Centralizer for downhole probes
An assembly for use in subsurface drilling includes a downhole downhole probe supported by a centralizer. The centralizer comprises a tubular member that extends around the downhole probe. A wall of the centralizer is fluted to provide inward contact points that support the downhole probe and outward contact points that bear against a bore wall of a section of drill string. The downhole probe may be supported for substantially its entire length.
1. A centralizer for use in subsurface drilling, the centralizer comprising:
an elongated tubular member having a wall formed to provide a cross-section that provides first outwardly-convex lobes, the first lobes arranged to contact a bore wall of a bore in a section of drill string at a plurality of spots spaced apart around a circumference of the bore wall; and
a plurality of inwardly-projecting portions, each of the plurality of inwardly-projecting portions arranged to support a probe in a centralized relationship to the bore wall;
wherein the first lobes are arranged to define a first plurality of fluid channels between the centralizer and the bore wall and the inwardly projecting portions are arranged to define a second plurality of fluid channels between the centralizer and the probe which is centralized by the centralizer.
2. A centralizer according to claim 1 wherein the inwardly-projecting portions comprise inwardly-projecting lobes that are inwardly-convex and outwardly-concave.
3. A centralizer according to claim 2 wherein a thickness of the wall is substantially uniform.
4. A centralizer according to claim 3 wherein the wall has a thickness in the range of about 0.1 to 0.3 inches.
5. A centralizer according to claim 4 wherein the wall has a thickness in the range of about 0.15 to 0.25 inches.
6. A centralizer according to claim 2 wherein, in cross-section, the centralizer has 2-fold rotational symmetry.
7. A centralizer according to claim 1 wherein the cross-section provides two first lobes.
8. A centralizer according to claim 1 wherein the cross-section provides two to eight first lobes.
9. A centralizer according to claim 1 wherein the plurality of first lobes are equally angularly separated around a longitudinal centerline of the centralizer.
10. A centralizer according to claim 1 wherein the first lobes are provided by longitudinally-extending ridges on an outer surface of the centralizer.
11. A centralizer according to claim 1 wherein the wall of the centralizer comprises a thermoplastic material.
12. A centralizer according to claim 11 wherein the thermoplastic material comprises a fibre-filled thermoplastic material.
13. A centralizer according to claim 12 wherein thermoplastic material comprises PEEK or PET.
14. A centralizer according to claim 1 wherein the wall is made of an electrically-insulating material.
15. A centralizer according to claim 1 wherein the wall is made of a composite of electrically-conductive and electrically-insulating materials.
16. A centralizer according to claim 1 wherein the first lobes extend along helical paths along the length of the centralizer.
17. A downhole assembly comprising:
a drill string section having a bore extending longitudinally through the drill string section;
a downhole probe located in the bore of the drill string section; and
a centralizer according to claim 1 in an annular region of the bore surrounding the downhole probe.
18. A downhole assembly according to claim 17 wherein the downhole probe comprises an electronics package.
19. A downhole assembly according to claim 17 wherein the downhole probe comprises a metal housing and the metal housing is harder than a material of the centralizer wall.
20. A downhole assembly according to claim 17 wherein the downhole probe comprises a telemetry signal generator.
21. A downhole assembly according to claim 17 wherein, when following a path around a cross section of the wall of the centralizer, the path has inner portions that contact the outside of the downhole probe but do not contact the inside of the bore that alternate with outer portions that contact the inside of the bore but do not contact the downhole probe.
22. A downhole assembly according to claim 17 wherein, in the centralizer, the inwardly-projecting portions comprise inwardly-projecting lobes that are inwardly-convex and outwardly-concave.
23. A downhole assembly according to claim 17 wherein the downhole probe comprises a layer of vibration damping material between a housing of the downhole probe and the centralizer.