable 22 along its length to
permanently monitor the well over its lifetime. Note, that
according to the present invention, cable 22 could be positioned
between a production tubing and a casing 16.
[0035] A cross-section of one preferred embodiment of the cable 22
is shown in FIG. 2, and from this it will be seen that the cable
cross-section 30 is of substantially elliptical shape. The
flat-pack design cable comprises sixteen conductors 32 arranged in
four groups 34, 36, 40, 42 of four conductors and two wire ropes
44, 46 at respective ends of the major dimension of the
cross-section. A filler material 50 surrounds and secures the
conductors 32 in fixed relation to the wire ropes 44, 46 and also
provides an external jacket 52 of the cable.
[0036] The cross-section of the cable is flattened and elongate
when compared to a conventional circular cable, reducing the
likelihood of the cable snagging on the casing when the cable is
placed downhole. Typically the cable 22 is placed downhole by
securing the cable to the outer wall of the casing or production
tubing using protectors and centralisers, and then fed downhole as
successive portions of casing or production tubing are inserted in
the borehole.
[0037] The electrical core of the cable 22 which provides power to
sensors downhole, consists of the four identically sized groups 34,
36, 40, 42 of metallic conductors 32. The conductors 32 are made
from solid strands of copper, each strand being externally plated
with a layer 54 of nickel so as to resist corrosion from any liquid
or gas contacting the conductors when the cable 22 is downhole.
Each conductor is electrically isolated from the three other
conductors in their respective group by an outermost coating 56 of
ethylene propylene copolymer insulation. Other insulators may be
chosen depending on a particular well's downhole
characteristics.
[0038] To further reduce electrical communication between
respective conductors within a group and to improve cable handling,
the conductors within each group are twisted together in a helix so
as to reduce electrical cross-talk between circuits within the
cable 22. When the conductors 32 are twisted together into this
helical arrangement, a polysulphide rubber compound filler 56 is
used to fill the voids in the helix and resulting groups are
encased within Mylar tape binder 58, and also Neo Nylon binder. In
this way, all interstitial spaces in the helix are filled and a
composite group of conductors is produced ready for assembly into
the cable 22. Cable 22 could also comprise coaxial cables for
increased bandwidth.
[0039] As shown in FIG. 2, the bundles 34, 36, 40, 42 of conductors
are placed adjacent one another with the two wire ropes 44, 46
spaced from the four adjoining bundles. A nitrile rubber jacket 52
surrounds and envelops the two wire ropes 44, 46 and the four
bundles of conductors to secure them in a fixed relationship. The
wire ropes and bundles are positioned along the longest axis, or
major dimension, of the cross-section, so maximising the number of
conductors that can be provided within the narrow cross-section
cable 22.
[0040] The nitrile rubber jacket and filler used in the groups
ensure that the cable is free of voids, so minimising any fluid
passage that might occur within the cable in the axial
direction.
[0041] Integral electrodes for sensing purpose can be moulded onto
the cable to limit interface problems between the cable and
electrodes. The solid copper conductors ensure that welding of
electrode wires running along the outside of the cable to the
conductors is relatively straightforward, such welding also being
assisted by the thermosetting qualities of the nitrile rubber
jacket which ensures it is less time consuming to weld electrodes
to the cable conductors. Where electrodes are welded onto the
conductors, the conductors are replated with nickel over the weld
area to ensure that a continuous layer of corrosion protective
coating is maintained.
[0042] The wire ropes 44, 46 have a greater diameter than each
composite bundle of conductors and so provide protection for the
bundles should the cable 22 be crushed transversely to its
direction of elongation, such as when installing the cable. The
wire ropes 44, 46 also provide axial strength and stiffen the cable
22, so improving rigidity and robustness of the cable when
positioning downhole. The stiffness is also of advantage when the
cable 22 is cemented into position within the borehole 12.
[0043] The wire ropes 44, 46 can be armoured single or
multi-conductor logging cables, or logging cable that includes one
or more optical fibres. Single-mode optical fibres 60, 62 are shown
included in the cable in FIG. 2, and are placed centrally within
each wire rope and encased in a stainless steel tube 64, 66 is
filled with gel which runs along the centre of the wire rope. The
optical fibres 60, 62 are thus protected from breakage both by the
cushioning effect of the gel and the rigid case provided by the
wire ropes 44, 46.
[0044] The cable jacket material, whilst typically nitrite rubber,
may be made of any other material which resists the conditions
downhole, although is desirably of a thermoset material that allows
for easy over-moulding of electrodes which may be attached to the
cable where resistivity measurements are required downhole.
[0045] In FIG. 3, a sectional view through the well along line
III-III of FIG. 1 is shown. This illustrates the position of the
cable 22 shown in FIG. 2, and compares this with a circular
cross-section cable. Typically the borehole has a diameter of
8{fraction (1/2)} inches, and the production tubing or casing 16,
placed centrally within the circular cross-section borehole 12 has
a diameter of 51/2 inches, so forming an annulus of 11/2 inches in
width. The respective diameters of the casing and borehole may
vary, for example a borehole of 121/4 inches with a casing of 95/8
inches may be used or a borehole of 81/2 inches, with a casing of
41/2 inches diameter.
[0046] The cable 22 is placed in annulus 70 formed between an outer
wall 72 of the casing and the wall 74 of the borehole 12. The cable
22 adjoins the casing 16 such that a major dimension 76 of the
cross-section of cable 22 runs at right angles to the borehole
radius, and thus extends generally along an arc within annulus 70.
A minor dimension 78 of the cross-section extends along part of the
borehole radius, with a gap 80 of length L left between the cable
22 and the wall 74 of the borehole. The gap 80 is much larger than
a gap 84 that would be achieved if a circular cross-section cable
86 were placed in the annulus 70.
[0047] Thus use of a flat-pack design cable, which has a flattened
cross-section, increases the spacing between the wall of the
borehole and the cable over the spacing that is possible with a
circular cross-section cable. The limited space between casing and
the wall of the borehole can thus be used more effectively,
particularly when cementing the cable permanently in position
downhole, for the reasons as discussed below.
[0048] Other flattened cross-sections are equally suitable to
achieve the increase in spacing between the cable and the wall of
the borehole, and two further cable cross-sections are illustrated
in FIG. 4.
[0049] Cable 90 has a crescent-shaped cross-section, with an inner
concave surface 92 of the crescent adjoining the casing wall 72.
This generally eliminates any gaps that may occur between the cable
and the casing wall 72, and avoids complications during cementing
of the cable in the annulus 70.
[0050] A further preferred embodiment of a cable in accordance with
the present invention is shown in FIG. 4, this third embodiment 94
being comprised of a central circular cross-section cable 96
modified in cross-section by the addition of wings 100, 102 which
are moulded onto the cable 96 so as to create an integral flattened
cross-section. As with crescent-like cable 90, one surface of the
cross-section is substantially concave and this surface is placed
so as to adjoin the casing wall 72.
[0051] The flattened cross-section of the cable has certain
advantages in connection with placing the cable permanently
downhole in the annulus between the casing and the wall of the
borehole. The flattened cross-section is less likely to catch on
the wall or casing and be damaged, and in particular provides
certain advantages when cementing the cable in place downhole.
[0052] Conventional cementing technology involves isolating the
inside of an oil well from a surrounding rock formation by running
casing inside the borehole. The outer diameter of the casing is
usually one or two inches smaller than the borehole diameter, and
cementing is required to displace the annulus of drilling mud,
which sits between the casing and the outer wall of the borehole,
with cement so that materials from the production stratum can only
leave the borehole through the casing in a controlled manner.
However successful cementing can be prevented where the distance
between the casing and the outer wall of the borehole varies,
whether due to the casing not being placed centrally in the
borehole or due to other bodies narrowing the distance.
[0053] The cementing process will now be briefly described with
reference to FIGS. 5 to 8.
[0054] In cementing as shown in FIG. 5, cement 110 is pumped down
the inside casing 16, where a rubber plug 112 separates the cement
110 from drilling mud 114. The rubber plug 12 is forced downhole by
the pressure of the cement 110, and when the rubber plug 112
reaches a bottom, or shoe 116, of the casing 16, it bursts under
pressure so that the cement and mud are then in contact for the
first time, see FIG. 6.
[0055] If the casing 16 is centralised in the hole 12, then the
cement 110 pushes the mud 114 out of the annulus 70 so that a
mud/cement interface 120, 122 is independent of angle 0 (see FIG.
7). This constant width annulus is the optimal geometry for mud
displacement.
[0056] However in reality the casing 16 is often distorted from a
true circular cross-section, see FIG. 7, resulting in an annulus of
varying width, such as with a wide gap 130 at .theta.=0 and a
narrow gap 132 at .theta.=180.degree.. In this situation, the fluid
will flow fast on the wide side of the annulus 70 and be static or
slow flowing on the narrow side, see FIG. 8. Because the mud 114
has a yield stress, the stress applied to the mud 114 from the
narrow side can be so small that the mud does not yield and remains
as an immobile solid. The cement will then only push the mud 114
from the wide side of the annulus, and the mud/cement interface
will vary over the annulus. The isolation of the inside of the well
has then failed and remedial work needs to be performed to ensure
full isolation.
[0057] Similar problems arise if a circular cable is attached to
the outside of the production casing, as this produces a wide and
narrow side to the annulus. Increasing the distance from the cable
to the wall of the borehole by reducing the width of the cable (as
with the first embodiment) improves the likelihood of a successful
cementing process as the different in flow about the annulus is not
as great. Similarly flattened or crescent-shaped cables reduce the
risk of leaving mud on the narrow side of the annulus, when
compared to a circular cable with the same cross-sectional
area.
* * * * *