Casing centralisation
The centralisation of casing and liners and also the correct positioning of the centralisers is an essential part of any casing or liner job. Without adequate centralisation it is possible that the casing can make contact with the wall of the hole preventing effective annular sealing by the cementation. Centralisers should preferably be pre-installed on the rack to save rig time. Two basic types of centraliser are used:
1. Spring type centralisers
- a) Large annuli: Centralisers are installed over a stop collar and should be of the Weatherford St-II, ST-III or St-IV type or similar.
- b) Close tolerance annuli: Centralisers are installed in 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 should be of the Weatherford ST-1 type or similar.
2. Rigid centralisers are used inside casing only. They are installed over stop collars, or in between stop collars or collared coupling and stop collar. The method used depends entirely on the centraliser design.
Centralisers should preferably be fixed over a stop collar. However, it is necessary to fit stop collars above and below the centralisers when using spring type centralisers of close tolerances, or when using rigid centralisers which do not have space to straddle a stop collar. If the centraliser can be pre-installed on the rack this should be done to save rig time. If pre-installation is not possible and when using collared type couplings, centralisers can be installed over the coupling.
20" and 185/s" casing The centralisers should be placed as follows:
Two spring centralisers on the shoe joint, one 5 ft above and one over or below the collar.
One spring centraliser on the 2nd and 4th joint.
13 3/s" casing The centralisers should be placed as follows:
One spring centraliser 5 — 10 ft above the shoe.
One spring centraliser on the 2nd, 3rd 4th and 5th joint.
One spring centraliser every five joints for open hole angles less than 20 degrees. One spring centraliser every two joints for open hole angles more than 20 degrees and one centraliser each joint in built up section for open hole angles more than 20 degrees.
One rigid centraliser below the hanger.
One spring centraliser inside the previous casing shoe.
9 5/§" and 7" casing The centraliser should be placed as follows:
Two spring centralisers on each of the first two joints over a stop collar for 9 5/s inch casing.
Two on each of the first two joints between stop collars for 7 inch casing. One spring centraliser on each joint over build up section. One spring centraliser every two joints for open hole angle less than 20 degrees. One spring centraliser every joint for open hole angle greater than 20 degrees. Two spring centralisers on each joint of 7 inch casing for open hole angles greater than 45 degrees.
One spring centraliser inside the previous casing shoe. One rigid centraliser one joint below the hanger.
7" and 4'/a" liner The centralisers should be placed as follows {excluding underreamed hole):
Two spring centralisers on each of the first two joints between stop collars. Two spring centralisers per joint in open hole. One rigid centraliser per joint over the liner lap.
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