A FRICTION REDUCING TOOL

Abstract: A centraliser having a first group of rollers (5) for reducing friction between the centraliser and an oil field tubular (7) and a second group of rollers (4) for reducing friction between the centraliser and the wall of a well. The axles of the first group of rollers (5) are generally aligned with the axis of the bore (2) to reduce rotational friction. The axles of the rollers of the second group of rollers (4) are generally transversed to the bore (2). The arrangement reduces rotational and axial drag between an oil field tubular (7) and the walls of a well, particularly when initiating rotation of an oil field tubular.

Description: A FRICTION REDUCING TOOL The Technical Field The present invention relates to a centraliser for use in drilling applications. More particularly, the present invention relates to a centraliser having externally projecting rollers to minimise axial drag and internally projecting rollers to minimise rotational drag between the centraliser and an oil field tubular such as a casing, liner or drill string. Background of the invention During the drilling of a well, centralisers may be employed to maintain the required alignment of the drill string. After a well has been drilled centralisers may be employed to centrally locate a liner or casing within the well. When cementing a liner within a well it is desirable that the liner is able to rotate to ensure optimum cement distribution. To date centralisers have typically consisted of a tubular body having radially projecting fins with the liner simply located within the bore of the centraliser. Chemical lubricants have been added to drilling mud to minimise the friction between the centralisers and the liner. Chemical lubricants are effective once the liner has been rotated with respect to the centraliser. However, the initial torque required to rotate the liner may be very great and may be beyond the limit of the rig or may exceed the torque that can be borne by the liner connections. It is also desirable to minimise the drag between a drilling string and the well bore when a centraliser is employed to reduce the torque required by the rig and minimise stress on components of the rig. Disclosure of the Invention It is therefore an object of the present invention to provide a centraliser that reduces friction between the centraliser and drilling string, casing or liner or at least provide the public with a useful choice. There is thus provided a centraliser comprising: a generally tubular body; a first group of rollers rotatably secured to the body at intervals about the body and positioned to project into the bore of the body to reduce rotational drag between the centraliser and an oil field tubular; and a second group of rollers rotatably secured about the outer periphery of the body which project beyond the outer periphery of the body to reduce axial drag between the centraliser and the walls of a well. The oil field tubular may be a drill string, liner or casing. The body of the centraliser may be of one part or two part construction. The rollers are preferably cylindrical rollers tapered at their ends. The axles of the second group of rollers are preferably substantially parallel with the axis of the bore in the body. The axles of the first group of rollers are preferably substantially transverse to the axis of the bore of the body. The rollers are preferably designed so that should an axle break the roller will be contained within the centraliser body. Brief Description of the Drawings The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 : is a perspective view of a centraliser according to the Invention; Figure 2: is a side view of the centraliser shown in figure 1 ; Figure 3: is a top view of the centraliser shown in figures 1 and 2 when fitted to a liner; Figure 4: shows the roller containment construction. Best mode for carrying out the invention Referring to figures 1 to 3 there is shown a centraliser having a generally tubular body 1 having a bore 2 there through and pedestals 3 about the periphery thereof. Rollers 4 are provided having their axles substantially transverse to the axis of bore 2 and projecting outwardly beyond pedestals 3. Rollers 4 are positioned to reduce axial drag between the centraliser and the walls of a well. Rollers 4 are preferably of a generally cylindrical form tapering at each end to better conform to the shape of the wall of a well. The axles of rollers 5 are substantially parallel to the axis of bore 2 and project into bore 2. Rollers 5 are positioned to engage the exterior surface of an oil field tubular such as a drill string, liner or casing. In the example shown in figure 3 rollers 5 engage a sleeve 6 provided on liner 7. Sleeve 6 may be formed of plastic or metal, although metal is preferred, to further assist in drag reduction. It will be appreciated that the rollers may be provided in different locations. For example, the positions of rollers 4 and 5 may be interchanged so that rollers 4 are provided within regions at the top and bottom and rollers 5 are centrally located. It is preferred that banks of two or more rollers 4 and 5 be provided for each orientation on each pedestal so as to better distribute load and maintain correct orientation. Body 1 may be of one or two part construction; two part construction being appropriate where a centraliser must be fitted to an assembled drilling string or liner or casing. Body 1 is preferably formed of metal or plastic. Rollers 4 and 5 are preferably formed of metal, ceramics, plastics or rubbers. Three or more pedestals are preferably provided about body 1 , although 5 pedestals are preferred. Referring now to figure 4 the roller containment design is shown in relation to a roller 4. Axle 9 secures roller 4 to body 1 . Roller 4 has projections 10 adjacent overhangs 1 1 . In assembly, roller 4 is inserted from within the centraliser from bore 2 into the position shown in figure 4. Axle 9 is then inserted through bore 12 and located within bore 13 in the position shown in figure 4. Axle 9 is then fixed in place by welding 14 or other suitable securing means. Projections 10 include recessed portions 1 5 so as not to place undue sheer force on axle 9 at the ends of roller 4. Should axle 9 fail, roller 4 will be contained by overhangs 1 1 within the centraliser. Rollers 5 are of a similar construction. This avoids broken components causing an obstruction. Axle 9 is preferably formed of steel and may be hardened by nitriding, case hardening or plating with electroless nickel/phosphorous/PTFE or other platings. Where such a centraliser is used in conjunction with the drilling string the rollers must be able to withstand high wear. In this case the interior bore of the rollers adjacent to the axle and exterior surface of the rollers are preferably coated with polycrystalline diamond compact (PDC). This is a synthetic diamond material that provides an extremely hard surface that withstands high wear. The centraliser may simply be placed on a section of drilling string, casing or liner or stock collars may be provided at either end to keep the centraliser at a fixed position. The invention therefore provides a centraliser that is effective to reduce axial and rotational drag, particularly rotational drag between an oil field tubular and the centraliser. Where in the foregoing description reference has been made to integers or components having known equivalents then such equivalents are herein incorporated as if individually set forth. Although this invention has been described by way of example it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope of the invention as defined in the appended claims.

Claims: CLAIMS: 1 . A centraliser comprising: a generally tubular body; a first group of rollers rotatably secured to the body at intervals about the body and positioned to project within the bore of the body to reduce rotational drag between the centraliser and an oil field tubular; and a second group of rollers rotatably secured about the outer periphery of the body which project beyond the outer periphery of the body to reduce axial drag between the centraliser and the walls of a well. 2. A centraliser as claimed in claim 1 wherein the axles of the first group of rollers are substantially parallel with the axis of the bore. 3. A centraliser as claimed in claim 1 or claim 2 wherein the axles of the rollers of the second group of rollers are generally transverse to the axis of the bore. 4. A centraliser as claimed in any one of the preceding claims wherein the first group of rollers are provided in banks of two or more rollers at spaced intervals about the body. 5. A centraliser as claimed in any one of the preceding claims wherein the second group of rollers are provided in banks of two or more rollers at intervals about the body. 6. A centraliser as claimed in any one of the preceding claims wherein the rollers are located within pedestals provided at spaced apart intervals about the body. 7. A centraliser as claimed in claim 6 wherein three or more pedestals are provided about the periphery of the body at spaced intervals. 8. A centraliser as claimed in any one of the preceding claims wherein the rollers are housed within cavities in the body. 9. A centraliser as claimed in any one of the preceding claims wherein the rollers of the first group of rollers are located within a central region of the centraliser and the rollers of the second group of rollers are located within regions at either end of the body. 10. A centraliser as claimed in any one of claims 1 to 8 wherein rollers of the second group of rollers are located within a central region of the body and rollers of the first group of rollers are located within regions at either end of the body. 1 1 . A centraliser as claimed in any one of the preceding claims wherein the rollers and body are dimensioned so that should an axle associated with a roller fail the roller remains contained within the body. 1 2. A centraliser as claimed in any one of the preceding claims wherein at least some rollers have internal bores for receiving axles that are coated with a polycrystalline diamond compact. 1 3. A centraliser as claimed in any one of the preceding claims wherein the exterior surfaces of at least some rollers are coated with a polycrystalline diamond compact. 14. A centraliser as claimed in any one of the preceding claims wherein at least some of the rollers are of generally cylindrical form and taper at each end. 5. A centraliser as claimed in any one of the preceding claims wherein the body is of one part construction. 6. A centraliser as claimed in any one of the preceding claims wherein the body is of two part construction. 7. A centraliser substantially as herein described with reference to the accompanying drawings.

Inventors: MURRAY GEOFFREY NEIL, MURRAY, GEOFFREY, NEIL
Applicants: AUSTOIL TECHNOLOGY LIMITED, MURRAY GEOFFREY NEIL, MURRAY, GEOFFREY, NEIL
Application year: 1998
Main classification: E21B17
IPC codes: E21B17