Offset Bow Centralizers Meet Underreamed Well Challenges
- Marius Boncutiu (Centek Group) | Richard Berry (Centek Group)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- September 2013
- Document Type
- Journal Paper
- 28 - 30
- 2013. Copyright is retained by the author. This document is distributed by SPE with the permission of the author. Contact the author for permission to use material from this document.
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Underreamed wells are among the toughest challenges to using a centralizer, a device that keeps the casing or liner in the center of a wellbore. Underreaming, the technique of enlarging of a wellbore beyond its originally drilled size, is a drilling method widely used to increase the openhole size, which may be required for various reasons.
Some well planners believe it is safest to drill unknown shallow formations with a small diameter bit and enlarge the pilot hole if no gas is encountered. Underreaming may also be performed if a small additional amount of annular space is desired, for example, if a liner must be run to protect against surge pressures.
A fundamental problem with under-reamed wells is achieving effective casing centralization in the underreamed section. The job of the centralizer is to center the casing to improve run in hole (RIH), allow easier pipe rotation, and to enable the cement column to circulate freely around the tubular and produce a robust cement seal to ensure zonal isolation.
Mud displacement is vital to achieving a good cement bond. The more central the pipe, the more efficient the mud displacement will be. In deviated and horizontal wells, if the tubular is not centralized, it will lie along the low side of the borehole and make cement circulation and the achievement of a uniform cement sheath difficult.
Poor centralization can also impair the cement bond by causing channeling, which can lead to various live annulus problems. Like any fluid, cement will take the easiest route in the annulus, and this can result in an inadequate seal if the casing or liner is not centralized. In addition, if the annular clearance is restricted in some sections, backpressure may result that requires a much reduced flow rate during cementation to avoid fracturing the formation and thereby losing fluid.
Offset Bow Centralizer
The Uros offset bow centralizer developed by Centek is designed for use in underreamed or washed-out well sections. The device significantly reduces initial insertion forces and drag when running through previously set casing. Once through this compressed stage, the offset bow centralizer will revert to its designed outer diameter in the open hole and thus maximize standoff without additional drag. The device achieves a reduction in drag compared with other centralizers because of its patented bow design, in which the high points of the bows are offset alternately without reducing the strength of the unit or its capacity to centralize the casing in the open hole.
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