(HOW MANY, WHAT TYPE, SHOE-TRACK CONFIGURATION?)
Casing centralisers are available in a bewildering variety of types and materials, each claiming to offer their own unique advantages. To be successful, it’s necessary to consider the casing running operation as a whole, with centralisation being only one of many considerations.
Designing a centraliser programme for a high angle well inevitably involves a compromise between the requirement to get the casing to bottom first time, while ensuring zonal isolation can be achieved. As a starting guide, it’s worth addressing the points below:
• What does the casing T&D road-map reveal? For example, is the run likely to be compromised at higher open hole friction factors, is rotation of the casing planned or even possible?
• Solid body centralisers are preferred over bow-springs for their increased robustness, particularly where rotation is anticipated.
• Is the use of centralisers made using low friction materials to reduce drag (may help inside previous casing, but not generally effective in open hole) and/or improve ability to rotate justified?
• If it’s necessary to run bow-springs (e.g. under-reamed hole), it’s important to include the centraliser run force in the drag modelling. The cumulative effect of this where large numbers of centralisers are being run can be significant. Note that even though the centralizers may have zero run force in the under-reamed hole, there may be a run force prior to exiting the previous casing, which must be accounted for during modelling. If these must be used, solid body designs are recommended over latch-and-lock.
• In general, centralizers run in open hole do not reduce drag on high angle wells (more likely to increase it), and it is preferred to minimise the number of centralisers to that required to achieve the necessary zonal isolation and mitigate known differential sticking risk.
• Avoid excessive shoe-track centralisation. This may increase the effective stiffness of the shoe-track and result in higher drag, particularly through the build section due to the “snow-plough” effect. The following is a good starting point for design discussion:
o 1st joint – one centraliser held in place with a stop collar ±2m above the shoe
o 2nd joint – no centralisation
o 3rd joint – one centraliser (free floating on joint)