Casing wear is very important as it effects the system integrity. A casing scheme is designed based on the properties of the chosen casing. Casing wear changes the thickness of casing walls and therefore changes its properties related to its capability to withstand the various loads exerted on it. This should be taken into consideration when designing wells.
The culprits which cause casing wear are, rotation, geometry of the well, drillpipe tension, etc. From this it can be seen that platform wells and deep wells are especially prone. Also axial misalignment of wellhead to casing and inaccurate rig skidding (note that the centres of wellhead and casing do not always line up!) all can add to the likelihood of casing wear occurring.
Casing wear can be predicted by the use of computer models and papers have been written on the subject which can assist in the design of wells
to minimise wear. Wear should be monitored by the use of ditch magnets for metal recovery, logs to measure casing ID's , and by examination of pulled casing. In re-entries, wear will usually be more significant than in new wells. Wear should be minimised by centralising the rig properly to the casing, using the right type of hardfacing, and possibly by the use of non-rotating protectors, placed in the most effective areas by modelling.
Some solutions to the problems associated with casing wear:-
Flexible Couplings. These are still at the conceptual stage but the idea is that they should allow the casing to form a smooth catenery whereby the 'usual' high casing wear spots (at the couplings) are eliminated.
Baker/Tristate Back-off Tool: This allows the removal of damaged casing to be unscrewing at a selected joint without the need to cut.
Drillquip Emergency Back Off Sub: This is run as part of the casing string as a coupling and has Mudline Suspension type threads which can be backed off to retrieve the damaged casing. Can also be used to retrieve the top part of the casing for sidetracking purposes.
FMC/Tristate External casing patch: This has replaced the old style sealing element of lead and rubber with a much more permanently effective metal to metal arrangement of wedges.
Other good tips:
- ID of wearbushing should be same as the hanger, which should be the same as the casing.
- Casing downsizing crossovers should have a transition taper of 4 degrees and not the usual 45 degrees.