WO2016030689A1

WO2016030689A1 - Improvements in or relating to centralisers

Info

Publication number: WO2016030689A1
Application number: PCT/GB2015/052492
Authority: WO
Inventors: James Edward Martin; Mark Mattison; Evangelos SAMIOTIS; Alexander WILKINSON
Original assignee: Reece Innovation Centre Limited
Priority date: 2014-08-27
Filing date: 2015-08-27
Publication date: 2016-03-03
Also published as: GB2544680A; GB201702982D0; US20170260816A1

Abstract

A centraliser (110) comprises longitudinally spaced collars (115, 120) connected by a plurality of springs (125), each of the springs (125) comprising two or more bow sections (127, 128, 129).

Description

IMPROVEMENTS IN OR RELATING TO CENTRALISERS

Centralisers are used in the oil, gas and water well drilling industries to centre a tubular member within a borehole (wellbore) or inside a previously installed larger tubular member (casing).

The purpose of a centraliser is to facilitate running casing to the desired depth and to assist in centring the casing in the wellbore. One of the main objectives of centralising a casing string is to facilitate good cement sealing within the annulus of the well bore between the outer surface of the casing and the formation wall, thereby isolating fluids from different zones.

A centraliser is a mechanical device that keeps casing from contacting the wellbore wall; maintaining a continuous 360-degree annular space around casing allows cement to completely seal the casing to the borehole wall.

There are two distinct classes of centralisers.

The older and more common is a simple, low-cost bow-spring design. Since the bow springs are slightly larger than the wellbore, they can provide complete centralisation in vertical or slightly deviated wells. However, they often struggle to support the weight of the casing adequately in deviated or horizontal wellbores.

The second type is a rigid blade design. This type is rugged and works well even in deviated or horizontal wellbores, but since the centralisers are smaller than the wellbore, they will not provide as good centralisation as bow-spring type centralizers in vertical wells. Rigid-blade casing centralisers are slightly more expensive and can cause trouble downhole if the wellbore is not in excellent condition. They are unable to compress and thereby unable to pass through narrow openings that may be caused when the wellbore collapses. This may result in expensive and time consuming operations to open up the hole in order to enable passage of the rigid blade centralizer.

Effective centralisation assists in the removal of drilling mud from the well bore annulus and helps ensure an even cement coat around the casing. Poor centralising of the casing within the wellbore will lead to inadequate cementing and will result in costly repair or potentially even 'killing' and losing the well.

In recent years the trend has been towards drilling more extended reach horizontal wells and so there is a real need to develop centralisers with a low moving force that can also support the weight of the casing and enable a good cement seal, whilst having the ability to pass through tight spots.

The ability to support the casing through larger openings is often also a benefit. Eccentric casing can lead to unequal or unbalanced annuli on the high and low side of the borehole resulting in mud pockets on the low side because the cement will tend to follow the path of least resistance.

I In summary, centralisers assist in the efficient installation of casing and enable cementing of the casing within the borehole.

There are many existing patents for differing designs of centralisers, and basic designs have existed for over seventy years. More recent patents often pertain to either the specific manufacturing process or materials used. The bow-spring centraliser is a well-established concept, and whilst prior art covers an extensive array of geometric designs variations, the fundamental principles of the form and shape remain largely the same.

Examples of bow-spring patents include US 6997254, US 8196670, US 3312285, US 2228648 and US 4909322.

The present invention seeks to provide improvements in or relating to centralisers.

A good centraliser for use in deviated or extended reach horizontal wells should be optimised to have a low moving force (minimum effort needed to push the casing through the well), a high restoring force (maximum support given to weight of the casing), the ability to pass through tight spots in collapsed or constricted boreholes (minimum outer diameter upon full compression), ability to increase diameter whilst passing through 'under-reamed' or 'washout' sections of the borehole (sections with a larger opening than intended), the ability to enable even and full flow of cement through and around the centralizer so as not to create voids or mud pockets that would require expensive remedial action.

In one aspect the present invention provides a centraliser comprising a plurality of springs, each spring comprising a larger, taller outward bow and a smaller, shorter outward bow, and an intermediate inward bow between the outward bows.

The present invention also provides a centraliser for keeping wellbore casing from contacting a formation wall, the centraliser comprising a plurality of springs, each spring comprising a larger, taller outward bow and a smaller, shorter outward bow, and an intermediate inward bow between the outward bows, the springs having a three phase action: i) whilst subject to low forces each spring acts as a larger, single bow spring against the formation wall; ii) with increased forces the inward bow contacts the casing to transform the spring into a shorter, stiffer bow spring; and iii) greater still forces cause the height of the two outward bows to equalise, transforming the spring into two smaller and stiffer bow springs.

According to a further aspect of the present invention there is provided a centraliser for keeping wellbore casing from contacting a formation wall, the centraliser comprising two longitudinally spaced collars connected by a plurality of springs, the springs comprising means for reducing pressure on the formation within the well by spreading the load over a greater contact area between the centraliser and the formation wall. The present invention also provides a centraliser for keeping wellbore casing from contacting a formation wall, the centraliser comprising two longitudinally spaced collars connected by a plurality of bow springs, the springs comprising means for reducing pressure on the formation within the well by spreading the load over a greater contact area between the centraliser and the formation wall, in which the cross-section of at least part of each box is curved.

Each bow may include a region of increased width. The cross section of the region of increased width may be curved. The region of increased width may be provided in the region of an apex of the bow.

This provides a stronger bow that does not deform easily, as well as providing a "ski" effect circumferentially as well as longitudinally across the formation i.e. there is no gouging and no cutting into the formation along the edge of the bow (which might otherwise occur if the bow apex were flat and had only two points of contact on the predominantly round bore hole).

The present invention also provides a centraliser comprising longitudinally spaced collars connected by a plurality of springs, each of the springs comprising two or more outward bow sections. The bows may be generally the same or different; for example the same or a different length and/or the same or a different resting height (for example radial extent) in use.

The present invention also provides a centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs comprising two or more curved sections, in which the, or at least two, of the sections have a different height in a resting position.

The present invention also provides a centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs having an undulating section.

The present invention also provides a centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs comprises an irregular curve.

The present invention also provides a centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs comprises one or more apices which contact a formation in use, the or each apex region being wider than the remainder of the spring.

The present invention also provides a centraliser comprising two longitudinally spaced collars connected by a plurality of springs, the springs contacting the collars at junctions, in which the junctions are rounded whereby to reduce stress.

The present invention also provides a centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs having a peak, in which the longitudinal position of peaks on at least two springs is different. In some embodiments alternate springs have different peak positions. For example there may be two spring peak positions which are longitudinally offset from each other.

In some embodiments the peaks are spaced circumferentially and/or rotationally and/or longitudinally. For example, in one embodiment six springs are provided, giving a first set of three points of contact which are 120° apart and a second set of three points of contact which are also 120° apart, but are also rotated 60° from the first set and are longitudinally offset from the first set.

The present invention also provides a centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs being generally helical. The or each spring may be a coiled spring.

A single helix that performs one complete revolution at the peak diameter may be provided. This may have significant advantages during manufacturing and in operation - one part, no interconnections, less to go wrong or break.

The diameter of the helices may change along the length thereof. For example the diameter of the helices may decrease from a mid-point towards the collars.

The present invention also provides a centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs being generally spiral. The or each spring may be a coiled spring.

The spring/s may have a generally circular cross-section. In some embodiments the cross section of spring/s may be polygonal, square, rectangular, hexagonal, diamond, elliptical, oval, egg-shape or trapezoidal. In embodiments with a non-circular cross-section the section may twist with the helix.

A circular cross-section may be preferred (for example because of ease of manufacture as well as inherent coil spring forces), but it may be beneficial to use another shape cross-section e.g. for friction reduction, for area spread, or maybe for manufacture (end collar twists may be better with flat edges).

Where a plurality of springs are provided all of the springs may have the same cross section; in some embodiments one or more different cross sections are present in a spring array.

The present invention also provides a centraliser comprising two longitudinally spaced collars connected by a plurality of springs, in which there is provided means for reduction of the co-efficient of friction of centraliser surfaces in contact with a formation and/or in contact with an casing outer wall.

In some embodiments at least part of the centraliser has a low-friction coating, for example

Molybdenum Disulphide or Diamond Like Carbon. The centraliser may be provided with one or more low-friction pads.

The centraliser may be provided with one or more wear pads. The wear pad may be a sacrificial pad to protect the spring s from damage, enabling it to reach its final resting place fully intact.

One or more low friction and/or wear pads may be fitted to a bow apex where they come into contact with the casing/formation. Alternatively or additionally they may be fitted to the ends (for example the front edges) of the end collars to protect against abrupt impacts of constrictions in a wellbore.

The centraliser may include surfaces with surface formations for reducing frictions. For example 3- dimensional patterns, a patterned surface, protruding dimples or bosses.

The surface formations may be formed on the surface by, for example, impressing, cutting, coating or moulding.

The surface formations may be provided externally on wall formation surface-contacting regions of the centraliser. Additionally or alternatively formations may be provided internally on casing/pipe contacting regions. In some embodiments surface formations are provided over substantially the entirety of the or each spring. In other embodiments formations are provided only in certain regions. For example, the surface formations may be provided only on the springs; or just spring peaks where present.

The present invention also provides a centraliser comprising one or more bows, the or each bow having a curved cross-section, and the or each bow having scalloped or wavy or ribbed edges for reducing the stress incurred as the bow flexes in use, whilst at the same time maintaining the overall contact area for skiing across a formation.

The present invention also provides a centraliser comprising a plurality of bow springs, each bow curving back in on itself at the two opposing ends to form collars that are directly underneath the bow.

The present invention also provides a centraliser comprising a plurality of bows, in which each bow loops around at both ends of arch/profile thereby creating a tube / hollow that in turn provides a pivot point enabling an axle to pass through and hence permitting free rotation of the bow ends.

The axle might be round bar in a ring to form the end collar of the centraliser assembly. This might in turn will be affixed to a traditional flat collar in order to maintain rigidity and prevent twisting and potential locking onto the pipe as the bows compress and stretch independently.

The present invention also provides a centraliser comprising one or more coiled springs with a diameter of the helix greater in the middle than at the two ends, and with a one or more revolutions of helix of equal diameter at the ends, and with one or more revolution at the centre at the largest diameter. The present invention also provides a centraliser comprising two or more coiled springs with diameter of the helix greater in the middle than at the two ends, and with a partial revolution or one or more (full or partial) revolutions of helix of equal diameter at the ends.

The pitch of the helix may be such that the plurality of helices are able to intertwine.

The coils at the ends may be welded together so as to create one centraliser assembly and maintain interaction between all helices. This may be similar to other helix designs herein with multiple helices, but with coiled ends, intertwined and simply welded together.

End collar coils may be slotted into tubular sections which in turn are welded onto an inner collar.

End collar coils may be bent twice to create an axial parallel section before returning to the original helical angle, thereby producing a circumferential mechanical stop for the next helix in the sequence. Alternatively the return may be such that the remainder of the bar is at 90deg to the axial direction, i.e. in the radial plane (this is what is drawn on the sketches. Whilst a return to the helical path may work, and may be employed, it may be preferable for prevention of dislodging if the return is in the radial plane, or maybe even better if it doubles back toward the centraliser slightly so that the end of the bar lodges into the kink of the next helix.

Two or more helices may be provided. For example three helices may be provided and interlocked by ensuring that the stop is set approximately 120 degrees from the end of the next helix and having the three helices interlock both circumferentially as well as axially. If, for example, four helices are provided then the "stop" will be set to the appropriate angle, in this case 90deg.

The present invention also provides a centraliser comprising longitudinally spaced collars connected by a plurality of springs, each of the springs comprising two or more bow sections, the transition between the bow and the end collar consisting of two bends of opposing direction to create an S-bend.

The present invention also provides a centraliser comprising one or more coiled springs with a plurality of sections differing in pitch and diameter such that a transition is made from an end collar section to a centralising bow section to a mid-collar to another bow section and then to an opposite end collar.

There may be two or more mid-collar sections. There may be two or more bow sections.

Each bow section may be offset in angle such that the resulting peaks are evenly spaced around the circle when viewed from the end axial orientation.

Each bow section may have a partial revolution so that the accumulative total revolution of all the bow sections is at least one full revolution of the circle. The present invention also provides a bow spring centraliser comprising a plurality of bow springs and two internally facing end collars.

A small radius curve/bend may be provided between each bow and the internally facing end collars which provides sufficient flexibility and movement around the transition area such that when the bow is compressed and loaded the stresses are distributed through the curve/bend and dissipated away from the transition radius, effectively allowing the bow to flex (almost pivot) somewhat rather than to crease at the transition area.

The present invention also provides a bow spring centraliser comprising a plurality of bow springs and two externally facing end collars.

A small radius curve/bend may be provided between each bow and the externally facing end collars which provides sufficient flexibility and movement around the transition area such that when the bow is compressed and loaded the stresses are distributed through the curve/bend and dissipated away from the transition radius, effectively allowing the bow to flex (almost pivot) somewhat rather than to crease at the transition area.

The present invention also provides a centraliser having an end collar with inward facing castellations. The present invention also provides a centraliser with a wavy or scalloped end collar.

The present invention also provides a centraliser with a twin phase bow design, the centraliser having a plurality of major bow springs extending between end collars, each major spring having a respective minor spring, the minor springs being positioned under the major springs, in a first loading phase just the major bow springs are compressed and in a second, greater loading phase the major bow springs are pressed onto their respective minor springs.

The end collars may be inward facing collars.

The present invention also provides a well bore having one or more centralisers as described herein.

Aspects and embodiments of the present invention aim to improve centraliser design and performance to achieve greater reliability, for example in extended reach wells.

Different aspects of the present invention may be used separately or together.

Further particular and preferred aspects of the present invention are set out in the accompanying independent and dependent claims. Features of the independent and dependent claims may be combined with the features of the other independent and dependent claims as appropriate, and in combination other than those explicitly set out in the claims.

The present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

Figure I . I is an isometric view of a centraliser formed in accordance with the present invention; Figure 1.2 is a side elevation of the centraliser of Figure I . I ; Figure 1.3 is an end view of the centraliser of Figure I . I ;

Figure 2 is an isometric view of a centraliser formed according to an alternative aspect; Figure 2.1 shows three phases of operation of a spring forming part of the centraliser of Figure 2; Figure 2.2 illustrates bow-spring stiffness during the three phases shown in Figure 2.1 ; Figure 3 is an isometric view of a centraliser formed according to a further aspect;

Figure 4.1 is an isometric view of a single helix spring coil centraliser formed according to the present invention;

Figure 4.2 is a side view of the centraliser of Figure 4.1 ; Figure 4.3 is an end view of the centraliser of Figure 4.1 ;

Figure 4.4 is a further side view of the centraliser of Figure 4.2, shown rotated 90 degrees;

Figure 5.1 is an isometric view of a single helix spring coil centraliser formed according to a further aspect;

Figure 5.2 is a magnified view of one end of the centraliser of Figure 5.1 ;

Figure 6.1 is an isometric view of a single helix spring coil centraliser formed according to a further aspect; Figure 6.2 is a magnified view of one end of the centraliser of Figure 6.1 ;

Figure 7.1 is an isometric view of a single helix spring coil centraliser formed according to a further aspect;

Figure 7.2 is a magnified view of one end of the centraliser of Figure 7.1 ; Figure 8.1 is an isometric view of a centraliser formed according to an alternative aspect; Figure 8.2 is a side view of the centraliser of Figure 8.1 ;

Figure 8.3 is an end view of the centraliser of Figure 8.1 with curvature shown exaggerated to illustrate the curvature;

Figure 8.4 is a magnified view of one bow of the centraliser of Figure 8.3 with curvature shown exaggerated to illustrate the curvature;

Figure 8.5 is a further side view of the centraliser of Figure 8.2, shown rotated 90 degrees;

Figure 8.6 is a magnified view of one end of the centraliser of Figure 8.1 illustrating rounded corners;

Figure 9.1 is an isometric view of a centraliser formed according to a further aspect;

Figure 9.2 is a side view of the centraliser of Figure 9.1 ;

Figure 9.3 is an end view of the centraliser of Figure 9.1 ;

Figure 9.4 is a further side view of the centraliser of Figure 9.2, shown rotated 90 degrees;

Figure 10.1 is an isometric view of a centraliser formed according to a further aspect;

Figure 10.2 is a side view of the centraliser of Figure 10.1 ;

Figure 10.3 is an end view of the centraliser of Figure 10.1 ;

Figure 10.4 is a magnified view of one bow of the centraliser of Figure 10.3;

Figure 10.5 is a further side view of the centraliser of Figure 10.2, shown rotated 90 degrees;

Figure I I . I is an isometric view of a centraliser formed according to a further aspect;

Figure I 1.2 is an end view of the centraliser of Figure I I . I ;

Figure I 1.3 is a magnified view of one bow of the centrliaser of Figure I 1.2;

Figure I 1.4 is a perspective view of the view of Figure I 1.3; Figure 12.1 is an isometric view of a single helix spring coil centraliser formed according to the present invention;

Figure 12.2 is a side view of the centraliser of Figure 12.1 ; Figure 12.3 is an end view of the centraliser of Figure 12.1 ;

Figure 12.4 is a further side view of the centraliser of Figure 12.2, shown rotated 90 degrees;

Figure 1 3.1 is an isometric view of a multiple helix spring coil centraliser formed according to a further aspect;

Figure I 3.2 is a side view of the centraliser of Figure I 3.1 ; Figure I 3.3 is an end view of the centraliser of Figure I 3.1 ;

Figure I 3.4 is a further side view of the centraliser of Figure I 3.2, shown rotated 90 degrees; Figure I 3.5 is a magnified view of one end of the centraliser of Figure I 3.1 ;

Figure 14.1 is an isometric view of an interlocked multiple helix spring coil centraliser formed according to a further aspect;

Figure 14.2 is a magnified view of one end of the centraliser of Figure 14.1 ; Figure 14.3 is a perspective view of the magnified view of Figure 14.2; Figure 14.4 is a side view of the centraliser of Figure 14.1 ; Figure 14.5 is an end view of the centraliser of Figure 14.1 ;

Figure 14.6 is a further side view of the centraliser of Figure 14.4, shown rotated 90 degrees; Figure l4.7 shows one helix forming part of the centraliser of Figure 14.1 ; Figure 14.8 is a side view of the helix of Figure 14.7;

Figure 14.9 is a side view of the helix of Figure 14.8 shown rotated 45 degrees; Figure 14. 1 0 is a side view of the helix of Figure 14.8, shown rotated 90 degrees;

Figure 1 5. 1 is an isometric view of a multiple helix spring coil centraliser formed according to a further aspect;

Figure 1 5.2 is a magnified view of one bow spring of the centraliser of Figure 1 5. 1 ; Figure 1 5.3 is a side view of the centraliser of Figure 1 5. 1 ; Figure 1 5.4 is an end view of the centraliser of Figure 1 5. 1 ;

Figure 1 5.5 is a magnified view of one of the springs of the centraliser of Figure 1 5.4;

Figure 1 5.6 is a further side view of the centraliser of Figure 1 5.3, shown rotated 90 degrees;

Figure 1 6. 1 is an isometric view of a multiple helix spring coil centraliser formed according to a further aspect;

Figure 1 6.2 is a side view of the centraliser of Figure 1 6. 1 ;

Figure 1 6.3 is an end view of the centraliser of Figure 1 6. 1 ;

Figure 1 6.4 is a magnified view of one of the springs of the centraliser of Figure 1 6.3;

Figure 1 6.5 is a further side view of the centraliser of Figure 1 6.2, shown rotated 90 degrees;

Figure 1 7. 1 is an isometric view of a centraliser formed according to a further aspect;

Figure 1 7.2 is a magnified view of one end of the centraliser of Figure 1 7. 1 ;

Figure 1 7.3 is a side view of the centraliser of Figure 1 7. 1 ;

Figure 1 7.4 is an end view of the centraliser of Figure 1 7. 1 ;

Figure 1 7.5 is a further side view of the centraliser of Figure 1 7.3, shown rotated 90 degrees;

Figure 1 8. 1 is an isometric view of a centraliser formed according to a further aspect;

Figure 1 8.2 is a magnified view of one end of the centraliser of Figure 1 8. 1 ; Figure 18.3 is a side view of the centraliser of Figure 18. 1 ;

Figure 18.4 is an end view of the centraliser of Figure 18.1 ;

Figure 18.5 is a further side view of the centraliser of Figure 18.3, shown rotated 90 degrees; Figure 1 9.1 is an isometric view of a centraliser formed according to a further aspect; Figure 1 9.2 is a magnified view of one end of the centraliser of Figure 1 9.1 ; Figure 1 9.3 is a side view of the centraliser of Figure 1 9. 1 ; Figure 1 9.4 is an end view of the centraliser of Figure 1 9.1 ;

Figure 1 9.5 is a further side view of the centraliser of Figure 1 9.3, shown rotated 90 degrees;

Figure 1 9.6 is an exploded view of the centraliser of Figure 1 9.1 ;

Figure 20.1 is an isometric view of a centraliser formed according to a further aspect;

Figure 20.2 is a magnified view of one end of the centraliser of Figure 20.1 ;

Figure 20.3 is a perspective view of the end of Figure 20.2;

Figure 20.4 is a side view of the centraliser of Figure 20. 1 ;

Figure 20.5 is an end view of the centraliser of Figure 20.1 ;

Figure 20.6 is a further side view of the centraliser of Figure 20.4, shown rotated 90 degrees.

Figures 21 .1 to 21 .4 show perspective, side and end views of a centraliser formed in accordance with an aspect of the present invention;

Figures 22.1 to 22.5 show perspective, side and end views and a magnified end view of a centraliser formed in accordance with the present invention;

Figure 23 shows a centraliser with a single internal stop collar, formed in accordance with the present invention and fitted onto a pipe;

Figures 24.1 to 24.4 show perspective, side, end and magnified end views of a centraliser formed in accordance with the present invention; Figures 25.1 to 25.3 show perspective, side and end views of a centraliser formed in accordance with the present invention, the centraliser being formed with a single internal stop collar and being shown fitted onto a pipe;

Figure 26.1 and 26.2 show perspective and side views of a centraliser with two external stop collars, the centraliser being formed in accordance with the present invention and shown fitted onto a pipe;

Figure 27.1 is a perspective view of a centraliser with a single internal stop collar, the centraliser being formed in accordance with the present invention and shown fitted onto a pipe;

Figure 27.2 shows the centraliser of Figure 27.1 removed from the pipe; and

Figure 27.3 is a side view of the centraliser of Figure 27.2.

Example embodiments are described below in sufficient detail to enable those of ordinary skill in the art to embody and implement the systems and processes herein described. It is important to understand that embodiments can be provided in many alternate forms and should not be construed as limited to the examples set forth herein.

Accordingly, while embodiments can be modified in various ways and take on various alternative forms, specific embodiments thereof are shown in the drawings and described in detail below as examples. There is no intent to limit to the particular forms disclosed. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims should be included. Elements of the example embodiments are consistently denoted by the same reference numerals throughout the drawings and detailed description where appropriate.

The terminology used herein to describe embodiments is not intended to limit the scope. The articles "a," "an," and "the" are singular in that they have a single referent, however the use of the singular form in the present document should not preclude the presence of more than one referent. In other words, elements referred to in the singular can number one or more, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes," and/or "including," when used herein, specify the presence of stated features, items, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, items, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein are to be interpreted as is customary in the art. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealized or overly formal sense unless expressly so defined herein. The device 10 of Figures I . I to 1.3 aims to reduce pressure on the formation within the well by spreading the load over a greater contact area between the centraliser and the formation wall, therefore reducing the risk of the bows digging into the formation and thereby maintaining a more centralised location of casing, whilst reducing the effective friction of the contact. The larger contact area and reduced pressure resulting from the design may be particularly useful for centraliser contact with softer formations, e.g. shale, chalk, clay, etc.

The device comprises two longitudinally spaced collars 15, 20 with six bow springs 25 connected therebetween. In other embodiments (not shown) devices may have less or more springs (for example two, three, four, five, seven, eight, nine or ten). In this embodiment two collars are provided; in other embodiments (not shown) more spaced collars are provided with springs therebetween.

Each of the springs 25 has an apex or peak 30 along its length. In this embodiment the springs are wider at the peaks (to increase surface area contact on wall formation material in use). The bow is also curved in cross-section to prevent side edges of the bow from digging/gouging into the formation circumferentially as well as longitudinally.

The lower sections of the bow are narrower to permit flexibility as the bow is acted upon, whilst the curvature enables the structure to maintain its shape and strength. The narrow sections towards the base of the bows also permit a good flow-by area for cementing due to the maintaining a larger void area between bows close to the inner casing/pipe (as viewed longitudinally).

Three of the springs have their peak closer to the collar 15; three of the springs have their peak closer to the collar 20. This means that alternate bow spring apexes are longitudinally offset, forming two rings of contact where each ring is formed from three springs making three points of contact. There are two principal benefits: ( I ) the two rings of contact present a restoring moment force onto the casing to aid centralization, and (2) the three springs initially meeting a constriction are compressed first and pass through the constriction before the trailing three springs are compressed, the centraliser's restoring forces would thus be better distributed longitudinally to ease passage through the constriction.

The device of Fig. I also aims to distribute stress better at the junctions 35 where each bow spring is connected to the centralised rings. The junctions 35 are rounded, which reduces the risk of centraliser mechanical failure.

The device I 10 of Fig. 2 aims to exert more centralising force on the casing as the compression increases.

Each spring 125 of the device has a double outward bow arrangement, with a larger, taller bow 127 and a smaller, shorter bow 129, and a single inward bow 128 between the outward bows. Each spring acts as a larger single and weaker bow-spring whilst subject to minimal forces [Phase I ] (as experienced on vertical or near vertical borehole sections), but as forces are increased (due to the weight of casing bearing onto the centraliser in more horizontal sections) the central inward facing bow will connect with the casing outer wall and the higher bow effectively transforms into a shorter, stiffer bow spring [Phase 2], and further still as the height of the two bows 127, 129 become equal the system effectively transforms into two smaller and stiffer bow-springs [Phase 3]. As shown in Fig 2.1 and Fig 2.2:

Phase I

RHS bow 127 only in contact with formation. Phase 2

RHS bow 127 only in contact with formation, and central inward facing bow 128 in contact with casing. Phase 3

RHS bow 127 and LHS Bow 129 in contact with formation, and central inward facing bow 128 in contact with casing.

The spring therefore has multiple bows; in this embodiment being a longer, prouder bow and a shorter, less proud bow. In other embodiments (not shown) more bows may be provided, for example three or more.

An alternative fundamental design of centraliser 210 is shown in Fig. 3, which aims to provide greater contact area with the formation in order to minimize gouging into the formation.

Up to 360degree circumferential contact area for support (restoring force in all radial directions) can be achieved depending on the number of revolutions and helices employed - whilst at the same time no linear point contact is present that might induce gouging, i.e. the contact area is constantly changing around the circumference of the borehole due to the helical design. In other words there is no linear point contact in the axial (or inner pipe direction).

It induces axial tension through helical coiled springs 225, thereby inducing restoring force on the formation. Torsional forces aid passage through constricted sections by effectively twisting and wrapping tightly around the casing. The spiral nature of the design aids the natural flow of the cement through and around the centralizer with minimal restriction to flow-by cross-sectional area - both important characteristics to minimize the chance of unwanted voids or cavities or mud pockets. A gentle increase in slope along the path of the spiral and the easy rotation of the helix design both aid to reduce starting and push forces through tight spots.

The helix / spiral provides a natural guide away, and around, obstacles or protruding rocks, and prevents snagging or sticking or lodging since the centraliser is free to rotate around the pipe. In Figures 4.1 to 4.4 a centraliser 310 formed according to an alternative embodiment is shown. In this embodiment a single helical coiled spring 325 with a circular cross section is provided. The ends 315, 320 of the spring 325 are coiled to form integral end collars. The end view (Figure 4.3) illustrates a substantially 360 degree contact whilst not being in the same plane i.e. a single coil contacts the full circle, but the contact is spread longitudinally. This arrangement induces axial tension through helical coiled springs 325, thereby inducing restoring force on the formation. Torsional forces aid passage through constricted sections by effectively twisting and wrapping tightly around the casing. The spiral nature of the design aids the natural flow of the cement through and around the centralizer with minimal restriction to flow-by cross-sectional area - both important characteristics to minimize the chance of unwanted voids or cavities or mud pockets. A gentle increase in slope along the path of the spiral and the easy rotation of the helix design both aid to reduce starting and push forces through tight spots.

Figures 5.1 and 5.2 show a centraliser 410 formed according to an alternative embodiment. The centraliser 410 is similar to the centraliser 310 of Figure 4, except that the spring 425 has a generally square cross section.

Figures 6.1 and 6.2 show a centraliser 510 formed according to an alternative embodiment. The centraliser 510 is similar to the centralisers 310, 410 of Figures 4 and 5, except that the spring 525 has a generally hexagonal cross section.

Figures 7.1 and 7.2 show a centraliser 610 formed according to an alternative embodiment. The centrliaser 610 is similar to the centralisers 310, 410, 510 of Figures 4 to 6, except that the spring 625 has a generally elliptical cross section.

Figures 8.1 to 8.6 show a centraliser 710 formed according to an alternative embodiment.

The device 710 shares some similarities with the device 10 of Figure I . The highest point 730 of each bow is curved longitudinally and transversely. In this embodiment the curvature of the wider contact area is greater.

A wide flat bow deforms in a much different way and a curved cross-section is advantageous in terms of retaining the bow structure as it is acted upon.

The curvature has two benefits:

a) to strengthen the bows 725 and permit the use of thinner material and/or narrower bow widths; and

b) it provides a ski affect in all directions, without the chance of a sharp edge digging sideways into the formation. In this embodiment the radius of the curvature after insertion into the hole would be equal or less than the radius of that hole. In this embodiment the apices 730 of all of the bows 725 are generally in the same place longitudinally. In other embodiments (not shown) a longitudinally offset array may be provided.

In addition, in this embodiment there are rounded corners 724 where the bows 725 meet the end collars 715, 720, as shown best in Figure 8.6.

Referring now to Figures 9.1 to 9.4 there is shown a centraliser 810 formed according to an alternative embodiment.

The device shares some similarities with the device 10 of Figure I . Three of the springs 825a have their peak closer to the collar 815; three of the springs 825b have their peak closer to the collar 820. This means that alternate bow spring apexes 830a, 830b are longitudinally offset, forming two rings of contact where each ring is formed from three springs making three points of contact. There are two principal benefits: ( I ) the two rings of contact present a restoring moment force onto the casing to aid centralization, and (2) the three springs initially meeting a constriction are compressed first and pass through the constriction before the trailing three springs are compressed, the centraliser's restoring forces would thus be better distributed longitudinally to ease passage through the constriction.

Figures 10.1 to 10.5 show a centraliser 910 formed according to an alternative embodiment.

The centraliser shares some similarities with the device 710 of Figures 8.1 to 8.6. In this embodiment each of the widened apices 930 has a plurality (in this case three, although one or more may be provided in other embodiments) of longitudinal ridges 931. The ridges reduce frictional contact on a casing in use.

Figures I I . I to I 1.4 show a centraliser 1010 formed according to an alternative embodiment. The centraliser 1010 comprises a plurality of generally straight bow springs 1025 connected at either end to a respective end collar 1015, 1020.

Each of the springs 1025 comprises a plurality of mutually spaced dimples 1026 pressed out from the inside face so that they extend radially outwards. In this embodiment the dimples 1026 are restricted generally to the highest region of each bow.

The dimples 1026 help to reduce frictional contact of the bows on a casing in use whilst maintaining an overall spread of load across the full width of the bow.

Referring now to Figures 12.1 to 12.4 there is shown a centraliser I I 10 formed according to an alternative embodiment. The centraliser I I 10 is similar to the centraliser I 10 of Figure 2 and accordingly has a plurality of springs I 125, with each spring 125 of the device have a double outward bow arrangement, with a larger, taller bow I 127 and a smaller, shorter bow I 129, and a single inward bow I 128 between the outward bows I 127, I 129 to give a generally sinuous configuration. In Figures 1 3.1 to I 3.5 there is shown a centraliser 1210 formed according to an alternative embodiment.

The centraliser 1210 comprises multiple (in this embodiment three) coiled springs 1225a, 1225b, 1225c. At the end of each spring the coil tightens to give one complete revolution. Collectively the three end coil termini 1227a, 1227b, 1227c, 1228a, 1228b, 1228c form an integral collar 1215, 1220.

Each spring has its greatest diameter generally centrally along the length thereof (although circumferentially offset from each other). As shown best on Figure 13.3, none of the coils has a full revolution at the centre of the centraliser; all three coils are required to give substantially 360 degree cover i.e. enough to contact in all directions. The benefit of a multi-helix is that each helix does not have to elongate as far to wrap around a pipe.

To secure the springs together, the end coil revolutions are welded together, as shown best in Figure I 3.5 in which welds 1229 adjoining the gap between coils is shown. This means that the coils cannot twist independently of each other.

In this embodiment the welds extend along the last 120 degrees of each helix end i.e. there are three weld beads (in other embodiments, not shown, more or less weld beads with a great or lesser circumferential extent may be used).

Figures 14.1 to 14.10 illustrate a centraliser 1 310 formed according to an alternative embodiment.

The centraliser 1 310 is similar to the centraliser 1 310 of Figure 1 3, with multiple helical coiled springs 1 325a, 1 325b, 1 325c. The ends of the springs also together form integral end collars 1 315, 1 320. However, in this embodiment there is no welding to secure the coils together. Instead, each coil terminus I 327a, I 327b, I 327c, I 328a, I 328b, I 328c includes a kink I 322 - a 90 degree return - and then continue for a further 120 degrees with a terminal tail I 323. Figures 14.7 to 14.10 show one spring I 325a separately for clarity.

Within both end collars the end of each kink butts up against the next, thus locking the springs longitudinally and circumferentially.

Referring now to Figures 15.1 to 15.6 there is shown a centraliser 1410 formed according to a further embodiment.

The centraliser 1410 is similar to the centraliser 710 of Figure 8 and includes a plurality of springs 1425 each having a widened apex region 1430.

In this embodiment the apex regions 1430 include a plurality of shallow scallops 1433 along both edges thereof giving a wave-like appearance. The scallops 1433 help to reduce stresses that might otherwise build up under load and without reducing the overall width of the increased width apex. In this embodiment the scalloped edges extend only in the region of the apex; in other embodiments the scallops may extend along different areas of the bows.

The stress reduction scallops are extremely useful in this embodiment because the apices have a curved cross section; therefore when the apices flatten and compress in use there is a need to alleviate the stresses at the outer edges of the bow.

Referring now to Figures 16.1 to 16.5 there is shown a centraliser 1510 formed according to a further embodiment.

The centraliser 1510 is similar to the centraliser 1410 of Figure 15 and includes a plurality of springs 1525 each having a widened, curved (longitudinally and transversely) apex region 1530.

In this embodiment each apex region has a single, central scallop 1534 along each edge. Again the scallop is provided to alleviate stresses under loading of the springs in use without reducing the overall width of the apex.

Figures 17.1 to 17.5 show a centraliser 1610 formed according to an alternative embodiment. The centraliser 1610 comprises a plurality (in this embodiment six) generally straight bow springs 1625 which merge at each end into an end collar 1615, 1620.

This embodiment in part addresses the need to reduce the stress and the inevitable counter-acting forces resulting from an integral collar where the bow arch transitions to that collar i.e. as the bow flattens and elongates the end collar, being attached via a radius in the opposing direction avoids the tendency to lift up and pivots effectively at the transitional radius.

To avoid this the transitions 1637 between the bow springs 1625 and each collar 1615, 1620 are looped almost 180 degrees back underneath the arch of the bow, thereby creating a pivot point where stress are minimal and follow the same direction. In other words there is a "heel" transition from the spring into the collar which allows the bows to pivot at both ends. In some embodiments the bow arch ends abruptly and pivots about the end point freely, although this may not always be possible whilst maintaining an integral all-in- one centraliser with end collars that hold it together circumferentially. This arrangement means that the collars are created on the inside of the centraliser.

Figures 18.1 to 18.5 show a centraliser 1710 formed according to an alternative aspect.

The centraliser 1710 comprises a plurality of generally straight bow springs 1725 with an end collar 1715, 1720 at either end. With the intention of reducing stress at the transition between the springs and the collars, the flat bows 1725 are curved around ("bent back") at both ends to form hook-like termini 1716. The end collars 1725 are also provided with hook-like termini 1717 which are interspersed between the bow termini 1716.

A bar/ring 1718 is passed through all of the termini 1716, 1717 to join the bows 1725 to the collars 1715, 1720. The bows can rotate freely with respect to the ring 1718.

The collars 1715, 1720 are again on the inside (longitudinally) of the centraliser. In other embodiments (not shown) the collars could be positioned on the outside (longitudinally) of the centraliser. An inside collar may, for example, be preferred if it can be used to create a stop for a stop collar (not shown) to hit against.

Figures 19.1 to 19.6 show a centraliser 1810 formed according to a further embodiment. The centraliser 1810 is similar to the centraliser 1710 of Figure 18.

In this embodiment there is no ring provided to join the springs 1825 and the collars 1815, 1820. Instead, the collar is provided with six circumferential slots 1819 and the spring termini 1816 are curled around so that they thread through respective slots to firmly engage the separate collars.

Referring now to Figures 20.1 to 20.6 there is shown a centraliser 1910 formed according to an alternative embodiment.

The centraliser comprises a plurality of bow springs 1925 with collar 1915, 1920 at either end. The transition between the springs and the collars is designed to reduce stresses in use. A generally S-shape kink 1922 is provided at the transitions to allow flexing.

As shown in the drawings, starting from the bow spring the first bend is a relatively sharp bend radially towards the centre of the inner pipe. This is followed by a 90 degree (other angles are possible) bend into the collar. The resulting formation has two main benefits: firstly to provide an opposing face for a single stop collar mounted centrally within the centraliser (as opposed to two at either end - this itself has two main benefits: to reduce manufacture cost; and most importantly to pull the centraliser through the hole rather than push and ease passage through constrictions); and the S-shape transition 1922 aims to reduce stress by allowing an element of pivot at the first transition.

Figures 21.1 to 21.4 shows a multiple offset helical spring centraliser design formed in accordance with the present invention.

A centraliser 2010 comprising of one or more coiled springs with a plurality of sections differing in pitch and diameter such that a transition is made from an end collar section 2001 to a centralising bow section 2002a to a mid-collar 2003 to another bow section 2002b and then to an opposite end collar 2004. There may be two or more mid-collar sections 2003a, 2003b (two are provided in this embodiment), and two or more bow sections 2002a, 2002b, 2002c (three are provided in this embodiment) where each bow section is offset in angle such that the resulting peaks are evenly spaced around the circle when viewed from the end axial orientation (I 20deg offset shown).

Each bow section may have partial revolution (I 20deg shown) so that the accumulative total revolution of all the bow sections is at least one full revolution of the circle, i.e. 360deg shown. The favoured number of bow sections is three, each offset by I 20deg, and the favoured number of mid-collar sections is two - this is considered the minimum to ensure even force distribution on the outer casing. The favoured pitch of the end collars and mid-collars is such that the resulting coil is closely wound, but it may be also a larger pitch to become an open coil. The diameter of the end and mid-collar sections is to suit the pipe/casing diameter so that it is free to move axially.

In this embodiment the transition from collar sections to bow sections is preferred to be soft and more gradual rather than a sharp and sudden transition for ease of manufacture [although the drawings show quite a sudden transition - partly due to the limitations of the CAD modelling tools used to produce the sketch].

The benefit of this design is partly in the manufacturing, i.e. the larger the pitch the more difficult it is to produce. For example it is difficult to achieve a pitch greater than 200mm whilst maintaining the spring properties and not lose its shape and form. Also, a full revolution across 200mm pitch results in a more closely wound coil that is not preferred ultimately due to the requirements of the centraliser to avoid obstruction downhole. For this reason it is preferred to perform a partial revolution at the same pitch, returning to a mid-collar before starting a new partial revolution bow section, so long as the result as viewed axially is an even distribution of contact points on the outer casing wall.

It is also beneficial in function as it helps maintain spring forces by having a shorter pitched coil that will not stretch so easily beyond its elastic limit, as well as allowing a more gradual angle of helix entering the outer casing thereby easing its path and reducing friction and snagging potential.

Figures 22.1 to 22.5 show a centraliser 21 10 which addresses stress relieving - of internally facing end collar designs.

A small radius curve / bend 2105b between the main bow 2105c and the internally facing end collar 2 l 05d provides sufficient flexibility and movement around the transition area 2105a such that when the bow is compressed and loaded the stresses are distributed through the curve / bend 2105b and dissipated away from the transition radius 2105a, effectively allowing the bow to flex somewhat rather than to crease at 2105a.

Figure 23 relates to a general discussion of a centraliser 2210 provided with inward facing end collars 2215a, 2215b formed in accordance with the present invention (for example the centraliser 21 10 of Figure 22).

Advantages of a centraliser with an inward facing end collar: a) Only one stop collar 2206 needs to be fitted to the inner casing / pipe 2207 rather than two. This also halves the total quantity to be manufactured and bought, making it cheaper, quicker and less to transport. Halving the effort and time required to install stop collars by the rig site operators is a major advantage. b) Another major functional advantage is that the stop collar effectively 'pulls' the centraliser down the hole rather than 'pushing'. This is mechanically beneficial and preferred as it helps minimise the chance of sticking / jamming, as the bows are allowed to flex / bend away from the point of force applied axially and so does not need to fight against it. The centraliser does not need to elongate in the opposing direction to the applied force, rather with it. An analogy might be made with pulling a string through a tube rather than fighting to push it through, or dragging a branch across the ground by the base rather than by the tips.

Disadvantage:

c) A disadvantage might be considered to be that the centraliser is restricted in its ability to compress as close to the inner pipe surface as with other traditional bow spring centralisers. In theory this is possibly a concern on really tightly constricted holes, but in reality / practice this may rarely be an issue as the well bore diameter should never be this constricted.

Figures 24.1 to 24.4 show a centraliser 2310 which address stress relieving - of externally facing end collar designs.

A small radius curve / bend 2305b between the main bow 2305c and the externally facing end collar 2305d provides sufficient flexibility and movement around the transition area 2305a such that when the bow is compressed and loaded the stresses are distributed through the curve / bend 2305b and dissipated away from the transition radius 2305a, effectively allowing the bow to flex somewhat rather than to crease at 2305a.

This reduces the tendency for the end collar to rise (or at least try to) as it counter acts the forces applied on the bow.

Further still the fillet 2305e adjoining the bow to the end collar is strictly kept well past (on the collar side) the curve 2305b so as to ensure the forces acting through the curve are kept in the same plane, which further reduces the material stresses in general as they are not combatting the cross sectional curvature of the collar. In other words the bow and the stress relieving curve at its heel are all rectangular in cross section rather than a complex 3D curved shape. This means that there is no requirement for the material to flex circumferentially at the same time that it might be required to flex axially or radially, i.e. stress points and fatigue points are much reduced.

Figures 25.1 to 25.3 show a centraliser 2410 with a double-acting / twin phase bow design (with inward facing collars). This embodiment originated as an exaggerated stress relieving curve, this embodiment has a curve / bump 2408 directly under the main bow 2409 effectively acting as a secondary bow spring - actually there are two, one at either each end of the main bow. They lead into an inward facing end collar, but in addition there is a radially support collar 2408a immediately after the heel of the main bow and before the secondary curve 2408. This is important in maintaining the overall shape of the centraliser and preventing distortion, breaking, entanglement, gouging, twisting, etc. of the ends of the main bow as it passes through the hole.

2408b is effectively the end collar that contacts with the stop collar 2406. 2408a is a secondary collar that ensures no distortion / lift / rise occurs during passage through the wellbore (hitting rocks etc.). Both sections 2408a and 2408b are rings that hold the bows together around the pipe, but it is only 2408b that meets with the stop collar.

In a similar function to that of the three phase triple bow of Figure 2, this twin phase is beneficial in providing a minimal centralising force in vertical sections of the well before adapting to a greater restoring force functionality as the main bow compresses into contact with the secondary, smaller bows underneath, which are stiffer due to their size as well as the fact that there are two of them working in tandem. This additional restoring force is required in more horizontal sections of the well when the weight of the pipe and cement becomes a factor.

Figures 26.1 and 26.2 illustrate a centraliser 2510 with a wavy or scalloped end collar. The end collar design is applicable to other aspects and embodiments of the present invention.

The end collars 251 I a and 251 I b are profiled to form a wave or scalloped edge so that the peak of each wave is the point of contact between the end collar 251 l b and the stop collar 2512b and so the area of contact is minimal. This reduces friction between the two opposing faces allowing the centraliser 2510 to spin around the inner casing/pipe 251 3 more freely. This is advantageous to the well drilling when rotating the casing so as to avoid accumulative forces from many centralisers.

Figures 27.1 to 27.3 show a centraliser 2610 having an end collar with inward facing castellations.

End collars 2614a, 2614b are profiled so that the portion between each bow 2625 around the circumference 2615 is elongated inwards to meet with a single stop collar 2616 situated internally within in the centraliser between the two opposing end collars 2614a, 2614b.

This provides the functionality of the single stop collar (i.e. pulling the centraliser rather than pushing, as well as only needing one rather than two), whilst presenting a potentially easier manufacture technique of the more traditional end collar (i.e. the heel of the bow 2617 is not doubled back in on itself (as in some other claims with single internal stop collars), which might present difficulties in manufacturing from a single sheet, as well as making the height from the casing greater at the heel when compressed. In other words this castellated design can flatten against the inner casing 2618 with a height equal to the material thickness only.

The profile of the contact face between the castellation 2615 and the stop collar 2616 is drawn here as a flat or square face, but this could equally be rounded or scalloped or wavy so as to provide minimal contact with the stop collar (as described in relation to Figure 26), with the bonus of less friction when casing is rotated.

In any of the aspects and embodiments of the present invention there may be consideration given to reduction of co-efficient of friction on centraliser surfaces in contact with the formation as well as between bow-spring ring / collar and the casing outer wall (bow-spring needs to revolve around the casing to allow casing rotation) through use of either low friction coatings, low friction pads or roller devices. Examples of suitable low friction coatings are: Molybdenum Disulphide, Diamond Like Carbon (DLC), which would be applied as thin coatings that would flex with the bow spring during its operation.

Another way to reduce the effective friction between the bow spring and the casing or formation wall is with the introduction of surface textures to reduce the immediate contact area with the casing / formation, whilst at the same time maintain the overall spread of load across the bow. Such surface textures might consist of dimples, or dome shape impressions, or longitudinal ridges.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment shown and that various changes and modifications can be affected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

1 ) A centraliser comprising longitudinally spaced collars connected by a plurality of springs, each of the springs comprising two or more bow sections.

2) A centraliser comprising a plurality of springs, each spring comprising a larger, taller outward bow and a smaller, shorter outward bow, and an intermediate inward bow between the outward bows.

3) A centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs comprising two or more curved sections, in which the, or at least two, of the sections have a different height in a resting position.

4) A centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs having an undulating section.

5) A centraliser for keeping wellbore casing from contacting a formation wall, the centraliser comprising two longitudinally spaced collars connected by a plurality of springs, the springs comprising means for reducing pressure on the formation within the well by spreading the load over a greater contact area between the centraliser and the formation wall.

6) A centraliser for keeping wellbore casing from contacting a formation wall, the centraliser comprising two longitudinally spaced collars connected by a plurality of bow springs, the springs comprising means for reducing pressure on the formation within the well by spreading the load over a greater contact area between the centraliser and the formation wall, in which the cross-section of at least part of each bow is curved.

7) A centraliser as claimed in claim 6, in which each bow has a region of increased width.

8) A centraliser as claimed in claim 7, in which the cross section of the region of increased width is curved.

9) A centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs comprises one or more top regions which contact a formation in use, the or each top region being wider than the remainder of the spring.

10) A centraliser comprising two longitudinally spaced collars connected by a plurality of springs, the springs contacting the collars at junctions, in which the junctions are rounded whereby to reduce stress.

1 1 ) A centraliser comprising two longitudinally spaced collars connected by a plurality of springs, each of the springs having a peak, in which the longitudinal position of peaks on at least two springs is different.

12) A centraliser as claimed in claim 8, in which alternate springs have different peak positions. I 3) A centraliser as claimed in claim I I or claim 12, in which there are two spring peak positions which are longitudinally offset from each other.

14) A centraliser comprising two longitudinally spaced collars connected by one or a plurality of springs, each of the springs being generally helical.

15) A centraliser as claimed in claim 14, in which the or each spring is a coiled spring.

16) A centraliser as claimed in claim 14 or claim 15, in which the diameter of the helices changes along the length thereof.

17) A centraliser as claimed in any of claims 14 to 16, in which the diameter of the helices decreases from a mid-point towards the collars.

18) A centraliser comprising two longitudinally spaced collars connected by one or a plurality of springs, each of the springs being generally spiral.

19) A centraliser as claimed in claim 18, in which the or each spring is a coiled spring.

20) A centraliser as claimed in any of claims 14 to 19, in which the springs have a generally circular cross-section.

21 ) A centraliser as claimed in any of claims 14 to 20, in which the pitch angle of the springs is generally constant.

22) A centraliser as claimed in any of claims 14 to 20, in which the pitch angle of the springs varies longitudinally.

23) A centraliser comprising two longitudinally spaced collars connected by a plurality of springs, in which there is provided means for reduction of the co-efficient of friction of centraliser surfaces in contact with a formation and/or in contact with an casing outer wall.

24) A centralizer as claimed in claim 23, in which at least part of the centraliser has surface formations.

25) A centraliser as claimed in claim 14, in which the surface formations are protruding dimples.

26) A centraliser as claimed in any of claims 23 to 25, in which at least part of the centraliser has a low- friction coating. 27) A centraliser as claimed in claim 26, in which the coating is Molybdenum Disulphide or Diamond Like Carbon.

28) A centraliser as claimed in any of claims 23 to 27, in which the centraliser is provided with one or more low-friction pads.

29) A centraliser as claimed in any of claims 23 to 28, in which the centraliser is provided with one or more wear pads.

30) A centraliser comprising one or more bows, the or each bow having a curved cross-section, and the or each bow having scalloped or wavy or ribbed edges for reducing the stress incurred as the bow flexes in use, whilst at the same time maintaining the overall contact area for skiing across a formation.

31 ) A centraliser comprising a plurality of bow springs, each bow curving back in on itself at the two opposing ends to form collars that are directly underneath the bow.

32) A centraliser comprising a plurality of bows, in which each bow loops around at both ends of arch/profile thereby creating a tube / hollow that in turn provides a pivot point enabling an axle to pass through and hence permitting free rotation of the bow ends.

33) A centraliser comprising one or more coiled springs with a diameter of the helix greater in the middle than at the two ends.

34) A centraliser as claimed in claim 33, in which the or at least one of the springs has one or more full or partial revolutions of helix of generally equal diameter at the ends.

35) A centraliser as claimed in claim 33 or claim 34, in which the or at least one of the springs has one or more full or partial revolutions at the centre at the largest diameter.

36) A centraliser comprising two or more coiled springs, the springs being intertwined at their ends to form end collars.

37) A centraliser comprising two or more coiled springs with diameter of the helix greater in the middle than at the two ends, and with a one or more revolutions of helix of equal diameter at the ends.

38) A centraliser as claimed in claim 36 or claim 37, in which the pitch of the helix is such that the plurality of helices are able to intertwine.

39) A centraliser as claimed in any of claims 36 to 38, in which the coils at the ends are welded together so as to create one centraliser assembly and maintain interaction between all helices. 40) A centraliser as claimed in claim 39, in which end collar coils are slotted into tubular sections which in turn are welded onto an inner collar.

41 ) A centraliser as claimed in any of claims 36 to 40, in which end collar coils are bent to create an axial parallel section before returning to the original helical angle, thereby producing a circumferential mechanical stop for the next helix in the sequence.

42) A centraliser as claimed in claim 41 , in which a return is formed which doubles back towards the inside of the centraliser.

43) A centraliser as claimed in claim 42, in which a return is formed such that the remainder of the bar is at 90deg or more to the axial direction.

44) A centraliser as claimed in claim 43, in which a return is formed such that the remainder of the bar is at substantially 90deg to the axial direction.

45) A centraliser as claimed in any of claims 41 to 44, in which a plurality of helices are provided and in which the stops are set at an angle defined by the number of helices, with an equal angular extent provided between each stop.

46) A centraliser as claimed in claim 45, in which three helices are provided and interlocked by ensuring that the stop is set approximately 120 degrees from the end of the next helix and having the three helices interlock both circumferentially as well as axially.

47) A centraliser as claimed in claim 46, in which four helices are provided and interlocked by ensuring that the stop is set approximately 90 degrees from the end of the next helix and having the four helices interlock both circumferentially as well as axially.

48) A centraliser comprising longitudinally spaced collars connected by a plurality of springs, each of the springs comprising two or more bow sections, the transition between the bow and the end collar comprising two bends of opposing direction to create an S-bend.

49) A centraliser comprising one or more coiled springs with a plurality of sections differing in pitch and diameter such that a transition is made from an end collar section to a centralising bow section to a mid- collar to another bow section and then to an opposite end collar.

50) A centraliser as claimed in claim 49, in which there are two or more mid-collar sections

51 ) A centraliser as claimed in claim 49 or claim 50, in which there are two or more bow sections 52) A centraliser as claimed in claim 51 , in which each bow section is offset in angle such that the resulting peaks are evenly spaced around the circle when viewed from the end axial orientation.

53) A centraliser as claimed in claim 51 or claim 52, in which each bow section has a partial revolution so that the accumulative total revolution of all the bow sections is at least one full revolution of the circle.

54) A bow spring centraliser comprising a plurality of bow springs and two internally facing end collars.

55) A centraliser as claimed in claim 54, in which a small radius curve/bend is provided between each bow and the internally facing end collars which provides sufficient flexibility and movement around the transition area such that when the bow is compressed and loaded the stresses are distributed through the curve/bend and dissipated away from the transition radius, effectively allowing the bow to flex (almost pivot) somewhat rather than to crease at the transition area.

56) A bow spring centraliser comprising a plurality of bow springs and two externally facing end collars.

57) A centraliser as claimed in claim 56, in which a small radius curve/bend is provided between each bow and the externally facing end collars which provides sufficient flexibility and movement around the transition area such that when the bow is compressed and loaded the stresses are distributed through the curve/bend and dissipated away from the transition radius, effectively allowing the bow to flex (almost pivot) somewhat rather than to crease at the transition area.

58) A centraliser having an end collar with inward facing castellations.

59) A centraliser with a wavy or scalloped end collar.

60) A centraliser with a twin phase bow design, the centraliser having a plurality of major bow springs extending between end collars, each major spring having a respective minor spring, the minor springs being positioned under the major springs, in a first loading phase just the major bow springs are compressed and in a second, greater loading phase the major bow springs are pressed onto their respective minor springs.

61 ) A centraliser as claimed in claim 60, in which the end collars are inward facing collars.

62) A centraliser substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

63) A well bore having one or more centralisers as claimed in any preceding claim.

64) A method of providing sprung centralising support in a well bore substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.