GB2525001A

GB2525001A - Centraliser

Info

Publication number: GB2525001A
Application number: GB1406387.9A
Authority: GB
Inventors: Andrew Keith Kirk; Nathan James Kirk
Original assignee: Downhole Products Ltd
Current assignee: Downhole Products Ltd
Priority date: 2014-04-09
Filing date: 2014-04-09
Publication date: 2015-10-14
Anticipated expiration: 2034-04-09
Also published as: GB201406387D0; GB2525001A9; GB2525001B

Abstract

A centraliser has a body with a bore to receive a tubular and has blades 75 with at least one groove 81 in the outer face of the blades 75, which channels fluids through the groove 81 axially along the blade 75. The groove 81 can have a different depth, width, volume or radius of curvature at one end as compared with the other, and can have a ramp at one end of the blade 81. Optionally, only one end of the groove 81 is open and the other (e.g. the upper) end is closed. The groove 81 can be parallel to an axis of the blade 75. More than one groove can be provided on the blade, including lateral grooves 82.

Description

Title: CENTRALISER The present application relates to a centraliser, particularly for use in centralising an elongate member in a bore of an oil or gas well.

Background to the invention

Centralisers are well known in the field of oil and gas drilling and production.

Centralisers are used to maintain a minimum stand-off or radial distance between the inner surface of a bore of a well, and a device being deployed (usually a tubular or string of tubulars) within the bore.

The function of the centraliser is to maintain a consistent radial spacing or stand-off between the outer surface of the device in the bore and the inner surface of the bore, so that the annulus between the device and the bore has a generally consistent radial dimension. This is desirable for a number of reasons. When running a string of casing into the well from the surface, centralising the string keeps it out of contact with the inner surface of the bore, which reduces frictional resistance to running in, and reduces differential sticking in open-hole sections, for example when part of the surface area of the string sticks to the inner surface of the bore because of pressure differentials between the high hydrostatic pressure in the bore and lower hydrostatic pressure in the rock being drilled.

In well comp'etion operations, the annulus between a tubular string and the inner surface of the bore is often filled with cement, and it is desirable that the layer of cement surrounding the Wbubr has a generally consistent radial dimension along the length of the tubular. Centralisers deployed between the outer surface of the tubular and the inner surface of the bore at intervals along the axis of the tubular maintain the stand-off so that the kyer of cement formed in the annulus has a generally consistent radial depth along the length of the tubular.

Centralisers can be of the solid body type) being cast or otherwise formed in a single piece. An example of this type of centraliser is described in our earlier granted patent US5797455, the disclosure of which is incorporated herein by reference, and which is useflul for understanding the invention.

Summary of the invention

According to the present invention, there is provided a centraliser having a body, the body having a bore adapted to receive a tubular member) the body having a central axis, the body having at least two blades, the blades being circumferentially spaced around the body, each of the blades having an outer face, wherein the outer face of at east one of the blades has at least one groove.

Optionally the blades extend radially with respect to the axis of the body.

Optionally the centraliser is a casing centraliser for use in an oil or gas well, for centralising casing or liner within the weHbore, which can be lined or unlined for example, with tubular casing or liner having a larger diameter, and surrounding or partially surrounding the casing or liner being centralised, and the tubular member is centralised within the bore of the casing or liner, but optionally the centraliser can also be used in un-lined bores. The bore is adapted to receive a casing or a liner, and the body of the centraliser is adapted to maintain the radial standoff between the outer surface of the casing and the inner surface of the wellbore, which can optionally be lined.

The blades optionally define between them at least one fluid channel extending between opposite axial ends of the body of the centraliser, between the outer face of the body of the centraliser and the inner surface of the wellbore. The blades optionally have sidewalls extending radially from the outer surface of the body of the centraliser, which optionally define the circumferentia' boundaries of the fluid channels between the blades. The blades optionally extend between opposite ends of the body, optionally parallel to the axis of the body, but in some cases the blades can extend helically around the body, i.e. axially and circumferentially between the ends of the body. Hence in some cases, the axis of the blade is not parallel to the axis of the body of the centraliser.

The outer face of the blade is optionally facing radially outwards with respect to the axis of the body. The outer face of the blade is optionally a plateau, and can optionally have a radius extending around at least a part of its circumference, which is optionally matched to the radius of the inner surface of the wellbore. The outer face of the blade optionally engages the inner surface of the welibore.

The body is optionally cast or moulded in a single piece with the blades being integrally formed as part of the body. In certain embodiments, the blades can be subsequently attached to the body, for example by welding fixings or adhesives, or by some other attachment mechanism. Either the blades or the body can be cast or moulded in the case of separate formation of the blades and the body.

The groove optionally channels fluids between the outer face of the blade and the inner face of the casing. The groove optionally extends at least partially between one axial end of the blade and the other. Optionally, fluid flows through the groove in an axial direction with respect to the axis of the body.

Optionally, at least one groove comprises a central groove.

Optionally the groove in the outer face of the blade extends radially into the face of the blade, towards the axis of the body of the centraliser, such that the outer face of the blade has a nominal radius which is greater than the radius in the groove.

Optionally the groove has a partially spherical cross section, for example hemispherical.

Optionally, the groove can have a consistent dimension along its length, but in some examples, the dimensions of the groove can change at different points along its length. For example, the groove can be deeper at one end than at the other.

Optionally, the groove can be wider at one end than at the other. Optionally the shapes of the sides of the groove can change between one end of the groove and the other. For example, the radius of curvature of the sides at one end of the groove can be different from the radius of curvature of the sides at the other end of the groove.

In certain embodiments, the volume of the groove changes from one end of the groove to the other. In such embodiments, the volume of the groove at the lower end of the groove is optionally higher than the volume of the groove at its upper end. Thus, when viewed in cross-section, the lower end of the groove optionally has a higher cross-sectional area than the upper end of the groove.

Optionally the groove opens into a ramp face atone end of the blade, optionally, the lower end of the blade. Optionally, the groove opens into a ramp face at each end of the blade, so that the opening of the groove into the ramp face has a smaller radius than the outermost face of the blade. Optionally, both ends of the groove can open into the ramp face at opposite axial ends of the blade. The openings at each end of the groove into the ramp face can be the same in certain examples, but in some cases, the opening of the groove at one end can be larger than the opening of the groove at the other end. For example, the opening of the groove at the upper end of the blade can be narrower than the opening of the groove at the lower end of the blade.

Optionally the ramp face can be scalloped, for example it may have a concave portion, in order to scoop fluid in the annulus and divert it more effectively onto the outer face of the blade.

Optionally, only one end of the groove is open. Optionally the lower end or the upper end of the groove optionally can be closed, or can reduce in depth until the depth of the groove at the upper end reduces to zero, having the same radius as the outermost face of the blade. Optionally, in such examples, the upper end of the groove is blind ended. Optionally, in such an arrangement, the upper end of the groove is effectively closed off from the upper end of the blade, and from any ramp face on the upper end of the blade.

In certain examples) the blade has an axis extending along its length, and the groove is optionally parallel to the axis of the blade. Optionally, the groove can be aligned with the axis of the blade, for example the groove can be coaxial with the axis of the blade. In certain examples, the groove extends at least partially along the midline of the blade. In certain examples, the groove is located at the centre of the blade, and is flanked on each side of the groove by plateau sections of the blade, which are optionally shaped to match the curvature of the interior surface of the casing within which the tubukr is being centralised.

Optionally, more than one groove is provided in the outer face of the blade. The plateau sections on either side of the groove can optionally incorporate at least one additional side groove to form at least one side channel which communicates with the central groove. Optionally, more than one side groove can be formed on each side of the central groove. Optionally, the side grooves can optionally extend helically with respect to the axis of the blade. Optionally the side grooves connect the central groove with the flow channels between circumferentially adjacent blades, thereby channelling fluid from the central groove to the flow channels between circumferentially adjacent blades. Optionally, the side grooves on one side of the central groove are arranged in the same helical direction, for example, connecting with the central groove at a first axial location with respect to the axis of the blade, and connecting with the flow channel between circumferentially adjacent blades at a second axial location. Optionally, the first axial location is below the second axial location, so that the side groove is arranged helically. Optionally, the side grooves on the other side of the central groove are arranged in the opposite helical direction, and optionally are mirror images of the side grooves on the first side of the central groove. Accordingly, in certain examples) the side grooves on each side of the central groove are symmetrical around the central groove, and are arranged helically with respect to the axis of the blade. Optionally, in such examples, the side grooves can optionally be arranged in a V-shape with an apex at the central groove, and the sides of the V diverging outwardly and upwardly from the apex towards the top end of the blade.

Optionally, the blade can be provided with ramps. In certain examples, ramps can be provided on the outer face of the blade. Optionally, ramps can be spaced apart axially along the blade. Optionally, ramps can be arranged adjacent to side grooves.

Optionally ramps can be arranged in pairs. Optionally, one ramp in each pair can be arranged on a respective side of the central groove, optionally at the same axial position on the outer surface of the blade. Optionally, a respective ramp can be arranged between a side groove and a plateau section. Optionally, a respective ramp is provided axially above at least one side groove, and axially below at least one plateau section. Optionally, the pattern of side groove, ramp, and plateau section repeats axially along the length of the outer face of the blade. Optionally, the central groove runs axially between the ramps and plateau sections on opposite sides of the outer surface of the blade, and optionally connects axially facing ramp sections at the top and bottom ends of the blade. Optionally, the ramps on the outer surface of the blade are inclined in an axial direction, with one low end near the bottom end of the blade, and one higher end near the top end of the blade.

Optionally, the lower end of the ramp is radially spaced from the axis of the body by substantially the same radial measurement as the central groove and the side grooves, in other words, the ramp optionally starts within at least one groove on the outer face of the blade. The higher end of the ramp optionally runs into the plateau section. Accordingly, the ramp optionally connects at least one groove with at least one plateau section on the outer surface of the blade. Optionally, the lower end of the ramp intersects at least one groove, such that the lower end of the ramp extends from a radial depth located within the groove. Optionally, the initiation point of the lower end of the ramp is above the bottom of the groove, which optionally extends radially deeper than the ramp. Optionally, the lower end of the ramp bisects at least one groove.

Optionally, at least one plateau section is immediately below at least one groove, advantageously a side groove. Optionally, at least one side groove is immediately below at least one ramp. Optionally, at least one ramp is immediately below at least one plateau section.

Optionally, the outer face of the blade can be provided with one or more protrusions extending radially outwardly from the outer face. Optionally, more than one protrusion can be provided on the outer face, optionally arranged on either side of the central groove, and advantageously arranged on the plateau sections of the outer face. Optionafly, the protrusions can be arranged in patterns on the outer face, and can be spaced from one another, optionally at regular intervals. Optionally, the patterns of the protrusions on the plateau sections of the outer face can diverge circumferentially outwardly from the central groove, and axially upward, towards the top end of the blade, in a similar manner to the V-shaped side grooves.

Optionally, the individual protrusions extend axially along the outer face of the blade, for at least a short distance that is substantially parallel to the axis of the centraliser, and optionally substantially parallel to the axis of the blade.

The invention also provides a centraliser assembly incorporating a tubular, a centraliser having a body with a bore adapted to receive the tubular, the body having a central axis, the body having at least two blades extending radially with respect to the axis of the body, the blades being circumferentially spaced around the body, each of the blades having an outer face, wherein the outer face of at least one of the blades has at least one groove.

The various aspects of the present invention can be practiced alone or in combination with one or more of the other aspects, as will be appreciated by those skilled in the relevant arts. The various aspects of the invention can optionally be provided in combination with one or more of the optional features of the other aspects of the invention. Also, optional features described in relation to one example or aspect can optionally be combined alone or together with other features in different examples or aspects of the invention.

Various examples and aspects of the invention will now be described in detail with reference to the accompanying figures. Still other aspects, features, and advantages of the present invention are readily apparent from the entire description thereof, including the figures, which illustrate a number of exemplary aspects and implementations. The invention is also capable of other and different aspects and implementations, and its several details can be modified in various respects, all

B

without departing from the present invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive.

Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope. Language such as "including" "comprising" "having" "containing" or "involving" and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable egal purposes.

Any discussion of documents, acts, materials, devices, articles and the like is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention.

In this disclosure, whenever a composition, an element or a group of elements is preceded with the transitional phrase "comprising", it is understood that we also contemplate the same composition, element or group of elements with transitional phrases "consisting essentially of', "consisting", "selected from the group of consisting of', "including", or "is" preceding the recitation of the composition, element or group of elements and vice versa.

All numerical values in this disclosure are understood as being modified by "about".

All singular forms of elements, or any other components described herein are understood to include plural forms thereof and vice versa. References to positional descriptions such as upper and lower and directions such as "up", "down" etc. in relation to the well are to be interpreted by a skilled reader in the context of the examples described and are not to be interpreted as limiting the invention to the literal interpretation of the term, but instead should be as understood by the skilled addressee, particularly noting that "up" with reference to a well refers to a direction towards the surface, and "down" refers to a direction deeper into the well, and includes the optional situation where a rig is above a wellhead, and the well extends down from the welihead into the formation, but also horizontal wells where the formation may not necessarily be below the wellhead.

Brief description of the drawings

In the accompanying drawings: figure 1 shows a side view of a centraliser on a tubular within a section of casing in a deviated bore; figure 2 shows a side view of a blade of the figure 1 centraliser; figure 3 shows an end view of the blade of figure 2; figure 4 shows a side view of the blade of figure 2; figure 5 shows a perspective view of the blade of figure 2; figure 6 shows a side view of a further centraliser on a tubular within a section of casing in a deviated bore; figure 7 shows an end view of the blade of the centraliser of figure 6; figure 8 shows a side view of the blade of figure 7; figure 9 shows an end perspective view of the blade of figure 7; figure 10 shows a side view of a further centraliser on a tubular within a section of casing in a deviated bore; figure 11 shows a side view of the blade of the blade of the centraliser of figure 10; figure 12 and figure 13 show end perspective views of the blade of figure 11; figure 14 shows an end view of the blade of the centraliser of figure 10; figure 15 shows a side view of the blade of figure 11; figure 16 shows an end perspective view of the blade of figure 11; figure 17 shows an end perspective view of a further alternative blade from a centraliser; and figure 18 shows an end perspective view of the blade of figure 17.

Description of examples of the invention

Referring now to the drawings, a centraliser 1 described with reference to figures 1 to S has a body 10 which is advantageously in the form of a cylindrical tube having a bore extending between two ends. The body 10 has a central axis. The bore through the body is arranged coaxially with the axis of the body 10, which receives a tubular T to be centralised within a wefibore of an oil or gas well. In figure 1, the welibore in which the tubing T and centraliser 1 are deployed is deviated [in other words the wellbore contains sections which are not precisely vertical). The wellhead (not shown) at the top of the weilbore is connected with the right hand side of figure 1, and the reservoir [not shown) at the bottom of the wellbore is connected to the left hand side of figure 1. Of course, the centraliser 1 is entirely suitable for deployment in non-deviated wellbores. The wellbore is optionally lined with tubular casing or liner C, having a larger internal diameter than the tubular T, although in some examples the casing C is optional, and the centraliser and tubular assembly is disposed in an un-lined borehole. The centraliser 1 is disposed on the outer surface of the tubular T, and in use occupies the annulus A between the outer surface of the tubular T and the inner surface of the casing or liner C. In practice, the centraliser 1 is secured onto the outer surface of the tubular Tin a relatively fixed axial position by attaching a stop collar (not shown) onto the outer surface of the tubular T to limit the axial freedom of movement of the centraliser 1 along the tubular T. Generally, and stop collar or other spatial anchoring device is fixed to the outer surface of the tubular T above and below the centraliser 1, thereby restricting the freedom of the centraliser ito move axially along the tubular T. Once the stop collar and the centraliser 1 are fixed onto the tubular T, the assembly of the tubular T with the centraliser 1 attached is pushed into the bore of the casing or liner C. The body of the centraliser 1 is optionally urged axially into the bore of the casing C by the stop collar which is fixed to the tubular T. Centralisers 1 are advantageously spaced axially along the tubular T at regular intervals in order to maintain the stand-off within the annulus A. The centraliser 1 has at least one blade 15 (optionally more than one blade 15] extending radially with respect to the axis of the body 10. The blades 15 extend radially outward from the outer surface of the generally tubular body 10.

Optionally, the blades 15 are formed as an integral part of the body 10, optionally by casting moulding or machining the centraliser 1 from a single piece of material, optionally a metal. Optionally, the blades 15 can be formed separately from the body 10, and attached subsequently, for example by welding.

The blades 15 maintain a minimum radial spacing (or "standoff') between the outer surface of the tubular T and the inner surface of the casing C. The centraliser lisa solid body centraliser, and the blades 15 and the body 10 are substantially non-compressible and non-resilient. In normal circumstances, the outer surfaces of the blades 15 bear directly against the inner surface of the casing C. The centraliser 1 shown in figure 1 only shows the surface detail of one of the blades 15, for reasons of clarity in the drawings, but it will be appreciated that all of the remaining blades on the centraliser 1 (shown schematically in figure 1) have the same configuration. Optionally the blades 15 are spaced at the same axial position on the centraliser body.

Each blade 15 is optionally arranged in a helix, and adjacent blades 15 are spaced circumferentially around the outer surface of the body 10, optionally at regular intervals around the body 10, and optionally at the same helical angle, so that each blade 15 extends axially between the top and bottom ends of the centraliser body 10, and circumferentially around the outer surface of the body 10.

The blades 15 optionally rise up out of the outer curved surface of the generally tubular body 10, creating channels 11 between circumferentially adjacent blades 15.

The channels 11 promote fluid flow in the annulus A in the direction of the arrows shown in figure 1, from the bottom of the well to the top. Fluid is circulated from the top of the well through the central bore of the tubular T, and is returned to the surface through the annulus A. The recirculation of fluid in this manner washes drill cuttings and other debris from the bottom of the well back to the surface for recovery and treatment as necessary. In addition to promoting fluid flow during recirculation of fluid from the surface, the flow channels 11 also channel the fluid in the well during insertion of the string of tubular T into the well, thereby reducing the hydraulic reaction force resisting the insertion of the string from the surface.

Each blade 15 has two sidewalls 16, 17 a bottom and a top face 18, 19, and an outer face 20. Optionally, the sidewalls 16 and 17 are radiussed (i.e. formed in an arc) at the point at which the meet the outer face 20. Optionally, the blade 15 has a central axis which is optionally not coaxial with the central axis of the centraliser, but extends helically with respect to the body 10. As best shown in figure 3, the sidewalls 16, 17 are optionally inclined inwardly toward the axis of the blade 15, and are optionally not precisely at 900 to the tubular outer surface of the body 10.

The sidewalls of the blades 15 extend optionally in a radial direction for a desired distance dependent on the desired stand-off for the annulus A. The outer face of the blades 15 optionally bears directly against the inner surface of the casing C. The outer face of the blades 15 is used as a bearing surface, in order to reduce the frictional coefficient of the string of tubulars T during insertion of the string into the hole. The bottom and top faces 18, 19 are optionally inclined, in the general form of a ramp which inclines from the nominal outer surface of the tubular body 10 at a generally consistent angle up to the outer face 20. Optionally, the top and bottom faces 19, 10 are asymmetrical. Optionally, the bottom face 18 extends axially further (with respect to the axis of the blade 15) on the side of the sidewall 17 than it does on the side of the sidewall 16. Optionally, the bottom face 18 is cut away on the side of the sidewall 16. The opposite arrangement optionally applies on the top face 19, which has an axial extension on the side of the sidewall 16, and is cut away at the sidewall 17. Accordingly, the blade 15 optionally has an asymmetric arrangement, at least between its top and bottom faces 19, 18. The top and bottom faces optionally act as scoops in order to divert fluid within the annulus A into the annular space between the outer face 20 of the blade 15, and the inner surface of the casing C. The outer face 20 of the blade 15 has a central groove 21. The central groove 21 optionally extends between the top and bottom faces 19, 18, and is optionally aligned with (for example parallel to) the axis of the blade 15, along the midline of the outer face 20, and facing radially outwards with respect to the axis of the body.

The central groove 21 cuts into the outer face 20, and so extends radially downward into the blade 15, so that the bottom of the centra' groove 21 is radially lower than the outer face 20 of the blade 15, and such that the outer face of the blade has a nominal radius which is greater than the radius in the central groove 21. The central groove 21 has a generally hemispherical cross-section, as best shown in figure 3. The lower end of the central groove 21 near to the bottom face 18 has a wider cross-section than the top of the groove near to the top face 19.

The outer face 20 of the blade therefore provides a plateau at the level of the outer face) optionally on either side of the groove 21, raised radially away from the intra-blade channels. The plateau has a radius extending circumferentially, which is matched to the radius of the inner surface of the wellbore, so that the outer face 20 engages the inner surface of the wellbore in use. In certain examples, the blade has an axis extending along its length, and the groove is optionally parallel to the axis of the blade.

The central groove 21 has a hemispherical cross-section. In some examples, the central groove 21 has a consistent dimension along its length, but in the present example, the dimensions of the central groove 21 vary along its length. The central groove 21 can optionally be deeper at the lower end of the blade than at the upper end of the blade. Additionally, or alternatively, the central groove 21 can be wider at the lower end than at the upper end. Furthermore, the shape of the sides of the groove 21 can change between one end of the groove and the other, and the volume of the groove changes from one end of the groove to the other. In this embodiment, the volume of the groove 21 at the lower end of the groove 21 is higher than the volume of the groove 21 at its upper end. Thus, when viewed in cross-section, the lower end of the groove 21 has a higher cross-sectional area than the upper end of the groove 21, and optionally a different radius of curvature in the walls of the groove 21. Thus the upper end of the groove 21 adjacent to the top face 19 optionally has a narrower throat than the lower end of the groove 21 adjacent to the bottom face 18, such that the groove 21 is tapered along its length.

The outer face 20 in this example has a series of protrusions in the form of ribs 24 extending radially outwardly from the outer face, and extending axially along the outer face 20 for a short distance that is substantially paraflel to the axis of the blade, and parallel to the groove 21. The protrusions in general and the ribs 24 in particular are optional and can be omitted in this example, in which case the outer face 20 is optionally substantially free from protrusions. The ribs 24 are optionally arranged on either side of the central groove 21, and on the plateau sections 2Op of the outer face 20, so that the extend radially outward from the plateau sections 2Op of the outer face 20, creating an annular space between the radially outward crest of the ribs 24, and the nominal diameter of the plateau section 2Op of the outer face 20.

In this example, the ribs 24 are arranged in patterns on the outer face 20, and are spaced from one another, at regular intervals. The patterns of the ribs 24 in this example diverge circumferentially outwardly from the central groove 21, and axially upward with respect to the blade, towards the top end of the blade, optionally forming a general V-shape with the axis of the V closer to the lower end of the blade.

The patterns of V-shaped arrangements of the ribs 24 are axially spaced along the blade 15, optionally at regular intervals along the axis. In this example, there isa short axial space between each repeating V. The radially outer crests of the ribs 24 engage the inner surface of the casing C, as best shown in figure 4, creating a small annular space between the nominal outer diameter of the plateau sections 20p of the outer face 20 and the inner diameter of the casing C. Thus the ribs 24 maintain a minimum annular stand-off between the plateau sections 20p of the outer face 20 and the inner diameter of the casing allowing and optionally channelling, fluid flow in the small annular space between the outer face 20 and the inner surface of the casing C. The central groove 21 channels fluids between the outer face 20 of the blade 15 and the inner face of the casing C. The groove 21 extends between one axial end of the blade and the other, thereby allowing fluid flow through the groove in an axial direction with respect to the axis of the body.

In use, as the string is being made up at the surface, the desired number of centralisers 1 are fixed in desired axial positions on the outer surface of the tubular T, generally as it is being made up into the string and inserted into the hole. The tubular T being inserted into the hole is a smaller diameter than the previous string of casing C which is already cemented in place below the wellhead, and the tubular T is being run into the hole in order to be cemented in pkce coaxially within the previous string of casing C once it has been run in. As the string of tubulars T is run into the drilled section of hole, the centralisers 1 on the outer surface of the tubular T maintain the stand-off, reducing friction between the suing of tubulars T and the inner surface of the casing C, and facilitating insertion of the new string into the hole. The operator can be confident that the centralisers 1 maintain a minimum annular stand-off between the outer surface of the tubulars and the inner surface of the casing C. The ribs 24 engage the inner surface of the casing C, and maintain a minimum annular spacing between the outer surface 20 of the centralisers 1 and the inner surface of the casing C. The ribs 24 help to reduce friction between the outer surface of the centraliser 1 and the inner surface of the casing C, thereby reducing the frictional resistance to insertion of the string from the surface.

When the string of tubulars T is run into the drilled section of the hole, the hole is optionally filled with fluids such as drilling fluid. As the string of tubulars T is run into the hole, it displaces the collected fluid, which flows up the hole, past the centraliser, in annulus between the outer surface of the tubular T and the inner surface of the casing C, as the string of tubulars T moves down relative to the stationary casing C. The fluid flow in the annulus is diverted into the wide flow channels 11 between the blades 15.

The ramps 18 of the thwer ends of the blades 15 divert fluid in the annulus radiafly outwards up the ramp and onto the outer face 20 of the Nade 15. As best shown in figure 3, the fluid is scooped up the ramp) and diverted into the central groove 21, which opens onto the surface of the ramp 18 at the lower end of the blade 15. The central groove 21 channels the fluid along the blade) maintaining it in the localised region between the outer face 20 and the inner surface of the casing C) creating a fluid "cushion" between the outer face 20 and the inner surface of the casing C, and further reducing friction between the string of tubulars T moving into the well and the stationary casing C. In certain examples of the centraliser 1, the groove 21 can maintain the same dimensions along its length, but in this example, the upper end of the groove 21 opening onto the ramp 19 at the top of the blade 15 has a smaller cross-sectional surface area than the lower end of the groove 21 opening onto the ramp 18 at the bottom of the blade 15. Optionally, the groove 21 is tapered along at least a portion of its length) although it is also possible to step the changes in cross-sectional surface area between the ends. In the centraliser 1, the groove 21 thus incorporates a fluid restriction between its upper and lower ends, which tends to increase the pressure of the fluid within the groove 21, as the string of tubulars T slides axially downwards through the bore of the casing C. The increased pressure of the fluid localised within the groove 21 enhances the cushion effect, using the force of the fluid flow past the centraliser 1 and directing it radially outwards to help to space the body of the centraliser 1 from the inner surface of the casing C. Any interaction between the ribs 24 on the outer face 20 of the blades 15 and the inner surface of the casing C is therefore reduced. The reduced interaction between the centraliser 1 and the inner surface of the casing C reduces even further the frictional resistance to continued axial insertion of the string of tubulars T into the casing C. Thus, the operator is affle to insert the string of tubulars T into the hole in the casing C with lower axial force requirements.

Optionally, only the lower end of the groove 21 is open. Optionally the upper end of the groove 21 can be closed. Optionally the upper end of the groove 21 can reduce in depth until the depth of the groove at the upper end reduces to zero, having the same radius as the outer face 20 of the blade 15. Optionally, the upper end of the groove 21 is blind ended) and is closed off from the upper end of the blade 21, and from the ramp face 19 on the upper end of the blade 20.

Referring now to figures 6 to 9, a further example of a centraliser will be described.

In this example, the centraliser 31 described with reference to figures 6 to 9 has a body 40 in the form of a cylindrical tube having a bore and a centra' axis. The bore is coaxial with the axis of the body 40, which receives a tubular T to be centralised within a weilbore of an oil or gas well. The wellbore is deviated. Other features of the centraliser 31 are common to the centraliser 1 described above) and the reader is referred to the earlier examples from which features can be adopted in this variation. The centraliser 31 is disposed on the outer surface of the tubular T, and in use occupies the annulus A between the outer surface of the tubular T and the inner surface of the casing or liner C and is secured onto the outer surface of the tubular T and then deployed into the bore of the casing or liner C as described above in relation to the centraliser 1.

The centraliser 31 has at least one blade 45 (optionally more than one blade 45] extending radially with respect to the axis of the body 40. The blades 45 extend radially outward from the outer surface of the generally tubular body 40, and are formed as an integral part of the body 40, optionally by casting, moulding or machining the centraliser 31 from a single piece of materiaL optionally a metal.

Optionally, the blades 45 can be formed separately from the body 40, and attached subsequently, for example by welding.

The blades 45 maintain a minimum radial spacing (or "standoff') between the outer surface of the tubular T and the inner surface of the casing C. The centraliser 31 is a solid body centraliser, and the blades 45 and the body 40 are substantially non-compressible and non-resilient. In normal circumstances, the outer surfaces of the blades 45 bear directly against the inner surface of the casing C. The centraliser 31 shown in figure 6 only shows the surface detail of one of the blades 45, for reasons of clarity in the drawings, but it will be appreciated that all of the remaining blades on the centraliser 31 (shown schematically in figure 6) have the same configuration.

Each blade 45 is optionally arranged in a helix, and adjacent blades are spaced circumferentially around the outer surface of the body 40, optionally at regular intervals around the body 40, and optionally at the same helical angle, so that each blade 45 extends axially between the top and bottom ends of the centraliser body 40, and circumferentially around the outer surface of the body 40. Optionally the blades 45 are spaced at the same axial position on the centraliser body.

The blades 45 optionally radially rise up out of the outer curved surface of the generally tubular body 40, creating channels 41 between circumferentially adjacent blades 45. The chann&s 41 promote fluid flow in the annulus A in the direction of the arrows shown in figure 6, from the bottom of the weil to the top. The flow channels 41 channel the fluid in the well during insertion of the string of tubular T into the well.

Each blade 45 has two sidewalls 46, 47 a bottom and a top face 48,49, and an outer face 50. Optionally, the sidewalls 46 and 47 are radiussed at the point at which the meet the outer face 50. Optionally, the blade 45 has a central axis which is not coaxial with the central axis of the centraliser, but extends helically with respect to the body 40. As best shown in figure 7, the sidewalls 46, 47 are optionally inclined inwardly toward the axis of the blade 45, and are optionally not precisely at 900 to the tubular outer surface of the body 40.

The sidewalls of the blades 45 extend optionally in a radial direction for a desired distance dependent on the desired stand-off for the annulus A. The outer face of the blades 45 optionally bears directly against the inner surface of the casing C. The outer face of the blades 45 is used as a bearing surface, in order to reduce the frictional coefficient of the string of tubulars T during insertion of the string into the hole. The bottom and top faces 48, 49 are optionally inclined, in the general form of a ramp which inclines from the nominal outer surface of the tubular body 40 at a generally consistent angle up to the outer face 50. Optionally, the top and bottom faces 49, 48 are asymmetrical. Optionally, the bottom face 48 extends axially further (with respect to the axis of the blade 45) on the side of the sidewall 47 than it does on the side of the sidewall 46. Optionally, the bottom face 48 is cut away on the side of the sidewall 46. The opposite arrangement optionally applies on the top face 49, which has an axial extension on the side of the sidewall 46, and is cut away at the sidewall 47. Accordingly, the blade 45 optionally has an asymmetric arrangement, at least between its top and bottom faces 49, 48. The top and bottom faces optionally act as scoops in order to divert fluid within the annulus A into the annular space between the outer face 50 of the blade 45, and the inner surface of the casing C. The outer face 50 of the blade 45 has a central groove 51. The central groove 51 is optionally aligned with (for example parallel to] the axis of the blade 45, along the midline of the outer face 50, and facing radially outwards with respect to the axis of the body. The central groove 51 cuts into the outer face 50, and so extends radially downward into the blade 45,so that the bottom of the central groove 51 is radially lower than the outer face 50 of the blade 45, and such that the outer face of the blade has a nominal radius which is greater than the radius in the central groove 51.

The outer face 50 of the blade provides a narrow plateau at the level of the outer face, and has plateau sections optionally in the form of side strips SOp on either side of the groove 51, at the edge of the outer face 50. The radially extending apexes of the side strips SOp are the highest points on the outer face 50, and so engage the inner surface of the casing C in use. The blade has an axis extending along its length, and the groove 51 is optionally parallel to the axis of the blade. Whereas the groove 21 in the previous example was narrow relative to the width of the outer face 20, the groove 51 is considerably wider, and takes up substantially more of the width of the outer face 50 of the blade 45 in this example. The plateau section of the outer face 50 is therefore limited to the side strips 50p at the edges of the groove 51, and the portion of the outer face 50 above the end of the groove 51. The lower end of the central groove 51 near to the bottom face 48 opens onto the bottom face 48 as in the previous example. However, the top of the groove 51 near to the top face 49 has a blind end and does not communicate with the top face 49, in contrast to the previous example. Thus, fluid flowing into the groove 51 from the lower end does not flow through the groove in an axial direction as it did in the previous example.

The groove 51 is also substantially deeper than the groove 21, at least at the lower end, and has sidewalls which are substantially vertical (i.e. extending in a radial direction with respect to the axis of the centraliser 31) and a floor which connects to the sidewalls at angles, rather than having a hemispherical cross-section as in the

previous example.

In this example, the central groove 51 can have consistent dimension along its length) but in some examples, the dimensions of the central groove 51 vary as in previous examples, for example, the central groove 51 can optionally be deeper at the lower end of the blade than at the upper end of the blade. Thus the shape of the groove 51 can change between one end of the groove and the other, and the volume of the groove changes from one end of the groove to the other. In this embodiment, the volume of the groove Si at the lower end of the groove 51 is higher than the volume of the groove 51 at its upper end. Thus, when viewed in cross-section, the lower end of the groove 51 has a higher cross-sectional area than the upper end of the groove 51, and optionally a different shape in the walls and floor of the groove 51. Optionally the upper end of the groove 51 adjacent to the top face 49 can narrow such that the groove 51 is tapered along its length.

Optionally the floor of the groove 51 can be ramped, so that the upper end of the groove 51 is shallower than the lower end, and optionally the floor of the groove 51 can arise radially to substantially the same radial height as the outer face 50.

However, in this example, the depth of the groove 51 shown in figures 6 to9 is relatively constant along its length.

The sidewalls of the groove 51 optionally have lateral grooves 52 extending between the central groove 51 and the outer surface of the sidewalls 46, 47, allowing fluid communication between the central groove 51 and the intra-blade channels 41 on the centraliser. The lateral grooves 52 permit the escape of fluid from the central groove 51, but optionafly, the cross-sectional area of the lateral grooves 52 is relatively small, and optionally only a small amount of fluid can escape through the lateral grooves 52. Optionally, the bteral grooves 52 can be tapered, so that the fluid outlet opening onto the sidewalls 46, 47 has a smaller cross-sectional area than the inlet of the lateral grooves 52 communicating with the centra' groove 51. Tapering the lateral grooves 52 a'so reduces the flow path for fluid to escape from the central groove 51.

Optionally, the lateral grooves 52 can be equally spaced along the sidewalls on either side of the central groove 51, as they are in this case, but in certain examples, the spacing between the lateral grooves along the axis of the blade can be different at opposite ends of the blade.

The central groove 51 channels fluids between the outer face 50 of the blade 45 and the inner face of the casing Gas in the previous example. The groove 51 extends from the lower axial end of the blade towards the other, thereby allowing fluid flow up the groove in an axial direction with respect to the axis of the body.

In use, as the string is being made up at the surface, the desired number of centralisers 31 are fixed in desired axial positions on the outer surface of the tubular T, generally as it is being made up into the string and inserted into the hole. The tubular T being inserted into the hole is a smaller diameter than the previous string of casing C which is already cemented in place below the wellhead, and the tubular T is being run into the hole in order to be cemented in pbce coaxially within the previous string of casing C once it has been run in. As the string of tubulars T is run into the drilled section of hole, the centralisers 31 on the outer surface of the tubular T maintain the stand-off, reducing friction between the string of tubulars T and the inner surface of the casing C, and facilitating insertion of the new string into the hole. The operator can be confident that the centralisers 31 maintain a minimum annular stand-off between the outer surface of the tubulars and the inner surface of the casing C. The sides of the plateau sections of the outer surface 50 engage the inner surface of the casing C, and maintain a minimum annular spacing between the outer surface 50 of the centralisers 31 and the inner surface of the casing C. When the string of tubulars T is run into the drilled section of the hole, the hole is optionally filled with fluids such as drilling fluid. As the string oftubulars T is run in, it dispkces the fluid already in the hole, which flows up the annulus between the outer surface of the tubular T and the inner surface of the casing C, as the string of tubulars T moves down relative to the stationary casing C. The upwardly flowing fluid in the annulus is largely channelled into the flow channels 41 between the blades 45, which provide a brge surface area, to promote free passage of fluid within the annulus.

The ramps 48 of the sower ends of the blades 45 act as scoops to divert fluid in the annulus up the ramp 48 and into the groove 51 cut into the outer face 50 of the blade 45. As best shown in figure 7, the fluid is scooped up the ramp, and diverted into the central groove 51, which opens onto the surface of the ramp 48 at the lower end of the blade 45. The central groove 51 channels the fluid along the blade, and this creates a fluid "cushion" between the outer face 50 and the inner surface of the casing C, further reducing friction between the string of tubulars T moving into the well, and stationary casing C. The upper end of the groove 51 is closed. Thus the groove 51 incorporates a fluid restriction between its upper and lower ends, which tends to increase the pressure of the fluid within the groove 51, as the string of tubulars T slides axially downwards through the bore of the casing C, moving axially downwards into the well relative to the stationary fluid column. The increased pressure of the fluid localised within the groove 51 enhances the cushion effect, using the force of the fluid flow past the centraliser 31 to help to space the body of the centraliser 31 from the inner surface of the casing C. Any interaction between the outer face 50 of the blades 45 and the inner surface of the casing C is therefore reduced. The reduced interaction between the centraliser 31 and the inner surface of the casing C reduces even further the frictional resistance to continued axial insertion of the string of tubulars T into the casing C. Thus, the operator is able to insert the string of tubulars T into the h&e in the casing C with lower axial force requirements.

Since the upper end of the groove 5115 closed off from the upper end of the blade 51, the fluid that is retained within the groove 51 cannot flow readily S of the groove 51, and hence is diverted sideways through the lateral grooves 52. The lateral grooves optionally extend radially only for part of the distance into the outer face 50, and optionally do not extend all the way to the floor of the central groove 51.

The small lateral grooves 52 present a relatively limited flow path for escape of the fluid from the central groove 51, which therefore creates a pressure differential between the relatively high pressure of fluid that is localised within the groove 51 and the relatively low pressure of fluid in the intra-blade flow channels 41. This pressure differential enhances the fluid cushion effect, and helps to maintain the annular spacing between the body of the centraliser and the inner surface of the casing. It also reduces the friction arising from impacts between the body of the centraliser and the inner surface of the casing, since those impacts are generally reduced in force as a result of the fluid cushion effect. Accordingly, the groove on the outer surface of the blade reduces frictional resistance to running the string into the hole, particularly in deviated wells.

It should be noted that in this example, it is possible to achieve good results without the lateral grooves 52, which are optional in this variation.

A further example of a centraliser will now be described with reference to figures 10 to 16. In the example shown in Figs 10-16, a centraliser 61 has a body 70 in the form of a cylindrical tube having a bore and a central axis. The bore is coaxial with the axis of the body 70, which receives a tubular T to be centralised within a wellbore of an oil or gas well. The wellbore is deviated. Various other features of the centraliser 61 are common to the centralisers 1 and 31 described above, and the reader is referred to the earlier examples from which features can be adopted in this variation. The centraliser 61 is disposed on the outer surface of the tubular T, and in use occupies the annulus A between the outer surface of the tubular T and the inner surface of the casing or liner C) and is secured onto the outer surface of the tubular T and then deployed into the bore of the casing or liner C as described above in relation to the centralisers land 31.

The centraliser 61 has at least one Nade 75 (optionally more than one blade 75) extending radially with respect to the axis of the body 70. The blades 75 extend radially outward from the outer surface of the generally tubular body 70, and are formed as an integral part of the body 70, optionally by casting moulding or machining the centraliser 61 from a single piece of material, optionally a metal.

Optionally, the blades 75 can be formed separately from the body 70, and attached subsequently, for example by wekling.

The blades 75 maintain a minimum radial spacing (or "standoff'] between the outer surface of the tubular T and the inner surface of the casing C. The centraliser 61 is a solid body centraliser, and the blades 75 and the body 70 are substantially non-compressible and non-resilient. In normal circumstances, the outer surfaces of the blades 75 bear directly against the inner surface of the casing C. The centraliser 61 shown in figure 10 only shows the surface detail of one of the blades 75, for reasons of clarity in the drawings, but it will be appreciated that all of the remaining blades on the centraliser 61 (shown schematically in figure 10) have the same configuration.

Each blade 75 is optionally arranged in a helix on the body 70, and adjacent blades are spaced circumferentially around the outer surface of the body 70, optionally at regular interva's around the body 70, and optionally at the same helical angle, so that each blade 75 extends axially between the top and bottom ends of the centraliser body 70, and circumferentially around the outer surface of the body 70.

Optionally the blades 75 are spaced at the same axial position on the centraliser body.

The blades 75 optionally radially rise up out of the outer curved surface of the generally tubular body 70, creating channels 71 between circumferentially adjacent blades 75. The chann&s 71 promote fluid flow in the annulus A in the direction of the arrows shown in figure 10, from the bottom of the well to the top. The flow channels 71 channel the fluid in the well during insertion of the string of tubular T into the well.

Each blade 75 has two sidewalls 76, 77 a bottom and a top face 78, 79, and an outer face 80. Optionally, the sidewalls 76 and 77 are radiussed at the point at which the meet the outer face 80. Optionally, the blade 75 has a central axis which is not coaxial with the central axis of the centraliser, but extends helically with respect to the body 70. As best shown in figure 14, the sidewalls 76, 77 are optionally inclined inwardly toward the axis of the blade 75, and are optionally not precisely at 900 to the tubular outer surface of the body 70.

The sidewalls of the blades 75 extend optionally in a radial direction for a desired distance dependent on the desired stand-off for the annulus A. The outer face of the blades 75 optionally bears directly against the inner surface of the casing C. The outer face of the blades 75 is used as a bearing surface, in order to reduce the frictional coefficient of the string of tubulars T during insertion of the string into the hole. The bottom and top faces 78, 79 are optionally inclined, in the general form of a ramp which inclines from the nominal outer surface of the tubular body 70 at a generally consistent angle up to the outer face 80. Optionally, the top and bottom faces 79, 78 are asymmetrical. Optionally, the bottom face 78 extends axially further (with respect to the axis of the blade 75) on the side of the sidewall 77 than it does on the side of the sidewall 76. Optionally) the bottom face 78 is cut away on the side of the sidewall 76. The opposite arrangement optionally applies on the top face 79, which has an axial extension on the side of the sidewall 76, and is cut away at the sidewall 77. Accordingly, the blade 75 optionally has an asymmetric arrangement, at least between its top and bottom faces 79, 78. The top and bottom faces optionally act as scoops in order to divert fluid within the annulus A into the annular space between the outer face 80 of the blade 75, and the inner surface of the casing C. The outer face 80 of the blade 75 has a central groove 81. The central groove 81 is optionally aligned with (for example parallel to) the axis of the blade 75, along the micfline of the outer face 80, and faces radially outwards with respect to the axis of the body. The central groove 81 cuts into the outer face 80, and so extends radially downward into the blade 75, so that the bottom of the central groove 8lis radially lower than the outer face 80 of the blade 75, and such that the outer face of the blade has a nominal radius which is greater than the radius in the central groove 81.

The outer face 80 of the blade provides a plateau at the highest point which engages the inner surface of the casing C in use, optionally in the form of spaced apart plateau sections 80p which are axially spaced along the blade. The blade has an axis extending along its length, and the groove 81 is optionally parallel to the axis of the blade. The groove 81 is substantially similar to the groove 21 in the previous example 31 and is narrow relative to the width of the outer face 20. The lower end of the central groove 81 near to the bottom face 78 opens onto the bottom face 78 as in the previous example, and the top of the groove 81 near to the top face 79 opens onto the top face 79, so fluid flowing into the groove 81 from the lower end flows through the groove in an axial direction as in the centraliser 31. The central groove 81 in some variations can have consistent dimension along its length, but in this example, the dimensions of the central groove 81 vary as in previous examples, for example, the central groove 81 can optionally be deeper, wider at the lower end of the blade than at the upper end of the blade, with different shape and volume, and having a higher cross-sectional area than the upper end of the groove 81, which narrows such that the groove 81 is tapered along its length, and the inlet of the groove 81 at the bottom is larger than the outlet at the top.

Optionally the floor of the groove 81 can be ramped, so that the upper end of the groove 81 is shallower than the lower end) and optionally the floor of the groove 81 can arise radially. However) in this example) the depth of the groove 81 shown in figures 10 to 16 is relatively constant along its length.

The sidewalls of the groove 81 optionally have lateral grooves 82 extending between the central groove 81 and the outer surface of the sidewalls 76. 77, allowing fluid communication between the central groove 81 and the intra-blade channels 71 on the centraliser. The lateral grooves 82 permit the escape of fluid from the central groove 81 as in the previous example 61, but optionally, the cross-sectional area of each of the lateral grooves 82 is smaller than the cross sectional area of the central groove 81. Optionally, at least some of the lateral grooves 82 can be tapered or stepped between an inlet opening into the central groove 81 and an outlet opening into the channel 71, so that the fluid outlet opening onto the sidewalls 76, 77 and communicating with the channel 71 has a smaller cross-sectional area than the inlet of the lateral grooves 82 communicating with the central groove 81. Tapering the lateral grooves 82 also reduces the flow path for fluid to escape from the central groove 81.

Optionally, the lateral grooves 82 can be equally spaced along the sidewalls on either side of the central groove 81, as they are in this case, but in certain examples, the spacing between the lateral grooves along the axis of the blade can be different at opposite ends of the blade. The lateral grooves 82 are optionally arranged on opposite sides of the central groove 81, and optionally extend axially and circumferentially between the central groove Bland the outer face of the sidewall of the centraliser 61. Optionally, all of the lateral grooves 82 are mutually parallel, and extend the same angle. Optionally the lateral grooves on opposite sides of the central groove Blare mirror images of each other, and form a general V-shape with an apex at the central groove, and the sides of the V diverging outwardly and upwardly from the apex towards the top end of the blade.

The central groove 81 channels fluids between the outer face 80 of the blade 75 and the inner face of the casing C as in the previous example. The groove 81 extends from the lower axial end of the blade towards the other, thereby allowing fluid flow up the groove in an axial direction with respect to the axis of the body. In addition to flowing axially through the central groove 81, fluid also passes into the lateral grooves 82 from the central groove 81. Thus fluid flow is directed by the grooves 81, 82 across a larger surface area of the outer face 80 of the blades 75, creating a wider "cushion" of fluid across the blade 75.

The blade 75 is provided with ramps 83, although in this embodiment the ramps 83 are optional, as are the lateral channels 82. Tn this example, ramps 83 are provided on the outer face 80 of the blade 75 spaced apart axially along the blade 75, and arranged adjacent to lateral grooves 82, optionally between a lateral groove 82 and a plateau section SOp. Ramps 83 are arranged in pairs, with one ramp in each pair on a respective side of the central groove 81, at the same axial position on the outer surface 80 of the blade 75 optionally axially above at least one lateral groove 82, and axially below at least one plateau section SOp. The pattern of lateral groove 82, ramp 83, and plateau section 80p repeats axially along the length of the outer face of the blade, optionally at regular intervals. The central groove 81 runs axially between the ramps 83 and plateau sections SOp on opposite sides of the outer surface of the blade, and connects the ramped top and bottom faces 79, 78 of the blade.

The ramps 83 are inclined in an axial direction, with one low end near the bottom end of the blade 75, and one higher end near the top end of the blade 75. The ramp intersects with a lateral groove 82 (optionally slightly above its deepest point) and rises in an axial direction to run into the plateau section SOp, connecting the two, and forming a fluid pathway for fluid to flow axially up the ramp from the lateral groove 82, onto the outer face of the plateau section SOp.

In use, as the string is being made up at the surface, the desired number of centralisers 61 are fixed in desired axial positions on the outer surface of the tubular T, generally as it is being made up into the string and inserted into the hole. The tubular T being inserted into the hole is a smaller diameter than the previous string of casing C which is already cemented in place below the wellhead, and the tubular T is being run into the hole in order to be cemented in place coaxially within the previous string of casing C once it has been run in. As the string of tubulars T is run into the drilled section of hole, the centralisers 61 on the outer surface of the tubular T maintain the stand-off, reducing friction between the string of tubulars T and the inner surface of the casing C, and facilitating insertion of the new string into the hole. The operator can be confident that the centralisers 61 maintain a minimum annular stand-off between the outer surface of the tubulars and the inner surface of the casing C. The sides of the plateau sections of the outer surface 80 engage the inner surface of the casing C, and maintain a minimum annular spacing between the outer surface 80 of the centralisers 61 and the inner surface of the casing C. When the string of tubulars T is run into the drilled section of the hole, the hole is optionally filled with fluids such as drilling fluid. As the string of tubulars I is run in, it displaces the fluid already in the hole, which flows up the annulus between the outer surface of the tubular T and the inner surface of the casing C, as the string of tubulars I moves down relative to the stationary casing C. The upwardly flowing fluid in the annulus is largely channelled into the flow channels 71 between the blades 75, which provide a large surface area) to promote free passage of fluid within the annulus.

The ramps 78 of the lower ends of the blades 75 act as scoops to divert fluid in the annulus up the ramp 78 and into the groove 81 cut into the outer face BOof the blade 75. As best shown in figure 7, the fluid is scooped up the ramp, and diverted into the central groove 81, which opens onto the surface of the ramp 78 at the lower end of the blade 75. The central groove 81 channels the fluid along the blade, and this creates a fluid "cushion" between the outer face 80 and the inner surface of the casing C, further reducing friction between the string of tubulars T moving into the well, and stationary casing C. Fluid flowing through the central groove 81 is also channelled into the lateral grooves 82, and is thereby diverted across a relatively large surface area of the outer face 80. Fluid flowing laterally through the lateral channels 82 is also diverted up the ramps 83, which are optionally moving axially downwards relative to the fluid as a result of the string of tubulars T being run into the hole. The ramps 83 impart radial thrust to the fluid in the lateral channels 82, urging it radially outwards relative to the body of the centraliser 61, creating a "jetting effect" thereby increasing the fluid cushion generated by the grooves 81, 82 on the outer face 80, and reducing even further the frictional resistance to axial insertion of the string of tubulars T into the hole.

The groove 81 incorporates a fluid flow restriction at numerous points between its upper and lower ends (for example, at the junctions between the central groove 81 and the lateral grooves 82, and that the outlets of the lateral grooves 82 as they open onto the sidewalls 76, 77) which tends to increase the pressure of the fluid within the groove 81, as the string of tubulars T slides axially downwards through the bore of the casing C, moving axially downwards into the well relative to the stationary fluid column, and which also helps to localise the high pressure fluid within the grooves, while at the same time allowing flow of fluid through the grooves. The increased pressure of the flowing fluid within the groove 81 also enhances the cushion effect, using the force of the fluid flow past the centraliser 61 to help to space the body of the centraliser 61 from the inner surface of the casing C. Any interaction between the outer face 80 of the blades 75 and the inner surface of the casing C is therefore reduced. The reduced interaction between the centraliser 61 and the inner surface of the casing C reduces even further the frictional resistance to continued axial insertion of the string of tubulars T into the casing C. Thus, the operator is able to insert the string of tubulars T into the hole in the casing C with lower axial force requirements.

Since the upper end of the groove 81 is closed off from the upper end of the blade 81, the fluid that is retained within the groove 81 cannot flow readily 8 of the groove 81, and hence is diverted sideways through the lateral grooves 82. The lateral grooves optionally extend radially only for part of the distance into the outer face 80, and do not optionally extend all the way to the floor of the central groove 81.

The small latera' grooves 82 present a relatively limited flow path for escape of the fluid from the central groove 81, which therefore creates a pressure differential between the rehtively high pressure of fluid that is localised within the groove 81 and the relatively low pressure of fluid in the intra-blade flow channels 71. This pressure differential enhances the fluid cushion effect, and h&ps to maintain the annular spacing between the body of the centraliser and the inner surface of the casing. It also reduces the friction arising from impacts between the body of the centraliser and the inner surface of the casing since those impacts are generally reduced in force as a result of the fluid cushion effect. Accordingly) the groove on the outer surface of the Nade reduces frictional resistance to running the string into the hole, particularly in deviated wells.

A further example of a centraliser will now be described with reference to figures 17 and 18. In the example shown in Figs 17 and 18, a centraliser has a body in the form of a cylindrical tube having a bore and a central axis as previous described.

Various other features of the centraliser are common to the centralisers 1, 31, 61 described above, and the reader is referred to the earlier examples from which features can be adopted in this variation. The centraliser has blades 105 similar to those previously described, with sidewalls, a bottom and a top face, and an outer face) a central groove 111 and lateral grooves 112 similar to the centraliser 61, but in which the ramps 83 are omitted (they could optionally be incorporated in this variation). The outer face 110 of the blade provides a plateau at the highest point which engages the inner surface of the casing C in use, typically in the form of spaced apart plateau sections hOp which are axially spaced along the blade, like the centraliser 61. Fluid flow is directed by the grooves 111, 112 across a large surface area of the outer face 110 of the blades 105, creating a wider "cushion" of fluid across the blade 105.

The outer face 110 in this example has a series of protrusions in the form of ribs 84 similar to the ribs 21 in the first example 1. For brevity these will not be recited again, but the earlier disclosures of these features in common with earlier examples are specifically imported with respect to this example, and the reader is referred to the earlier parts of this disclosure for the details.

In use, as the string is being made up at the surface, the desired number of centralisers are fixed in desired axial positions on the outer surface of the tubular T, generally as it is being made up into the string and inserted into the hole. The tubular T being inserted into the hole is a smaller diameter than the previous string of casing C which is already cemented in place below the wellhead, and the tubular T is being run into the hole in order to be cemented in pkce coaxially within the previous string of casing C once it has been run in. As the string of tubulars T is run into the drilled section of hole, the centralisers on the outer surface of the tubular T maintain the stand-off, reducing friction between the string of tubulars T and the inner surface of the casing C, and facilitating insertion of the new string into the hole. The operator can be confident that the centralisers maintain a minimum annular stand-off between the outer surface of the tubulars and the inner surface of the casing C. The sides of the plateau sections of the outer surface 110 engage the inner surface of the casing C, and maintain a minimum annular spacing between the outer surface 110 of the centralisers and the inner surface of the casing C. When the string of tubulars T is run into the drilled section of the hole, the hole is typically filled with fluids such as drilling fluid. As the string of tubulars I is run in, it displaces the fluid already in the hole, which flows up the annulus between the outer surface of the tubular T and the inner surface of the casing C, as the string of tubulars I moves down relative to the stationary casing C. The upwardly flowing fluid in the annulus is largely channelled into the flow channels between the blades 105, which provide a large surface area, to promote free passage of fluid within the annulus.

The ramps of the lower ends of the blades 105 act as scoops to divert fluid in the annulus up the ramp and into the groove 111 cut into the outer face 110 of the blade 105. The fluid is scooped up the ramp, and diverted into the central groove 111, which opens onto the surface of the ramp at the lower end of the blade 105. The central groove 111 channels the fluid along the blade, and this creates a fluid "cushion" between the outer face 110 and the inner surface of the casing C, further reducing friction between the string of tubulars T moving into the well, and stationary casing C. Fluid flowing through the central groove 111 is also channelled into the lateral grooves 112, and is thereby diverted across a relatively large surface area of the outer face 110, thereby reducing the frictional resistance to axial insertion of the string of tubulars T into the hole.

The groove 111 incorporates a fluid flow restriction at numerous points between its upper and lower ends (for example, at the junctions between the central groove 111 and the lateral grooves 112, and that the outlets of the lateral grooves 112 as they open onto the sidewalls] which tends to increase the pressure of the fluid within the groove 111, as the string of tubulars T slides axially downwards through the bore of the casing C, moving axially downwards into the well relative to the stationary fluid column, and which also helps to localise the high pressure fluid within the grooves, while at the same time allowing flow of fluid through the grooves. The increased pressure of the flowing fluid within the groove 111 also enhances the cushion effect, using the force of the fluid flow past the centraliserto help to space the body of the centraliser from the inner surface of the casing C. Any interaction between the outer face 110 of the blades 105 and the inner surface of the casing C is therefore reduced. The long narrow ribs 114 reduce even further the interaction between the centraliser and the inner surface of the casing C, which reduces the frictional resistance to continued axial insertion of the string of tubulars T into the casing C. Thus, the operator is able to insert the string of tubulars T into the hole in the casing C with lower force, reducing wear on equipment, and allowing insertion of greater densities ofcentralisers into holes.

The features of the centraliser of figures 1-5 can be combined with the features of the centraliser of figures 6-9, the centraliser of figures 10-16, and/or figures 17 and 18. Indeed, the features of any of the examples described herein can be combined with one another in modifications of the invention.

Claims

Claims 1 A centraliser having a body, the body having a bore adapted to receive a tubular member, the body having a central axis, the body having at least two blades, the blades being circumferentiafly spaced around the body, each of the blades having an outer face, wherein the outer face of at least one of the blades has at least one groove.
2 A centraliser as claimed in claim 1, wherein the outer face of the blade face radially outwards from the axis of the body.
3 A centraliser as claimed in claim 1 claim 2, wherein the groove channels fluids over the outer face of the blade.
4 A centraliser as claimed in any one of claims 1 to 3, wherein at least one groove on the outer face of the blade comprises a central groove aligned with a central axis of the blade.
A centraliser as claimed in any one of claims ito 4, wherein the groove in the outer face of the blade extends radially into the face of the blade, towards the axis of the body of the centraliser, such that the outer face of the blade has a nominal radius which is greater than the radius in the groove.
6 A centraliser as claimed in any one of claims 1 to 5, wherein the groove has a cross section which is at least partiafly spherical.
7 A centraliser as claimed in any one of claims ito 6, wherein the at least some of the dimensions of the groove change along its length.
8 A centraliser as claimed in any one of claims ito 7, wherein the groove is deeper at one end of the groove than at the other end of the groove.
9 A centraliser as claimed in any one of claims 1 to 8, wherein the groove is wider at one end than at the other.A centraliser as claimed in any one of claims ito 9, wherein the volume of the groove changes from one end of the groove to the other.ii A centraliser as claimed in any one of claims ito 10, wherein the groove incorporates a fluid flow restrictor between one end of the groove and the other.12 A centraliser as claimed in any one of claims 1 to 11, wherein the thwer end of the groove has a higher volume than the upper end of the groove.13 A centraliser as claimed in any one of claims ito 12, wherein the groove opens onto a ramp face at the lower end of the blade.14 A centraliser as claimed in any one of claims 1 to 13, wherein the groove opens onto a ramp face at each end of the blade.A centraliser as claimed in claim 14, wherein the opening of the groove at the upper end of the blade is narrower than the opening of the groove at the lower end of the blade.16 A centraliser as claimed in any one of claims 13 to 15, wherein a ramp face of the blade comprises a scalloped portion.17 A centraliser as claimed in any one of claims ito 16, wherein the thwer end of the groove is open and the upper end of the groove is dosed.18 A centraliser as claimed in any one of claims 1 to 17, wherein the groove reduces in depth athng its length from the lower end to the upper end.19 A centraliser as claimed in any one of claims ito 18, wherein the depth of the groove at the upper end of the blade approaches the same radius as the outermost face of the blade.20 A centraliser as claimed in any one of claims 1 to 19, wherein the upper end of the groove has a blind end, closed off from the upper end of the blade.21 A centraliser as claimed in any one of claims 1 to 20, wherein the blade has an axis extending along its length, the groove is parallel to the axis of the blade, and wherein the groove is flanked on each side by plateau sections of the blade.22 A centraliser as claimed in any one of claims ito 21, wherein the outer face of the blade has more than one groove.23 A centraliser as claimed in any one of claims 1-22, wherein the outer face of the blade has at least one central groove, and at least one additional lateral groove which communicates with the central groove.24 A centraliser as claimed in claim 23, wherein more than one lateral groove is formed on each side of the central groove.A centraliser as claimed in any one of claims 23 to 24, wherein the or each lateral groove connects the central groove with the outer wall of the blade.26 A centraliser as claimed in any one of claims 23 to 25, wherein the word each lateral groove extends helically with respect to the central groove.26 A centraliser as claimed in claim 25, wherein the side grooves on a respective side of the central groove are arranged in the same helical direction.27 A centraliser as claimed in claim 25 or 26, wherein lateral grooves on opposite sides of the central groove are arranged in opposite helical directions.28 A centraliser as claimed in claim 27, wherein lateral grooves on opposite sides of the central groove are mutually symmetrical around the central groove.29 A centraliser as claimed in any one of claims 24 to 28, wherein the lateral grooves are arranged in a V-shape with an apex at the central groove, and the sides of the V diverging outwardly and upwardly from the apex towards the top end of the blade.30 A centraliser as claimed in any one of claims 1 to 29, wherein the blade comprises one or more ramps.31 A centraliser as claimed in claim 30, wherein at least one ramp is provided on the outer face of the blade.32 A centraliser as claimed in claim 30 or 31, incorporating more than one ramp, wherein ramps are spaced apart along the blade in an axial direction with respect to the axis of the blade.33 A centraliser as claimed in any one of claims 30 to 32, wherein at least one ramp is arranged adjacent to at least one lateral groove.34 A centraliser as claimed in any one of claims 30 to 33, wherein ramps are arranged in pairs, with one ramp in each pair on a respective side of the central groove, at the same axial position on the outer surface of the blade.A centraliser as claimed in any one of claims 30 to 34, wherein a respective ramp is arranged axially between a side groove and a plateau section.36 A centraliser as claimed in any one of claims 30 to 35, wherein a respective ramp is arranged axially above at least one side groove, and axially below at least one plateau section on the outer face of the blade.37 A centraliser as claimed in claim 36, wherein the pattern of side groove, ramp, and plateau section repeats axially along the length of the outer face of the blade.38 A centraliser as claimed in claim 36 or 37, wherein a central groove runs axially between the ramps and plateau sections on opposite sides of the outer surface of the blade) and connects axially facing ramp sections at the top and bottom ends of the blade.39 A centraliser as claimed in any one of claims 30 to 38, wherein at least one ramp on the outer surface of the blade is inclined in an axial direction, with one low end near the bottom end of the blade, and one higher end near the top end of the blade.A centraliser as claimed in any one of claims 30 to 39, wherein the lower end of the ramp starts within at least one groove on the outer face of the blade.41 A centraliser as claimed in any one of claims 30 to 40, wherein the higher end of the ramp runs into the plateau section.42 A centraliser as claimed in any one of claims 30 to 41, wherein the ramp connects at least one groove with at least one plateau section on the outer surface of the blade.43 A centraliser as claimed in any one of claims ito 42, wherein the outer face of the blade has one or more protrusions extending from the outer face in a radial direction with respect to the axis of the centraliser.44 A centraliser as claimed in claim 43. wherein at least one protrusion is arranged on at least one plateau section of the outer face.A centraliser as claimed in claim 43 or 44, wherein more than one protrusion is provided on the outer face.46 A centraliser as claimed in claim 45, wherein a plurality of protrusions is arranged on opposite sides of the groove.47 A centraliser as claimed in claim 45 or 46, wherein the protrusions are arranged in patterns on the outer face.48 A centraliser as claimed in any one of claims 45 to 47, wherein at least some protrusions on the outer face are spaced from one another at regular intervals.49 A centraliser as claimed in claim 47 or claim 48, wherein the patterns of the protrusions on the outer face diverge circumferentially outwardly from the central groove, and axially upward, towards the top end of the blade.A centraliser as claimed in any one of claims 43 to 49, wherein at least one protrusion extends axially along the outer face of the blade, for at least a short distance that is substantially parallel to the axis of the centraliser, and substantially parallel to the axis of the blade.51 A centraliser assembly incorporating a tubular, a centraliser having a body with a bore adapted to receive the tubular, the body having a central axis, the body having at least two blades extending radially with respect to the axis of the body, the blades being circumferentially spaced around the body) each of the blades having an outer face, wherein the outer face of at least one of the blades has at least one groove.52 A centraliser as claimed in claim 51. wherein the blades extend radially with respect to the axis of the body.53 A centraliser as claimed in claim Si or 52, wherein the blades define between them at least one fluid channel extending between opposite axial ends of the body of the centraliser, between the outer face of the body of the centraliser and the inner surface of the wellbore.54 A centraliser as claimed in any one of claims 51 to 53, wherein the blades have sidewalls extending radially from the outer surface of the body of the centraliser, which define the circumferential boundaries of the fluid channels between the blades.A centraliser as claimed in any one of claims 51 to 54, wherein the blades extend helically around the body.56 A centraliser as claimed in any one of claims 51 to 55, wherein the outer face of the blade is shaped to match the curvature of the interior surface of the casing within which the tubular is being centralised.57 A centraliser as claimed in any one of claims ito 56, wherein the centraliser is a casing centraliser for use in an oil or gas well, for centralising casing or liner within the wellbore.58 A centraliser as claimed in claim 57, wherein the wellbore is lined with tubular casing or liner having a brger diameter than the tubular being centralised, and at least partially surrounding the casing or liner being centralised, and wherein the tubular member is centralised within the bore of the casing or liner.59 A centraliser as claimed in claim 57 or claim 58, wherein the bore of the body of the centraliser is adapted to receive a casing or a liner, and the body of the centraliser is adapted to maintain the radial standoff between the outer surface of the casing and the inner surface of the weilbore.A method of inserting a string of casing into a welibore, the method comprising providing a pkirality of centralisers according to any one of claims 1-60 on the surface of the casing so that the bore of the centraliser receives the casing to be centralised, running the casing into the welibore, and channelling fluid in the welibore into the at least one groove on the outer surface of the blade, thereby creating a fluid cushion between the outer surface of the blade and the inner surface of the welibore.Amendments to the C'aims have been ffled as follows:-Claims 1 A centraliser having a body, the body having a bore adapted to receive a tubular member, the body having a central axis, the body having at least two blades, the blades being circumferentially spaced around the body, each blade having an axis extending along the length of the blade, each of the blades having an outer face, wherein the outer face of at least one of the blades has at least one groove extending at least partially between one axial end of the blade and the other, each groove being arranged parallel to the axis of its blade, and wherein each groove is adapted to channel fluids along at least a part of the length of the blade.2 A centraliser as claimed in claim 1, wherein the outer face of the blade faces radially outwards from the axis of the body.3 A centraliser as claimed in claim 1 claim 2, wherein the groove channels fluids over the outer face of the blade, thereby allowing fluid flow through the groove in an axial direction with respect to the axis of the body.4 A centraliser as claimed in any one of claims 1 to 3, wherein at least one groove on the outer face of the blade comprises a central groove aligned with a central axis of the blade.A centraliser as claimed in any one of claims ito 4, wherein the groove in the outer face of the blade extends radially into the face of the blade, towards the axis of the body of the centraliser, such that the outer face of the blade has a nominal radius which is greater than the radius in the groove.6 A centraliser as claimed in any one of claims 1 to 5, wherein the groove has a cross section which is at least partially spherical.7 A centraliser as claimed in any one of claims ito 6, wherein the at least some of the dimensions of the groove change along its length.8 A centraliser as claimed in any one of claims ito 7, wherein the groove is deeper at one end of the groove than at the other end of the groove.9 A centraliser as claimed in any one of claims i to 8, wherein the groove is wider at one end than at the other.A centraliser as claimed in any one of claims i to 9, wherein the volume of the groove changes from one end of the groove to the other. ioii A centraliser as claimed in any one of claims ito iO, wherein the groove incorporates a fluid flow restrictor between one end of the groove and the other.12 A centraliser as claimed in any one of claims ito ii, wherein the lower end iS of the groove has a higher volume than the upper end of the groove.13 A centraliser as claimed in any one of claims i to i2, wherein the groove opens onto a ramp face at the lower end of the blade.14 A centraliser as claimed in any one of claims i to i3, wherein the groove opens onto a ramp face at each end of the blade.A centraliser as claimed in claim i4. wherein the opening of the groove at the upper end of the blade is narrower than the opening of the groove at the lower end of the blade.16 A centraliser as claimed in any one of claims i3 to 15, wherein a ramp face of the blade comprises a scalloped portion.17 A centraliser as claimed in any one of claims ito i6, wherein the lower end of the groove is open and the upper end of the groove is closed.18 A centraliser as claimed in any one of claims ito 17, wherein the groove reduces in depth along its length from the lower end to the upper end.19 A centraliser as claimed in any one of claims 1 to 18, wherein the depth of the groove at the upper end of the blade approaches the same radius as the outermost face of the blade.A centraliser as claimed in any one of claims 1 to 19, wherein the upper end of the groove has a blind end, closed off from the upper end of the blade.21 A centraliser as claimed in any one of claims ito 20, wherein the blade has an axis extending along its length, the groove is parallel to the axis of the blade, and wherein the groove is flanked on each side by plateau sections of the blade.22 A centraliser as claimed in any one of claims 1 to 21, wherein the outer face of the blade has more than one groove.23 A centraliser as claimed in any one of claims i-22, wherein the outer face of the blade has at least one central groove, and at least one additional lateral groove which communicates with the central groove.24 A centraliser as claimed in claim 23, wherein more than one lateral groove is formed on each side of the central groove.25 A centraliser as claimed in any one of daims 23 to 24, wherein the or each lateral groove connects the central groove with the outer wall of the blade.26 A centraliser as claimed in any one of daims 23 to 25, wherein the or each lateral groove extends helically with respect to the central groove.26 A centraliser as claimed in claim 25. wherein lateral grooves on a respective side of the central groove are arranged in the same helical direction.27 A centraliser as claimed in claim 25 or 26, wherein lateral grooves on opposite sides of the central groove are arranged in opposite helical directions.28 A centraliser as claimed in claim 27, wherein lateral grooves on opposite sides of the central groove are mutually symmetrical around the central groove.29 A centraliser as claimed in any one of daims 24 to 28, wherein the lateral grooves are arranged in a V-shape with an apex at the central groove, and the sides of the V diverging outwardly and upwardly from the apex towards the top end of the blade.A centraliser as claimed in any one of claims ito 29, wherein the blade comprises one or more ramps.31 A centraliser as claimed in claim 30, wherein at least one ramp is provided on the outer face of the blade.32 A centraliser as claimed in claim 30 or 31, incorporating more than one ramp, wherein ramps are spaced apart along the blade in an axial direction with respect to the axis of the blade.33 A centraliser as claimed in any one of daims 30 to 32, wherein at east one ramp is arranged adjacent to at least one lateral groove.34 A centraliser as claimed in any one of claims 30 to 33, wherein ramps are arranged in pairs, with one ramp in each pair on a respective side of the central groove, at the same axial position on the outer surface of the blade.35 A centraliser as claimed in any one of daims 30 to 34, wherein a respective ramp is arranged axially between a lateral groove and a plateau section.36 A centraliser as claimed in any one of daims 30 to 35, wherein a respective ramp is arranged axially above at least one lateral groove, and axially below at least one plateau section on the outer face of the blade.37 A centraliser as claimed in claim 36, wherein the pattern of lateral groove, ramp, and plateau section repeats axially along the length of the outer face of the blade.38 A centraliser as claimed in claim 36 or 37, wherein a central groove runs axially between the ramps and plateau sections on opposite sides of the outer surface of the blade, and connects axially facing ramp sections at the top and bottom ends of the blade.39 A centraliser as claimed in any one of claims 30 to 38, wherein at least one ramp on the outer surface of the blade is inclined in an axial direction, with one low end near the bottom end of the bbde, and one higher end near the top end of the blade.A centraliser as claimed in any one of claims 30 to 39, wherein the lower end of the ramp starts within at least one groove on the outer face of the blade.41 A centraliser as claimed in any one of daims 30 to 40, wherein the higher end of the ramp runs into the plateau section.42 A centraliser as claimed in any one of claims 30 to 41, wherein the ramp connects at least one groove with at least one pkteau section on the outer surface of the blade.43 A centraliser as claimed in any one of claims 1 to 42, wherein the outer face of the blade has one or more protrusions extending from the outer face in a radial direction with respect to the axis of the centraliser.44 A centraliser as claimed in claim 43, wherein at least one protrusion is arranged on at least one plateau section of the outer face.A centraliser as claimed in claim 43 or 44, wherein more than one protrusion is provided on the outer face.46 A centraliser as claimed in claim 45, wherein a plurality of protrusions is arranged on opposite sides of the groove.47 A centraliser as claimed in claim 45 or 46, wherein the protrusions are arranged in patterns on the outer face.48 A centraliser as claimed in any one of claims 45 to 47, wherein at least some protrusions on the outer face are spaced from one another at regular intervals.49 A centraliser as claimed in claim 47 or claim 48, wherein the patterns of the protrusions on the outer face diverge circumferentially outwardly from the central groove, and axially upward, towards the top end of the blade.50 A centraliser as claimed in any one of claims 43 to 49, wherein at east one protrusion extends axially along the outer face of the blade, for at least a short distance that is substantially parallel to the axis of the centraliser, and substantially parallel to the axis of the blade.51 A centraliser assembly incorporating a tubular, a centraliser having a body with a bore adapted to receive the tubular, the body having a central axis) the body having at least two blades extending radially with respect to the axis of the body) the blades being circumferentially spaced around the body) each blade having an axis extending along the length of the blade, each of the blades having an outer face, wherein the outer face of at least one of the blades has at least one groove extending at least partially between one axial end of the blade and the other, each groove being arranged parallel to the axis of its blade, and wherein the groove is adapted to channel fluids along at least a part of the length of the blade.52 A centraliser assembly as claimed in claim 51, wherein the blades extend radially with respect to the axis of the body.53 A centraliser assembly as claimed in claim 51 or 52, wherein the blades define between them at least one fluid channel extending between opposite axial ends of the body of the centraliser, between the outer face of the body of the centraliser and the inner surface of the wellbore.54 A centraliser assembly as claimed in any one of claims 51 to 53, wherein the blades have sidewalls extending radially from the outer surface of the body of the centraliser, which define the circumferential boundaries of the fluid channels between the blades.A centraliser assembly as claimed in any one of claims 51 to 54, wherein the blades extend helically around the body.56 A centraliser assembly as claimed in any one of claims 51 to 55, wherein the outer face of the blade is shaped to match the curvature of the interior surface of the casing within which the tubular is being centralised.57 A centraliser assembly as claimed in any one of claims ito 56, wherein the centraliser is a casing centraliser for use in an oil or gas well, for centralising casing or liner within the weilbore.58 A centraliser assembly as claimed in claim 57, wherein the wellbore is lined with tubular casing or liner having a larger diameter than the tubular being centralised, and at least partially surrounding the casing or liner being centralised, and wherein the tubular member is centralised within the bore of the casing or liner.59 A centraliser assembly as claimed in claim 57 or claim 58, wherein the bore of the body of the centraliser is adapted to receive a casing or a liner, and the body of the centraliser is adapted to maintain the radial standoff between the outer surface of the casing and the inner surface of the wellbore.A method of inserting a string of casing into a wellbore, the method comprising providing a plurality of centralisers according to any one of claims 1-60 on the surface of the casing so that the bore of the centraliser receives the casing to be centralised, running the casing into the wellbore, and channelling fluid in the welibore into the at least one groove on the outer surface of the blade, thereby creating a fluid cushion between the outer surface of the blade and the inner surface of the weilbore.