GB2361019A - Polyamide Casing Centraliser - Google Patents

Abstract

A casing centraliser 1 comprises an annular body 2 with a plurality of ribs 3 extending longitudinally on the outer surface. A longitudinal bore extends through the annular body 2. The centraliser 1 is formed by casting a polyamide material, preferably using a monomer casting technique with centrifugal or gravity poured raw material. The ribs 3 may be hollow and contain ports 4 which allow the circulation of fluid through the ribs 3. Conduction means such as power lines or optical fibres may be contained within the ribs 3. The ribs 3 may also spherical members (8, fig 3) to reduce friction with the wellbore in use.

Description

1 1 IMPROVED CASING CENTRALISER 2 3 2361019 The present invention relates

to centralisers as are typically for centering casing and to assist in the task of cementing casing in oil or gas wells, wherein the centraliser is made from a cast polymer material.

It is conventional practice after drilling an oil or gas well to run tubing, known as casing, into the well bore to act as a liner. The casing stabilises and prevents the bore from collapsing inwards. The casing is run into 12 the newly formed bore from the surface, and the annular 13 space between the casing and the bore is then filled with cement. Typically, cement is pumped to the bottom of the bore from where it is displaced into the annular space between the external surface of the casing and the interior of the well bore. The cement acts as a sealant and also to structurally support the casing in place.

16 17 18 19 20 It will be appreciated that before the casing is cemented 21 in place, it is important that the casing is held 22 substantially central within the bore. This allows a 23 strong cement bond to be formed around the casing by 2 1 ensuring that an even thickness of cement is displaced 2 around the casing. Ensuring that the casing is in a 3 central position within the bore also prevents 4 channelling of the cement or the creation of void spaces, as often occurs if the casing rests against the wall of 6 the wellbore during the cementing procedure. 7 8 It is known in the art that fitting a centraliser to the 9 casing will support the casing or liner off the well bore 10 wall. It will be appreciated that providing standoff is 11 particularly important as well bores are never truly 12 vertical and often contain sharp bends and deviations in 13 direction. Typically, one or more centralisers are 14 mounted on the outside of the casing as it is lowered 15 into the bore to centre the casing. The centralisers act 16 to reduce friction and create standoff from the well bore 17 which allows the casing to be rotated and cemented in a 18 satisfactory manner. 19 20 21 22 23 24 25 26 27 28 29 There are several types of centraliser known to the art. One commonly used centraliser is the bow type, which comprises hollow cylindrical components or two axially spaced apart collars which are interconnected by spring bows. So called rigid or solid centralisers have a fixed outer diameter and are therefore less flexible than the spring bow type centraliser. It is common to include a number of "blades' around the body of solid centralisers. These are typically raised solid structures which extend longitudinally around the centraliser body which engage 30 the well bore wall and optimise standoff and therefore 31 aid the centralising action of the tool. 32 3 1 It is also becoming more common to incorporate rollers on 2 the body of centralisers to aid the rotation of the 3 centraliser and therefore prevent "sticking" and also to 4 reduce friction, torque and drag which are afforded to 5 the centraliser as it contacts the wall of the well bore. 6 Whilst these rollers reduce the resistance and torque 7 afforded to the casing and centraliser, they are limited 8 in efficiency as the rollers must be mounted on either a vertical or horizontal axis.

11 Conventionally, centralisers are made from a strong metal 12 material. It will be appreciated that centralisers are 13 subject to huge loads, such as impact and shear forces, 14 and accordingly must be durable and hardy. However 15 although metal centralisers are able to withstand high 16 loads and the hostile downhole environment, they are not 17 particularly flexible. This is a disadvantage if the 18 centraliser encounters obstructions as it is run through 19 the well bore on the casing. For example it is not 20 uncommon for ridges or ledges to form in the wall of a 21 bore during drilling, or for debris to accumulate in as section of the well. Metal centralisers are generally not flexible enough to yield to such obstructions, and therefore there is a risk that the progress of the casing procedure will be halted and that the casing upon which the centraliser is mounted will become stuck. A further problem of metal centralisers lies in the fact that they are prone to corrosion. This is a particular problem as acid is often pumped into well bores to stimulate and 30 clean the well. 31 32 33 The most important criteria for any material to be used in the manufacture of centralisers is that the material 4 1 must be strong enough to withstand the shear forces and 2 high loads which are afforded to a centraliser in use, 3 but must also flexible enough to manoeuvre obstructions 4 in the well bore and yield under extreme load. The use 5 of plastic centralisers has been suggested for this 6 purpose. Previous attempts to manufacture plastic 7 centralisers have commonly relied on two manufacturing 8 processes, namely injection moulding and extrusion. 9 These involve physically changing processed raw material 10 plastic "pellets". The pellets are heated to a suitable 11 temperature such that they are malleable for extrusion 12 and then ejected under pressure into the desired shape of 13 a casing centraliser. However it has been found that 14 plastic centralisers manufactured in this manner are 15 restricted in their ability to withstand force. In 16 addition the "pellets" which are produced by the chemical 17 industry for extrusion and injection moulding have 18 undergone several condensate and extrusion processes to 19 deliberately thin viscosity to enable processing in 20 standard machinery. The high pressures and chilling used 21 in these processes induces an amorphous non-crystalline 22 structure to be produced and causes further degradation 23 of the material which detrimentally affects the final 24 strength and wear resistance of the centraliser. 25 26 27 28 29 30 31 32 33 It is an object of the present invention to provide a casing centraliser which is hard wearing but has sufficient compliance and resilience to negotiate a restriction in the wall of a well bore. A linked aim is to provide a centraliser which does not corrode. In particular, in the present invention it is recognised that the method of manufacture previously used to produce plastic centralisers can be improved so as to produce 9 1 centralisers which have superior flexibility and 2 durability. It has been found that the described method 3 is best carried out using polyamide material. 4 It a further object of the present invention to provide a 6 casing centraliser which has superior means for reducing 7 torque and friction afforded to the centraliser as it is 8 advanced downhole. In particular it is an object of the present invention to provide a centraliser which has sufficient compliance and resilience to negotiate any 11 unplanned restrictions in the well bore.

12 13 21 According to the present invention there is provided a 14 centraliser for mounting on casing comprising an annular 15 body having a plurality of blades around said annular 16 body, and a longitudinal bore extending through the 17 annular body, characterised in that the annular body is a 18 cast polyamide material. 19 20 Preferably the longitudinal bore makes a clearance fit around the casing on which the centraliser is attached.

22 23 Preferably the polyamide is nylon. 24 25 26 Preferably the plurality of blades are hollow.

27 Preferably each of the plurality of blades has a number 28 of integral ports which allow the flow of fluid into, and 29 out of the hollow blade.

31 Preferably the blades may contain conduction means. 32 33 Optionally the conduction means are power lines.

6 1 Alternatively the conduction means are optical fibres.

3 4 Optionally the blades extend parallel to the length of the annular body.

Alternatively the blades extend longitudinally and 8 helically around the annular body.

9 Preferably the blades are shaped in such a manner that they are separated by void areas which define a flow path 12 which permits the relative by pass of fluid between each 13 of the blades.

14 16 17 18 19 21 22 23 24 26 27 28 29 31 32 33 Optionally the annular body is rotatably mounted on the casing.

Preferably the centraliser has low friction means.

Most preferably said low friction means are ball bearings.

Preferably said ball bearings are mounted on a racer.

Preferably said ball bearings are mounted on the blades of the annular body.

Preferably the centraliser has connection means for mounting the centraliser on the casing.

Most preferably said connection means are threaded end connections which mate with the casing.

7 1 2 3 4 Preferably the centraliser has an internal diameter which is at least equal to, or greater than the diameter of the casing on which the centraliser is mounted.

Preferably the dimensions of the centraliser are not 6 limited and could be adapted to be suitable for use with 7 any size of casing. 8 According to a second aspect of the present invention there is provided a method of manufacturing a cast 9 10 11 12 13 14 Optionally the polyamide centraliser is cast by 15 centrifugally casting a liquid polyamide material.

polyamide centraliser wherein a liquid polyamide is cast into an annular shaped body by monomer casting.

16 17 Alternatively the polyamide centraliser is cast by 18 gravity pouring a liquid polyamide material. 19 20 21 22 23 24 25 26 27 28 29 Most preferably said ball bearings and annular body are 30 manufactured in one procedure. 31 Preferably the liquid polyamide is directly polymerised to a solid.

Preferably the annular shaped body has a number of ball bearings.

Preferably said ball bearings are made from polyamide material.

8 1 An example embodiment of the invention will now be 2 illustrated with reference to the following Figures i 3 which: 4 5 Figure 1 illustrates a cast polyamide centraliser in 6 accordance with the present invention; 7 Figure 2 illustrates a cast polyamide centraliser with an 8 alternative port configuration; and 9 Figure 3 illustrates a cast polyamide centraliser with 10 friction reducing means.

11 12 Referring firstly to Figure 1 a cast polyamide 13 centraliser is generally depicted at 1 and comprises a 14 polyamide annular body 2, which is essentially 15 cylindrical and hollow and which can be mounted onto a 16 casing string (not shown). The polyamide used may be 17 nylon. The annular body 2 is at least the same size as 18 the casing it is to be attached to and makes a tight fit 19 around the casing (not shown). The annular body may be 20 able to rotate or may remain stationary relative to the 21 casing. The annular body 2 has a plurality of raised 22 blades 3 which extend longitudinally and helically around 23 the annular body 2. The blades are also produced from a 24 polyamide material, typically the same polyamide as the 25 annular body 2. The blades 3 provide maximum standoff 26 27 28 29 30 31 32 33 when the centraliser 1 is in position on a casing or liner (not shown) in a well bore. Although the blades illustrated in Figure 1 extend longitudinally and helically around the annular body 2 it will be appreciated that in an alternative embodiment the blades may extend longitudinally and parallel to the annular body 2. An advantage of the present invention is that the blades 3 are hollow and have a number of ports 4 into and 9 1 out of which fluid can flow. The fluid may include 2 drilling fluid, cleaning fluid, carrier fluids or gels 3 used for gravel packing operations such as are 4 continuously passed through the section of well being bored to lubricate the drilling apparatus and wash out the bore. The ports 4 may be positioned on the wall facing surface 5 of the blades or, in an alternative embodiment shown in Figure 2, may be located at the ends of the blade 6. Because the blades are hollow, they may be used as a conduit or housing to protect conduction means such as power lines, electrical cables or optical fibres. A further advantage is that the plastic hollow blades have a greater flexibility and compliance than convention metal solid blades and can therefore negotiate any obstructions countered in the wall of the bore.

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 An important aspect of the present invention is the 26 process by which the plastic centraliser is produced. 27 Previous attempts to produce a plastic centraliser have 28 used extrusion techniques or injection moulding with pre 29 processed raw material plastic pellets. However it is generally recognised that such plastic centralisers are 31 less efficient at withstanding the huge loads and impacts 32 encountered in the downhole environment. In the present 33 invention, monomer casting is used to produce a superior The advantage of the present invention over the prior art is that the polyamide material offers a substantially lower co-efficient to friction whilst being run in the well, and has superior compliance and resilience to metal centraliser. The polyamide centraliser also has superior ability to yield to any obstructions met in the path of the casing, over metallic centralisers.

2 1 and more efficient plastic centraliser. In the present invention it is also recognised that the described manufacturing process is best carried out using a 4 polyamide material. The monomer casting is carried out using three lactums, namely caprolactum, capryllactam and 6 laurinlactum along with catalysts and activators. The 7 polyamide produced by monomer casting has a greater 8 crystalline structure, superior viscosity and greater 9 molecular weight than injected or extrusion materials.

The polyamide therefore has an increased strength and 11 wear resistance and is better suited to withstand the 12 torque and drag afforded to a centraliser in the downhole 13 environment. The castings may be produced from 14 centrifugal casting, or gravity pouring for greater density. The cast polyamide manufactured by this process 16 can also survive higher temperatures and pressures. A 17 particular advantage of this manufacturing process lies 18 in the fact that in order to monomer cast the polyamide 19 centraliser, a liquid polyamide has to be used. This is 20 21 22 23 24 25 26 27 28 29 30 31 in contrast to earlier attempts to produce a plastic centraliser which used pre-processed plastic "pellets". Therefore in the present invention, the manufacturing costs are reduced as there is no requirement for preprocessing the material, and the polyamide is directly polymerised from a liquid chemical form to a solid object in its mould.

Figure 3 illustrates a cast polyamide centraliser 1 which has a plurality of ball bearings 8 located on the externally facing surface of the annular body 2. In the depicted embodiment the ball bearings 8 are located on blades 9 which extend longitudinally and parallel to the 33 annular body. The ball bearings 8 minimises the torque 11 1 and friction which is afforded to the annular body 2 of 2 the centraliser, particularly when the centraliser is 3 rotating, or is being rotated in the borehole or inside 4 of a casing string Most importantly, the ball bearings 8 5 reduce the friction afforded in any direction on the tool 6 1. This is in contrast to roller reamers which have been 7 used in the past to reduce friction, but which can only 8 be mounted on a vertical or horizontal axis and can 9 therefore only minimise friction which is afforded in 10 these directions. In one embodiment the ball bearings 11 may be incorporated into the annular body 2 as a ball bearing racer which allows the ball bearings to %race' around the outer circumference of the centraliser.

13 14 15 It is recognised in the present invention that the ball 16 bearings may be manufactured from the same material as 17 the annular body, that is polyamide and therefore the 18 body 2 and blades 3, and the ball bearings 8 may be 19 manufactured at the same time and as a whole. 20 21 22 23 24 25 26 27 28 29 30 31 The advantage of the present invention is that by using a casting process to manufacture a centraliser from a polyamide material, it has been found that a non-metallic centraliser can be produced which is durable, resilient and compliant and which does not have the inherent problems of conventional metal centralisers, notably the risk of corrosion, and low flexibility. Furthermore, the blades which allow optimum standoff of the centraliser from the well bore are hollow and therefore have increased flexibility and can be used to house power lines and other conduction means.

12 1 2 3 4 5 6 7 8 9 10 11 An alternative production technique could be that of 12 reaction moulding which is also a system of low pressure 13 casting by polymerisation in the mould using catalysts in 14 a similar manner to monomer casting, but with the advantages of more complicated and defined sections. 16 17 Further modifications and improvements may be 18 incorporated without departing from the scope of the 19 invention herein intended.

A further advantage is that the use of ball bearings in the present invention aids rotation of the centraliser and minimises friction in all directions.

The present invention is described for use on casing. However it is recognised that the centraliser may be adapted for use on any type of work string which is used in a downhole bore, such as liner, tubing or drill string.

13

Claims (1)

1 Claims:

2 4 5 6 7 8 9 2. 10 11 1. A centraliser for mounting on casing comprising an annular body having a plurality of blades around said annular body, and a longitudinal bore extending through the annular body, characterised in that the annular body is a cast polyamide material.

A centraliser as claimed in Claim 1, wherein the longitudinal bore makes a clearance fit around the casing on which the centraliser is attached.

12 13 3. A centraliser as claimed in Claim 1 or Claim 2, 14 wherein the polyamide is nylon.

17 18 19 20 21 22 23 24 6. 25 26 27 28 29 30 31 32 4. A centraliser as claimed in any one of the preceding Claims, wherein the plurality of blades are hollow.

5.

7.

8.

A centraliser as claimed in Claim 4, wherein each of the plurality of blades has one or more integral ports which allow the flow of fluid into, and out of the hollow blade.

A centraliser as claimed in any one of the preceding Claims, wherein the blades contain conduction means.

A centraliser as claimed in Claim 6, wherein the conduction means are power lines.

A centraliser as claimed in Claim 6, wherein the conduction means are optical fibres.

14 1 9. A centraliser as claimed in any one of the preceding 2 Claims, wherein the blades extend parallel to the 3 length of the annular body.

4 10. A centraliser as claimed in any one of Claims 1 to 6 8, wherein the blades extend longitudinally and 7 helically around the annular body.

8 9 11 12 13 14 12.

16 17 18 19 21 22 23 24 26 27 28 29 31 32 A centraliser as claimed in any one of the preceding Claims, wherein the blades are shaped in such a manner that they are separated by void areas which define a flow path which permits the relative by pass of fluid between each of the blades.

A centraliser as claimed in any one of the preceding Claims, wherein the annular body is rotatably mounted on the casing.

13. A centraliser as claimed in any one of the preceding Claims, wherein centraliser has low friction means.

14. A centraliser as claimed in Claim 13, wherein said low friction means are ball bearings.

15. A centraliser as claimed in any one of the preceding Claims, having connection means for mounting the centraliser on the casing.

16. A centraliser as claimed in Claim 15, wherein said connection means are threaded end connections which mate with the casing.

17. A centraliser as claimed in Claim 15 or 16, wherein the centraliser has an internal diameter which is at least equal to, or greater than the diameter of the casing on which the centraliser is mounted.

3 4 6 18. A method of manufacturing a cast polyamide 7 centraliser wherein a liquid polyamide is cast into 8 an annular shaped body by monomer casting.

9 19.

12 13 14 20. 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 A method as claimed in Claim 18, wherein the polyamide centraliser is cast by centrifugally casting a liquid polyamide material.

A method as claimed in Claim 18, wherein the polyamide centraliser is cast by gravity pouring a liquid polyamide material.

A method as claimed in any one of Claims 18 to 20, wherein the liquid polyamide is directly polymerised to a solid.

A method as claimed in any one of Claims 18 to 20, wherein the annular shaped body has a number of ball bearings, wherein said ball bearings are also made from polyamide material, and wherein the ball bearings and annular body are manufactured in one procedure.