The centralisation of casing and liners as well as the correct positioning of the centralisers are essential parts of any casing or liner job. The main reason for centralisation is to ensure a homogeneous distribution of cement around the casing. No centralisation or poor centralisation will cause channelling of the cement and hence produce a poor cement bond.
Wall Sticking
When running casing, adequate use of centralisers reduces the chance of wall sticking.
Deviated Wells
In a deviated well because of the increased amount of support, the casing requires to stay in the centre of the hole, a more dense distribution of centralisers is required than in straight holes - especially in build-up sections.
Types of Centralisers
There are two main types of centraliser:
Spring (Bow) Centralisers
Rigid Centralisers
Spring (Bow) Centralisers
Large Annuli (diameter difference above 2” ). Centralisers are installed over a stop collar and shall be of the Spring (Bow) type.
Close Tolerance Annuli (diameter difference less than 2”). Centralisers are installed between stop collars such that they can fully collapse. In the case of a collared coupling, the upper stop collar is replaced by the coupling. Centralisers are of the Spring (Bow) type.
Rigid Centralisers
Rigid centralisers are used inside casing only. They are installed over stop collars or between stop collars or collared couplings and stop collars. The method of installation depends entirely upon the centraliser design.
Note: When ordering rigid centralisers for surface casing, care must be taken to ensure that they will pass through the riser/landing ring.
Centraliser Installation
Whenever possible, centralisers shall be installed over stop collars; however, it is necessary to fit centralisers above and below stop collars when
using close tolerance centralisers in case of a spring type or when the design of the centraliser does not allow the installation at the stop collar (e.g. solid sleeve centralisers)
Where possible, centralisers shall be pre-installed on the rack to save time.
Stop collars are available in sizes from 4.1/2” to 20”. The type will be determined by the supplier.
Example of Centralisation Programme for Various Casing String Sizes
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Onshore |
Offshore |
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Casing Size |
Spring Bow Centralisers |
Rigid Centraliser |
Spring Bow Centralisers |
Rigid Centraliser |
|
20in |
One per joint on first three joints |
None |
One per joint on first three joints |
|
|
13.3/8in |
One per joint on first five joints |
None |
One per joint on first seven joints. One per joint in 13.3/8in / 20in annulus up to seabed. |
Three spaced evenly in 13.3/8in / 20in annulus from seabed to sea level. |
|
9.5/8in |
One per joint on first four joints. One every +/- 200 ft in open hole. Three spread evenly in 9.5/8in / 13.3/8in annulus. |
One inside 13.3/8in casing shoe. |
Three every two joints for first 1 500 ft. One per joint over rest of open hole. One every three joints in 9.5/8in / 13.3/8in annulus. |
One above and below MSIPC in 9.5/8in / 13.3/8in annulus. |
|
7in liner |
Two every joint in open hole. |
One below liner hanger in 9.5/8in casing |
Two each on shoe track and one every other joint in open hole. |
One every two joints in 7in / 9.5/8in overlap. |
MSIPC: Multi Stage Inflatable Packer Collar