Under-reamed wells are among the toughest challenges any casing centraliser has to face. Cliff Berry reports.
Under-reaming is a drilling technique for enlarging the diameter of a borehole in order to increase the potential for achieving a good cement job. A major problem with under-reamed wells is getting effective casing centralisation in the under-reamed section. Conventional bow-spring centralisers can have the correct outer diameter to fit the under-reamed section, however, due to methods of construction they often get damaged in passing though narrower casings. The bows get compressed, lose elasticity and cannot expand to the correct diameter of the under-reamed hole.
Solid or rigid centralisers were intended to deal with this, but because of the fixed diameter, the solid centraliser is undersized to the previous casing, let alone the enlarged open hole, as is the case with the under-reamed section. Because the centraliser is too small it will lie on the low side producing much less effective cementation than would a precisely fitting bow spring designed for that wellbore, keeping the pipe more central.
The latest centralisers are manufactured from a single piece of heat-treated steel giving a hardened and tempered surface that results in greatly reduced torque and drag losses. These centralisers offer high fatigue strength for axial forces and radial side loads on bows during tubular rotation.
When running casing, losses can result from not getting to bottom due to friction or mechanical interference, getting packed off or stuck, while extra trips and side-tracks, all add to well costs. In addition, poor cementation due to inadequate stand-off causes improper cement placement resulting in production delays and early water entry directly attributable to incorrect tubular placement in the well-bore. Here are two typical examples of centraliser failure:
- Germany, land rig, September 2008: 7-in liner, 5400m horizontal well - liner length 1900m. The centralisers collapsed due to axial load. Result: 30 days downtime. Cost: $14m plus. Solution: Fit centralisers offering flexibility with improved stand-off and high axial loading capability to ensure couplings are kept off the wellbore wall.
- UK North Sea, Southern Sector, October 2007: 7-in liner, 80 degree inclination, liner length 1100 ft. The liner was stuck in hole, the centraliser packed off and the web broke. The liner was pulled. Result: Fishing trip and seven days downtime. Cost: $6m. Solution: Improve flow by area, improve stand off, and improve flexibility. Run simulations and dynamics. Fit appropriate centralisers.
Before choosing a centraliser careful consideration must be given to the flow by area, desired stand-off, well strength and geometry, the zonal isolation required, and the degree of centraliser flexibility needed to traverse known formations, as well as an evaluation of start and running forces.
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Cliff Berry is with Centek Limited, Newton Abbot, Devon, UK. www.centekltd.co.uk
