BACKGROUND TO INVENTION After a well has been drilled to its final depth it is necessary to secure the final borehole section by either lining the borehole with a tubular known as the casing/liner, whereby the annular space between the casing/liner and the borehole is filled with a cement slurry (termed a cased hole completion) leaving the borehole section open (termed an open hole completion).

In the first. scenario reduction tubing is then run into the lined hole and is secured at the bottom of the well with a sealing device termed a"packer"that seals the annulus so formed between the production tubing and the casing. At the top of the well the production tubing is fixed to a wellhead/christmas tree combination. The' production tubing is used to evacuate the hydrocarbon, the annulus so formed (termed the'A'annulus) being monitored to establish the condition of the production tubing. (A pressure rise in the annulus may indicate a hole in the production tubing and a requirement for the well to be secured).

In the second scenario, the final borehole section is left open and screens are run. Screens are perforated tubing with either slits or holes. These screens once in position act as a passageway in a procedure to fill the void between the borehole wall and the screen by placing sand around the screen. The sand acts as a filter and as a support to the borehole wall. The term normally used

in this operation is"gravel packing". Typically the sand/screen are"washed"after placement with a hydrochloric acid/sulphuric or nitric acid mixture. The screens tend to be weak and susceptible to bending and rupture during installation; this buckling limits the length available to be run or the screens being off centre in the hole, thus a non-uniform section of sand is placed around the pipe. Sometimes screens are expanded to the borehole wall.

Once the casing has been cemented the screens are in place the production tubing is run. Data gauges may be fitted to the bottom of the production tubing to give downhole measurements on an on-going basis. The production tubing may either be used as a conduit for the signal or a cable may be run along an outside of the production tubing. The production tubing is sometimes made of a higher grade material than the casing/liner as the production tubing is exposed to hydrocarbons, production water and gases during its life may corrode.

In either of the above cases the tubing needs to be prevented from contacting an inner wall of the casing.

In the event that a signal in the form of an electrical impulse is transmitted using the production tubing as a conductor, if contact, exists between the production tubing and casing an"earth"occurs thus stopping the signal being transmitted to surface equipment. Further, if the production tubing is made out of a material differing from the casing material, galvanic corrosion may occur in the event that the production tubing contacts the outer casing.

Prior art centralisers for screens and production tubing are typically made of aluminium, zinc or steel.

Among the disadvantages of such centralisers are: (a) Metals are conductive so inhibiting the use of electrical communications via the tubing/screen;

(b) Production tubing is generally made of a relatively expensive zinc alloy/high chromium steel compared to less expensive casing metals; thus galvanic corrosion may occur due to the presence of salt water; (c) Acid used to wash the production face causes metal centralisers to begin to dissolve and form a film or precipitate over fine slits in the screen, impeding flow of wellbore fluids into screen.

It is an object of at least one embodiment of the present invention to obviate or mitigate one or more of the aforementioned disadvantages.

SUMMARY OF INVENTION According to a first aspect of the present invention there is provided a centraliser for centralising production tubing within a casing or liner within a wellbore,'the centraliser having at least one substantially electrically nonconductive portion so as to substantially isolate the production tubing and the casing or liner one from the other.

The centraliser may act to isolate the production tubing and the casing or liner one from the other both physically and electrically.

The centraliser may seek to prevent any electrical signal passing through the production tubing from earthing to the casing/liner or vice versa and seeks to ensure no galvanic corrosion can occur between either.

Preferably the at least one nonconductive portion may be selected from a material comprising a polymeric material such as a plastics material, rubber, or an elastomeric material, or a ceramic material, cermet or submicron grained cemented carbide.

Each material has a number of advantages over the other.

The plastics material may have a Youngs modulus (modulus of elasticity) of 1, 000,000 psi or lower, and thus may expand.

Preferably the plastics material provides one or more of the following material characteristics as tested by ASTM (American Society for Testing and Materials):

Youngs Modulus 550,000 psi or 600,000 psi or higher (ASTM Test-Ref D638) Tensile strength 10, 000 psi or higher (ASTM Test-Ref D638) Friction Factor 0.35 or lower (Co-efficient of Friction) ASTM Test-Dry (thrust washer) against steel Izod input test (notched) 1. 6 and preferably 3.2 ft -lb/in or higher (ASTM Test Ref D256) HDT (Heat Deflection greater than 185°C (ASTM or Distortion Temperature) Test Ref D648 at 66 psi) Chemical resistance Able to withstand chemical attack from most common reagents found in a drilling environment, eg hydrocarbons, brines, weak alkalis and weak acids Specific gravity 1.28 In one implementation the first material may be a

polyphthalamide (PPA), eg a glass-reinforced heat stablilised PPA such as AMODEL, eg AMOEL-AT-1116 HS resin available from BP Amoco, (see http://www. bpamocoenqpolymers. com).

In another implementation the plastics material may be a polymer of carbon monoxide and alpha-olephins, such as ethylene.

Advantageously the plastics material may be an aliphatic polyketone made from co-polymerisation. of ethylene and carbon monoxide-optionally with propylene.

Advantageously the material may be CARILON (Trade Mark) available from Shell Chemicals. CARILON (Trade Mark) is a class of semi-crystalline thermoplastic materials with an alternating olefin-carbon monoxide structure.

In a further implementation the plastics material may be a nylon resin.

Advantageously the plastics material may be an ionomer modified nylon 66 resin.

The materials may alternatively be a nylon 12 resin such as RISLAN (Trade Mark) available from Elf Atochem.

In a yet further. alternative implementation the plastic material may be a modified polyamide (PA).

The material may be a nylon compound such as DEVLON (Trade Mark) available from Devol Engineering Ltd.

The material may be of the polyetheretherketone family, eg PEEK (Trade Mark) available from Victrex plc.

The plastics material may be ZYTEL (Trade Mark) available from ZYTEL in a class of nylon resins which, includes unmodified nylon homopolymers (e. g. PA66 and PA612) and copolymers (e. g. PA 66/6 and PA 6T/MPMDT, etc) plus modified grades produced by the addition of heat stabilizers, lubricants, ultraviolet screens, nucleating agents, tougheners, reinforcements, etc. The majority of resins have molecular weights suited for injection mouldings, roto-moulding and some are used in extrusion.

Alternatively the plastics material may be VESCONITE (Trade Mark) available from Vesco Plastics Australia Pty Ltd.

Alternatively the plastics material may be polytetrafluoroeth (yl) ene (PTFE).

In such case the material may be TEFLON (Trade Mark) or a similar type material.

TEFLON filled grades of PEEL CARILON (Trade Mark) may be used. These materials are suitable for roto- moulding which is a favoured method of manufacture for economic reasons for larger component sizes, eg greater than 9 5/8".

The ceramic material, may be, for example, zirconia, titania and/or alumina. The ceramic material may be toughened by the addition of a further material, for example zirconia, with the addition of alumina.

The centraliser may comprise a tubular body.

The tubular body may have a bore extending longitudinally therethrough.

The body may provide an outermost surface and an innermost surface, and the outermost surface may provide a plurality of raised portions.

The raised portions may be in the form of longitudinally extending blades or ribs or may alternatively be in the form of an array of nipples or lobes.

Adjacent raised portion may define a flow path therebetween such that fluid flow paths are defined between first and second ends of the tubular body.

Where the raised portions comprise longitudinal blades, such blades may be formed, at least in part, substantially parallel to an axis of the tubular body.

Alternatively the blades may be formed in a longitudinal spiral/helical path on the tubular body.

Each raised portion may provide a casing/liner contacting surface.

The bore through the body may be a clearance fit around a production tubing intended to be centralised by the centraliser.

In one embodiment an outermost and/or innermost surface of the tubular body may comprise a coating formed on an inner tubular body.

The coating (s) may be selected from a polymeric material such as a plastics material, rubber, or an elastomeric material, or a ceramic material, cermet or submicron grained cemented carbide, and advantageously comprise CARILON, AMODEL (Trade Marks) or similar type material.

The inner tubular body may be made substantially of a metallic material such as steel, zinc, zinc alloy, bronze, lead bronze, or preferably from aluminium or aluminium alloy. ;' In another embodiment the raised portions may be formed in one material and the tubular body from a second material.

For example, the one material may be selected from a polymeric material such as a plastics material, rubber or an elastomeric material, or a ceramic material., cermet or submicron grained cemented carbide, and advantageously comprise CARILON, AMODEL (Trade Marks), or the like type material. The tubular body may, in this embodiment, be made of a metallic material.

In a further embodiment the outermost surface may comprise part of a tubular body; the tubular body may be made from a material selected from a polymeric material such as a plastic material, rubber, or an elastomeric material, or a ceramic material, cermet, or submicron grained cemented carbide and advantageously may be CARILON, AMODEL (Trade Marks), or similar type material.

Preferably the centraliser may be of a unitary construction-that is formed in one piece and in one material.

Alternatively the centraliser may include a reinforcing means such as a cage, mesh, bars, rings and/or the like, which may be metallic.

The centraliser may, be formed by an injection moulding process. Alternatively the centraliser according to the first aspect of the present invention may be formed from a casting process.

Advantageously the centraliser according to the first aspect of the present invention may be formed from an injection moulding or roto-moulding process.

Advantageously the centraliser may be formed with the innermost surface providing at least one and preferably a plurality of spaced apart longitudinally extending tapered recesses. The recess (es) may taper from an upper end and towards a lower end of the centraliser.

It will be appreciated that the polymeric material mentioned above may include filler materials, and is known in the polymer art.

According to a second aspect of the present invention there is provided a wellbore apparatus to isolate production tubing from a surrounding casing or liner, the apparatus including a length of production tubing and at least one centraliser the at least one centraliser having at least one substantially electrically nonconductive portion, so as to isolate the production tubing and the casing or liner one from the other.

Preferably the at least one nonconductive portion may be selected from a material comprising a polymeric material such as plastics material, rubber, or an elastomeric material, or a ceramic material, cermet or submicron grained cemented carbide.

The well casing/liner is preferably of hollow tubular form.

Further the at least one centraliser may comprise a tubular body.

Preferably the centraliser may be of a unitary construction.

Alternatively the centraliser may include a reinforcing means such as a cage, mesh, bars, rings and/or the like, which may be metallic.

The centraliser may be formed by the injection moulding or roto-moulding process.

The at least one centraliser may be located so as to surround the production tubing.

The at least one centraliser may be located relative to the production tubing by means of a collar.

The at least one centraliser may be located relative to the production tubing and may be rotatable relative to the casing around longitudinal axis thereof.

According to a third aspect of the present invention there is provided a method of isolating a length of production tubing from a casing/liner in a well bore the method comprising the steps of: providing a well-bore having a casing/liner; providing a length of production tubing; providing at least one centraliser, the/each centraliser having a substantially electrically nonconductive portion to isolate the production tubing and the casing/liner one from the other. locating the at least one centraliser on the production tubing at a desired position so as to provide a centralising apparatus; and placing the centralising apparatus within the borehole.

Preferably the nonconductive portion may be a surface of the centraliser which is selected from a material comprising a plastics material or a ceramic material, cermet or submicron grained cemented carbide.

According to a fourth aspect of the present invention there is provided a centraliser for centralising a screen within a wellbore, the centraliser having at least one substantially nonconductive portion so as to substantially isolate the screen and a perforated casing or borehole one from the other.

Screens are typically perforated lengths of production tubing. The perforations are usually a regular pattern of suits or holes.

Preferably the at least one nonconductive portion may be selected from a material comprising a polymeric material, plastics material, rubber, or an elastomeric material or a ceramic material, cermet or submicron grained cemented carbide.

Each material has a number of advantages over the other.

The plastics material may have a Youngs modulus of 1, 000, 000 psi or lower.

Preferably the plastics material provides one or more of the material characteristics as tested by ASTM (American Society for Testing and Materials) as defined in the table hereinbefore.

In one implementation the plastics material may be a polyphthalamide (PPA) such as AMODEL (Trade Mark), eg AMODEL-AT-1116 HS resin available from BP Amoco (see http ://www. bpamocoenqpolymers. com) In another implementation the plastics material may be a polymer of carbon monoxide and alpha-olefins, such as ethylene.

Advantageously the material may be an aliphatic polyketone made from co-polymerisation of ethylene and carbon monoxide-optionally with propylene.

Advantageously the material may be CARILON (Trade Mark) available from Shell Chemicals. CARILON (Trade Mark) is a class of semi-crystalline thermoplastic materials with an alternating olefin-carbon monoxide

structure.

In a further implementation the plastics material may be a nylon resin.

Advantageously the plastics material may be an ionomer modified nylon 66 resin.

The material may alternatively be a nylon 12 resin such as RISLAN (Trade Mark) available from Elf Atochem.

In a yet further alternative implementation the plastics material may be a modified polyamide (PA).

The material may be a nylon compound such as DEVLON (Trade Mark) available from Devol Engineering Limited.

The material may be of the polyetherketone family, eg PEEK (Trade Mark) available from Victrex plc.

The plastics material may be ZYTEL (Trade Mark) available from ZYTEL (Trade Mark) in a class of nylon resins which, includes modified nylon homopolymers (e. g. PA66 and PA612) and copolymers (e. g. PA 66/6 and PA 6T/MPMDT, etc) plus modified grades produced by the addition of heat stabilizers, lubricants, ultraviolet screens, nucleating agents, tougheners, reinforcements, etc. The majority of resins have molecular weights suited for injection mouldings, roto-moulding and some are used in extrusion.

Alternatively the plastics material is VESCONITE (Trade Mark) available from Vesco Plastics Australia Pty Ltd.

Alternatively the material may be polytetrafluoroeth (yl) ene (PTFE).

In such case the material may be TEFLON (Trade Mark), or a similar type material.

TEFLON filled grades of PEEL CARILON (Trade Mark) may be used. These materials are suitable for roto- moulding which is a favoured method of manufacture for economic reasons for larger component sizes, eg greater than 9 5/8".

The ceramic material, may be, for example, zirconia, titania and/or alumina. The ceramic material may be toughened by the addition of a further material, for example zirconia, with the addition of alumina.

The screen casing centraliser may comprise a tubular body.

The tubular body may have a bore extending longitudinally therethrough.

The body may provide an outermost surface and an innermost surface, and the outermost surface may provide a plurality of raised portions.

The raised portions may be in the form of longitudinally extending blades or ribs or may alternatively be in the form of an array of nipples.

Adjacent raised portions may define a flow path therebetween such that flow paths are defined between first and second ends of the tubular body.

Where the raised portions comprise longitudinal blades, such blades may be formed at least in part substantially parallel to an axis of the tubular body.

This arrangement is particularly advantageous for screen centralisers.

Alternatively the blades may be formed in a longitudinal spiral/helical path on the tubular body.

. Each raised portion may provide a wellbore contacting surface.

The bore through the body may be a clearance fit around a screen intended to be centralised by the centraliser.

In one embodiment an outermost and/or innermost surfaces of the tubular body may comprise a coating formed on an inner tubular body.

The coating (s) may be selected from a polymeric material such as a plastics material, rubber, or an elastomeric material, or a ceramic material, cermet or submicron grained cemented carbide, and advantageously

comprise CARILON, AMODEL (Trade Marks), or similar type material.

The inner tubular body may be made of a metallic material such as steel, zinc, zinc alloy, or preferably from aluminium or aluminium alloy.

In another embodiment the raised portions may be formed from one material and the tubular body from a second material.

For example, the one material may be selected from a polymeric material, such as a plastics material, rubber, or an elastomeric material, cermet or submicron grained cemented carbide and advantageously comprise the tubular body may in this embodiment be made of a metallic material.

In a further embodiment the outermost surface comprises part of a tubular body, and the tubular body may be made from a material selected from a polymeric material such as a plastic material such as a rubber, or elastomeric material, or a ceramic material, and advantageously may be CARILON, AMODEL (Trade Marks), or similar type material.

Preferably the centraliser may be of a unitary construction-that is formed in one piece and in one material.

Alternatively the centraliser may include a reinforcing means such as a cage, mesh, bars and rings and/or the like, which may be metallic.

The centraliser may therefore be formed from an injection moulding process. Alternatively the screen centraliser according to the fourth aspect of the present invention may be formed from a casting process.

Advantageously the screen centraliser according to the fourth aspect of the present invention may be formed from an injection moulding or roto-moulding process.

Advantageously the screen centraliser may be formed with the innermost surface providing at least one and

preferably a plurality of spaced apart longitudinally extending tapered recesses. The recess (es) may taper from an upper end towards a lower end of the centraliser.

It will be appreciated that the polymeric material mentioned above may include filler materials, as is known in the polymer art.

According to a fifth aspect of the present invention there is provided a wellbore apparatus including a screen and at least one screen centraliser located thereupon, the at least one centraliser having a substantially nonconductive portion so as to isolate the screen and a perforated casing or borehole one from the other.

Preferably the at least one nonconductive portion may be selected from a material comprising a polymeric material such as a plastics material, rubber or an elastomeric material or a ceramic material, cermet or submicron grained cemented carbide.

The screen is preferably of a hollow tubular form.

Further the at least one centraliser may comprise a tubular body.

Preferably, the centraliser may be of a unitary construction.

Alternatively the centraliser may include a reinforcing means such as a cage, mesh, bars, rings and/or the like which may be metallic.

The centraliser may be formed by an injection moulding or roto-moulding process.

The at least one centraliser may be located so as to surround the screen.

The at least one centraliser may be located relative to the screen by means of a collar.

The at least one centraliser may be located relative to the screen and may be rotatable relative to the screen around a longitudinal axis thereof.

According to a sixth aspect of the present invention there is provided a method of gravel packing a screen

within a wellbore or perforated casing, the method comprising the steps of: providing a screen; providing at least one screen centraliser according to the fourth aspect of the present invention ; locating the at least one centraliser on the screen at a desired position so as to provide a centralising apparatus; placing the centralising apparatus within the wellbore or within a perforated casing of the wellbore; pumping sand or the like into an annular space between an exterior of the screen and the wellbore or perforated casing.

According to a seventh aspect of the present invention there is provided an insulating sheath adapted for providing electrical isolation across an annulus formed between two tubulars within a borehole.

Preferably the sheath is made at least partly from a electrically insulating material so as to provide a non- electrically conductive path between an inner surface thereof which contacts a first of the two tubulars, and an outermost surface thereof which contacts a second of the two tubulars.

The electrically insulating material may comprise a plastic or polymeric material, rubber, or an elastomeric material, or a ceramic material, cement or a submicron grained cemented carbide.

Advantageously the electrically isolating material substantially comprises a polyphthalamide (PPA), a polyketon or a polyamide (PA).

The sheath may further be adapted to act as a centraliser to substantially centralise the first tubular within the second tubular.

According to an eighth aspect of the present invention there is provided a well-bore apparatus including a first tubular within a second tubular, and at

least one sheath according to the seventh aspect provided therebetween.

According to a ninth aspect of the present invention there is provided a method of providing electrical isolation across an annulus formed between two tubulars within a borehole, the method including the step of providing a sheath according to the seventh aspect between the two tubulars.

According to a tenth aspect of the present invention there is provided a well including a borehole, having a well-bore apparatus according to the eighth aspect.

BRIEF DESCRIPTION OF DRAWINGS Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, which are: Figure 1 a perspective view from one side and above of a centraliser according to a first embodiment of the present invention; Figure 2 (a) a side view of a centraliser according to a second aspect of the present invention; Figures 2 (b) & (c) cross-sectional views of the centraliser of Figure 2 (a) along section line A-A and B-B respectively; Figures 3 (a) & (b) perspective views from one side and above of centralisers according to third and fourth embodiments of the present invention; Figures 4 (a) & (b) perspective views from one side and above of centralisers according to fifth and sixth embodiments of the

present invention; Figure 5 a screen centralising apparatus according to a seventh embodiment of the present invention; Figure 6 a perspective view of a production tubing centralising apparatus positioned within a casing/liner for completing a well according to an eighth embodiment of the present invention; and Figures 7 (a) & (b) are perspective views of a screen centralising apparatus positioned within a borehole and perforated casing, respectively, for gravel packing a well according to a ninth embodiment of the present invention.

DETAILED DESCRIPTION OF DRAWINGS Referring initially to Figure 1, there is shown a first embodiment of a sheath in the form of a centraliser, generally designated 10, for centralising production tubing within a casing or liner within a wellbore, according to the present invention. The centraliser 10 has at least one portion of electrically nonconductive material so as to electrically isolate inner and outer surfaces of the centraliser 10 one from the other. This material comprises a polymeric material such as a plastics material, rubber or elastomeric material or a ceramic material, cermet or submicron grained carbide. In one form of the embodiment the material is a thermoplastic polymer, particularly a polymer of carbon monoxide and alpha-olefins and more particularly CARILON (Trade Mark) available from Shell Chemicals, as will hereinafter be discussed in greater

detail. In an alternative form the material is a polyphthalamide (PPA), eg AMODEL (Trade Mark) available for BP Amoco. In a further alternative form of the embodiment the material is polytetrafluoroeth (yl) ene (PTFE), and particularly TEFLON, or a modified polyamide (PA). In a yet further alternative form of the embodiment the material is a ceramic material, for example, selected from zirconia, titania and/or alumina perhaps toughened with titanium carbide, or alternatively a titanium based ceramic, perhaps with additions of aluminium/boron and nitrogen, or alternatively silicon nitride.

CARILON (Trade Mark) is a semi-crystalline aliphatic polykeytone as disclosed in Shell Chemical Literature available from their web-site bttp://www. shell chemical. com as at 10 November 1998 and included herein by reference.

According to the literature CARILON (Trade Mark) is characterised by the following: Short moulding cycles and good mould definition Low warpage and no need for post-moulding conditioning Superior resilience and snapability Very good impact performance over a broad temperature range Very good chemical resistance and barrier performance Very good hydrolytic stability Good friction/wear characteristics and low noise generation Additionally CARILON (Trade Mark) is an insulator being substantially electrically conductive.

A range of CARILON (Trade Mark) is used depending on the performance required and the fabrication method, i. e. extrusion or injection moulding. The current range is:

SC : 2544-97-CARILON D26CX100-- Advanced extrusion grade SC ; 2545-97-CARILON D26FX100--General purpose extrusion grade SC : 2546-97-CARILON D26HM100-- General purpose injection moulding grade SC : 2547-97-CARILON D26VM100--High- flow injection moulding grade SC : 2548-97-CARILON DB6G3A10--15% Glass reinforced general- purpose injection moulding grade SC : 2549-97-CARILON DB6GA10--30% Glass reinforced general- purpose injection moulding grade <BR> <BR> SC ; 2550-97-CARILON DB6FOA10--Flame retarded (V-O), injection moulding grade SC : 2551-97-CARILON DB6F5G40--Flame retarded (V-O), 20% glass reinforced, injection moulding grade SC : 2552-97-CARILON DB6F1G40--Flame retarded (V-1) tracking resistance 15% glass reinforced injection moulding grade SC : 2533-97-CARILON DA6L1A10 Lubricated injection moulding grade SC : 2554-97-CARILON DA6P2L10--High

performance lubricated injection moulding grade SC : 2557-97-CARILON DB6G6P30-- Lubricated glass reinforced injection moulding grade AMODEL-AT-116 HS resin available from BP Amoco, (see http ://www. bpamoco) is a glass-reinforced heat stabilised PPA.

For some environments ZYTEL (Trade Mark) can be used. ZYTEL (Trade Mark) is a nylon resin available from Du Pont which can be injection moulded, and is disclosed on their web-site http://www. dupont. com as at 12th November 1998, included herein by reference. Currently thirteen grades of ZYTEL (Trade Mark) can be used, namely: 408L NCO Ionomer modified nylon 66 resin 450HSL BK 152 Olefinic/rubber modified nylon 66 resin 3189 NCO10 Cube blend, stiff, rubber modified nylon 66 resin FN718 010 Flexible grafted ionomer modified nylon 66 resin FN714 NCO10 Very flexible grafted ionomer modified nylon 66 resin CFE4003HS BK245 Heat stabilised toughened black nylon resin CFE4004HS NCO10 Heat stabilised toughened nylon 66 resin CFE4005HS BK246 Heat stabilized highly toughened

black nylon 66 resin CFE4006HS NCO10 Heat stabilized highly toughened nylon 66 resin which are toughened nylons and ST801 NC010 Grafted rubber modified nylon 66 resin ST801W NCO10 Grafted rubber modified nylon 66 resin ST901L NC095 Grafted rubber modified nylon 66 resin ST901L NC010 Grafted rubber modified amorphous nylon resin which are super tough nylons.

A further alternative plastic material which can be used in VESCONITE (Trade Mark). It is available from Vesco Plastics Australia Pty Ltd. VESCONITE (Trade Mark) exhibits greater hardiness, lower friction, negligible water absorption and higher chemical resistance than nylon. VESCONITE (Trade Mark) can be machined. Of better quality is VESCONITE HILUBE (Trade Mark) which can be injection moulded.

The centraliser 10 comprises a tubular body 12. The tubular body 12 has a bore 14 extending longitudinally therethrough. The body 12 is provided with an outermost surface 16 and an innermost surface 18. In this embodiment the surfaces 16,18 are smooth and the ends 20,22 are rounded so the centraliser 10 acts as a bush.

The thickness of the walls of the centraliser 10 are sufficient so that, depending on the material selected, substantially no electrical charge can be transmitted between the innermost surface 18 and the outermost surface 20 or vice versa.

Reference is now made to Figures 2 (a)- (c), which

illustrate a sheath or centraliser 10a, according to a second embodiment of the present invention. Like parts of the centraliser 10 of Figure 1 have given the same nomenclature but are suffixed with the letter'a'.

Centraliser l0a is provided with a plurality of raised portions 24a on the outermost surface 16a. Illustrated are four raised portions 24a in the form of blades 26a, but equally any number of blades could be used to provide a stand-off from the innermost surface 18a. The blades 26a are longitudinally extending from one end 16a to the other end 16a. The blades 26 are substantially parallel to an axis of the tubular body 12a and substantially equally spaced apart around the body 12a.

As can best be seen from Figure 2 (a), bore 14a through body 12a is a clearance fit around a tubular section 28a. The tubular section 28a could be a length of production tubing or screen, intended to be centralised by the centraliser 10a and electrically isolated from objects placed against the outermost surface 16a of the centraliser 10a, e. g. casing/liner/borehole wall.

In this embodiment the body 12a and blades 26a are made in a single (unitary) piece from a polymeric material such as a plastics material, rubber or elastomeric material, or a ceramics material, cermet or submicron grade carbide and particularly CARILON (Trade Mark) or AMODEL (Trade Mark). It will, however, be appreciated that the centraliser 10a may be nonunitary and may include reinforcing means.

In an alternative embodiment, the outermost and/or innermost surfaces 16a, 18a of the centraliser l0a may be selected from a polymeric material such as a plastics material, rubber or elastomeric material, such as ceramic material, cermet or submicron grained cement carbide, and advantageously comprises CARILON (Trade Mark) or AMODEL (Trade Mark). In such an embodiment the

outermost/innermost surfaces 16a, 18a may comprise coatings formed on an inner tubular body. The inner tubular body 12a may be of a metallic material, such as steel, zinc, zinc alloy, bronze or lead bronze, or preferably from aluminium or aluminium alloy. The coating over the metallic material is sufficiently thick to provide electrical isolation between the tubular body material or the tubular body material and an outer casing/liner in which the tubular 28a is centralised.

An embodiment of a centralising apparatus is shown in Figure 2 (a) of the drawings. Onto a length of production tubing 44a is mounted at least one centraliser 10a, the centraliser l0a being as described hereinbefore.

Centraliser 10a is of unitary construction and located to surround production tubing 44a. Collar 46a mounted on production tubing 44a restricts longitudinal movement of centraliser 10a along tubing 44a while still allowing centraliser 10a to rotate around the tubing 44a along a longitudinal axis thereof.

Referring now to Figure 3 (a) and 3 (b) of the drawings there is illustrated sheaths or centralisers lOb, lOc according to third and fourth embodiments of the present invention. In Figure 3 (a) centraliser lOb comprises a tubular body 12b, which has a longitudinally extending bore 14b therethrough. Outermost surface 16b, innermost surface 18b and ends 20b, 22b are smooth and these may be made of a polymeric plastic, elastomer or rubber material. Between the innermost surface 18b and the outermost surface 18b is located a reinforcing structure 15b. In this embodiment the structure is a cage 15b and the cage is made of a metal. Other arrangements of reinforcing means such as mesh, bars or rings could also be used.

Figure 3 (b) shows a fourth embodiment of the present position. Sheath or centraliser lOc comprises a tubular body 12c having a longitudinally extending bore 14c

therethrough. The body 12c is made of a rigid material such as a metal to provide a supporting base onto which blades 26c are attached as raised portions 24c. The blades 26c are as described for the fifth embodiment, Figure 4 (a)-see hereinafter. The blades 26c are made of a low friction material, for example, a polymeric material such as a plastic, elastomer or rubber. The blades 26c are attached to the outermost surface 16c of the body 12c by bonding, bolts, screws or the like fixing means at connection points 31c.

Reference is now made to Figures 4 (a) and 4 (b) of the drawings which show sheaths or centralisers according to fifth and sixth embodiments of the present invention respectively. Like parts to those of the centraliser 10 in Figure 1 are given the same nomenclature but suffixed d'and e'respectively. The embodiments show variations of the innermost and/or outermost surfaces of the centralisers 10d 10e.. In Figure 4 (a) the outermost surface 16d of the centraliser lOd has a plurality of equally spaced longitudinally extending raised portions 24d. These blades 26d or raised portions 24d compromise three sections. Upper and lower sections 30d and 32d have outer surfaces which taper from a full height of the blade 26d to the tubular body 12d on its outer surface 16d. Upper sections 30d of all blades 26d are substantially parallel. Similarly lower sections 32d of all blades 26d are substantially parallel. A centre portion 34d is substantially trapezoidal in cross- section. The centre portion 34d defines a helical path around the tubular body 12d. In this embodiment five blades 26d are equally spaced around the tubular body 12d. The innermost surface 18d of the centraliser lOd is further provided with a plurality of spaced apart longitudinally extending tapered recesses 36d. In this embodiment the recesses 36d taper from the upper end 20d towards the lower end 22d. The recesses 36d facilitate

ease of release of the centraliser lOd forms a mould during manufacture where the centraliser 10d is made from moulding techniques such as injection moulding.

In the sheath or centraliser 10e of Figure 4 (b).

The outermost surface 16e is provided with a plurality of raised portions 38e in the form of an array of equally spaced nipples or lobes 38e. The nipples 38e are, in the embodiment, of substantially inverted inclined teardrop shape. The precise shape and dimensions of the nipples 38e may be designed to achieve a"flow-by"and"bearing surface"of optimum efficiency."Flow-by"is required in gravel packing where the sand should not fall out of suspension early. This allows a uniform distribution of sand around the centraliser 10e to ensure a good filter is established through the sand between the producing formation and a screen.

Reference is now made to Figure 5 of the drawings which depicts a wellbore screening apparatus, generally indicated by reference numeral 40f, according to an embodiment of the present invention. Screening apparatus 40f includes a screen 42f, 43f with a section of production tubing 44f therebetween. The at least one sheath or centraliser 10f is mounted on the production tubing 44f. The screen 42f, 43f is of hollow tubular form. The at least one centraliser lOf is according to one of the embodiments described hereinbefore.

Centraliser 10f is cut down section B-B, see Figure 2, and bonded/welded/bolted around the tubing 44f.

Alternatively an outer shell containing the blades 26f can be slipped over the screens 42f, 43f and once the centraliser 10f is in position inner collets can be inserted into the centraliser body 12f thereby forming a bearing onto which the outer body 12f rotates. In a further embodiment the at least one centraliser lOf is located relative to the screen 42f, 43f by a collar which restricts longitudinal movement of the centraliser lOf on

the screen 42f, 43f. The centraliser lOf is then rotatable relative to the screen 42f, 43f along a longitudinal axis thereof.

Reference is now made to Figure 6 of the drawings which shows a centraliser 10g, in use, providing electrical isolation between a length of production tubbing 44g and a casing/liner 48g in a wellbore 50g, by the following method steps of: providing a well casing/liner 48g; providing a length of production tubing 44g; providing at least one centraliser lOg, the/each centraliser lOg having a substantially electrically nonconductive portion so as to isolate the production tubing 44g and the casing liner 48g one from the other; locating the at least one centraliser lOg on the production tubing at 44g at a desired position'so as to provide a centralising apparatus 52g; and placing the centralising apparatus 52 within the well-bore 50g.

When the wellbore 50g is finally drilled to its target depth a final casing or liner string 48g is run and cemented in place, by pumping cement in an annular space between the borehole 50g and the casing 48 the drilling phase is finished. With the wellbore 50g in this condition well fluids are generally cleaned up from the drilling fluid to clear brines or seawater. Now with the well clear, the final string production tubing 44g or is run. Whilst the tubing 44g may appear externally to be similar to casing and liner 48g it is different in that the connections are higher strength gas tight connections. This tubing 44g is run with a sealing/anchoring device called a'packer'54g which is set normally of the order of 1000 ft from a bottom of the borehole, the tubing 44g contains a surface controlled sub surface safety valve, (not shown) normally run within 1000ft of surface, and terminates a christmas tree.

The centraliser 10g ensures that no metal to metal contact is made between the tubing 44g and the casing 48g to seek to ensure that: (a) No galvanic corrosion occurs between the outer casing 48g and the inner tubing string 44g (sometimes the production tubing is made of extremely expensive in colloy/high Chromium steels- differing in EMF from the casing, thus sets up a cell); (b) Earthing to the outer casing 48g is prevented in case the tubular string 44g is being used to conduct electrical impulses back to surface from downhole pressure/temperature gauges.

Alternatively, in use, the centraliser 10g may aid completion of a well. This is illustrated in Figure 6 also. This method of completing a well comprises the steps of: providing a length of production tubing 44g; providing at least one centraliser 10g ; locating the at least one centraliser 10g on the production tubing 44g at a desired position so as to provide a centralising apparatus 52g; placing the wellbore centralising apparatus 52g within a cased or lined wellbore 50g; and securing a bottom 56g of a length of production tubing 44g with a packer 54g to seal the tubing 44g to the casing/liner 48g. The packer 54g is also at least in part of a non-conductive material.

Referring to Figures 7 (a) and (b) in an alternative embodiment, in use, a centraliser 10h, i may aid in the gravel packing of a screen 62h, i in a well. This method of gravel packing a wellbore includes the steps of: providing a screen 62h, i;

providing at least one centraliser 10h, i; locating the at least one centraliser 10h, i on the screen 62h, i to provide a centralising apparatus 64h, i; placing the centralising apparatus 64h, i within a borehole 50h or perforated casing 66i; and placing sand 78h, i into an annular space between an exterior of the screen 62h, i and the wellbore 50h or perforated casing 66i.

A principal advantage of using centralisers 10h, i during gravel packing is that it provides for the gravel to be placed uniformly around the screen thus providing for an even filter surface. In addition, as screens 62h, i are relatively weak due to the slits cut in them running long lengths is prone to failure at a point when the well is most vulnerable from a cost perspective.

Centralisers 10h, i support the screens 62h, i and the use of materials hereinbefore described provides low friction of ease of insertion into a well. A further advantage of the materials is used is that once the screens are in place, acid is normally pumped down the hole to'wash' the production face of the borehole. Metal centralisers are disadvantage in that they tend to dissolve and form a film or precipitate over the fine slits in the screen, thus impeding the flow of wellbore fluids into the screens.

It will be appreciated by those skilled in the art that the embodiments of the invention hereinbefore described are given by way of example only, and are not meant to limit the scope of the invention in any way. It is noted that the term"centraliser"has been used herein; however it will be appreciated that the device also acts as a"glider". It will be further be appreciated that the embodiments providing straight blades are particularly suitable as screen centralisers.

It will also be appreciated that although the disclosed embodiments act advantageously as centralisers/isolators, the invention need not perform as a centraliser, and embodiments are envisaged which perform only as insulating sheaths.