General guidelines to minimise casing wear include:
- limiting build up rates
- deepening kick-off points
- optimising drillstring design to reduce contact loads or employing suitable hardbanding
- minimising rotating hours
- modifying operating practices to reduce well tortuosity and rotating hours
- using drillpipe protectors or torque reducing subs in critical zones.
Drill Pipe Protectors and Torque Reducing Subs
It is recognised that in some cases, due to conflicting well design issues, it is not technically feasible or economic to eliminate casing wear. In these cases the use of drill pipe protectors or torque reducing subs to prevent casing wear in the critical zones of the well should be considered.
Various service companies have computer programmes to optimise placement and installation frequency to ensure adequate coverage and prevent overloading of these devices.
All types of mechanical protectors and torque reduction subs operate by giving ‘stand-off ‘ which is necessary for prevention of casing wear.
Wellbore Devices
Wellbore devices include:
- conventional drillpipe protectors(elastomeric)
- non-rotating drillpipe protectors
- bearing subs
- torque reducing subs
- specialised drillpipe (with integral stabilisers placed on the joint body).
Casing Crossovers
Combinationcasing string crossovers can be worn by the drillpipe 18o taper if designed too short. This can be reduced by hardbanding the crossover and making the taper as shallow as possible.
Where crossovers join two different weights or sizes of casing it is necessary to ensure that the transition is ‘well supported’ in order to prevent bending and localised doglegs. Support is provided with suitable centralisation backed up with cement.
Surveying
Case histories outside Company have identified, that in the majority of cases, the surveying methods used, failed to identify doglegs. This can occur when the doglegs are shorter than the survey interval, i.e., 40’ dogleg with 100 ft survey spacing.
If casing wear is considered as a potential risk, then the survey interval should be shortened from, for example, every 90 ft to 30 ft. Doglegs will go undetected if the survey stations fall on either side of a dogleg which imparts no overall change in wellpath direction - such as ‘S’ shaped or symmetrical doglegs.
If one of the survey stations falls within these doglegs, the ‘apparent’ dogleg severity will be less than the ‘real’ dogleg severity. Higher survey station densities are needed to overcome this problem.
Mud Types
Relatively speaking water based muds are much more abrasive than oil based muds. The increased solids content and barite content of weighted muds tends to reduce the potential for casing wear as compared to unweighted muds.
Sidetrack / Re-entry
Casing wear should be considered as for a conventional well in the planning phase of a sidetrack or re-entry. The point here is that there will be additional rotating hours in the original well which could already have an existing potential wear problem that can only to be compounded by the additional work. If needed, USIT log can be run to measure casing wear before sidetracking.
Backreaming
Backreaming is a serious consideration and is the most likely operation to cause casing wear by maximising the tension and side-wall forces through doglegs at the same time as the string is being rotated.
A diligently planned well with low build up rates and low drillpipe tension may in fact be subject to unsuspected casing wear by the practice of backreaming.
Wireline Wear
Even after the well has been successfully drilled and completed, casing wear can still be a problem with wireline runs through tubing or liner. Operations engineers should bear this in mind when planning the well.
Centralisers programme
Centralisers should not be placed over the casing couplings in sections prone to casing wear. The casing couplings tend to support the casing away from the wellbore if uncemented.
If the stand-off is enhanced by a centraliser this produces a hump or mini-shoulder for the Drill Pipe 18o taper to wear against.