Offset Bow Centralizers Meet Underreamed Well Challenges

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.

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 underreamed 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.

An oilfield service and product supplier operating offshore Norway has specified the use of the offset bow centralizer in underreamed and deviated wells since August 2011. Even when using two centralizers per pipe joint, the company has consistently been able to reach target depth while maintaining the desired standoff in the open hole. The company is running the device in several fields and has expanded its use to normal wells, because of the fluid and cement displacement benefits that it allows.

Offset bow centralizers are increasingly used in Norwegian offshore well operations. These wells often have a high risk of washed-out sections, i.e., openhole sections that are larger than the original hole size, which are generally caused by soft or unconsolidated formations. However, even in gauge holes, offset bow centralizers are used to ensure high standoff.

Case Studies


A well operator offshore Norway ran 7-in.×9½-in. offset bow centralizers through 9⅝-in. 53.5-lbm/ft casing into an 8½-in. open hole with a total depth of 3600 m. Twice the operation reached a depth of 3560 m before it was necessary to pull the liner. Both times the liner was partially rotated in the open hole on the way in and was rotated for about 12 hours at 20 rev/min in the cased hole on the way out. The centralizers also had to be run through a whipstock window.

Each time after pulling the liner, the drilling and cement teams inspected all the centralizers and stop collars (Fig. 1). The only visible damage was some bending of the set screw sockets. The operator was able to run the same centralizers again and eventually install them at target depth. The offset bow centralizers were run through the whipstock window on five occasions without snagging or packing out on the edges. By comparison, an oversized conventional bow spring centralizer would have exhibited considerably more drag during pullout.

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Fig. 1—Undamaged centralizers were pulled twice from a well at 40 m above total depth and then run successfully for a third time to the target level.

The offset bow centralizers are also being used in various Latin American operations. In an Ecuadorean well, 7-in.×9⅞-in. centralizers were run through previously set 8.535-in. inside diameter casing into a 10.4-in. average diameter open hole. The installed centralizers achieved a standoff higher than the customer’s requirement of 70% along the target zone. In addition, there was less risk of differential sticking, an even annular flow of fluid, and a good well cleanout, with an improved cement job ultimately achieved. The centralization was defined using an analysis of openhole logs, standoff calculations, and liner tally. Centralizers were distributed two per joint along M1 sandstone.

It is necessary to understand the insertion forces and the cumulative running forces, as both affect the RIH performance because the bows are squeezed while passing through the smaller inner diameter of the previously set casing. If needed, the first few joints can be prefilled with mud to increase the string’s weight and enable it to be run in under its own weight from the beginning when less weight is present. This makes it possible to overcome the initial insertion and running forces without pushing the pipe.

In the Ecuador case, when all the casing was run and the liner hanger was installed, the pickup weight was 100,000 lbf and the slackoff weight was 70,000 lbf, which indicated 30,000 lbf of drag. Fig. 2 shows the drag calculation results, which are close to the real values observed while running the casing. In this case, the running force for each centralizer was approximately 1,000 lbf.

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Fig. 2—The drag calculation results were close to the real values while running the casing.

There was no restriction while running the liner through the open hole, and the bottom was reached without problems. During circulation, there was no indication of debris accumulation caused by drag. According to the circulation parameters, the well showed a good cleanout. By having acceptable drag conditions when passing through the previously set casing, low drag conditions in the open hole, and maximizing standoff in an enlarged annulus, the offset bow centralizers proved to be a major aid to reaching bottom and obtaining a good cement job in the Ecuadorean well.

Summary of Benefits


Offset bow centralizers result in greatly reduced torque and drag losses, and because they are heat treated, abrasive wear caused by running to depth and rotating the tubular is eliminated. Reducing torque ensures that casings can be rotated without wear in cased and open holes at deeper levels than would otherwise be possible. The ability to rotate a pipe can also greatly assist in mud removal. Typically, a rotational speed of 6 to 10 rev/min is all that is needed.

The centralizers are individually designed to fit each wellbore, rather than generally designed for specified gauge holes. Secure stop collars prevent centralizer movement on run in or pullout. The choice of a centralizer depends on a number of factors such as the expected flow by area, the desired standoff, the strength and geometry of the formation, the required zonal isolation, the centralizer flexibility needed to traverse known formations, and the estimated start and running forces.

Ensuring a lasting, effective annular seal in the wellbore is vital to maintaining oil and gas production. Preventing water inflow is extremely important and requires good zonal isolation. Achieving a long-term annular seal is difficult, especially in long extended reach wells. The use of offset bow centralizers can aid the cementing process in underreamed and conventional extended reach wells.