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Get Centred, Get TD

16th December 2014

Correct centralisation of tubular casings is crucial to the success of a well, as incorrect centralisation can result in, amongst other problems, channelling and poor cementation - centraliser failures can lead to rig downtime, pulled casing, stuck pipe, fluid migration, side-tracks and even loss of wells, explains Cliff Berry, VP Global Business Development at Centek Group

Get Centred, Get TD
Centek’s centralisers are designed to fit gauge hole conditions, and secure stop collar technology ensures there is no centralizer movement on run in or pull out of hole

The great majority of centraliser failures are due to choosing an incorrect unit for the job. Everyday someone, somewhere, is pulling casing and leaving debris in the hole for the simple reason that the wrong type of centraliser was used for a specific application.

Single-piece, non-welded centralisers of the type pioneered by Centek, have addressed the failings of multipart and solid body centralisers with some designs producing benefits such as zero start force and zero running force that reduce drag significantly. Reducing torque ensures that casings can be rotated easily, in both cased and open holes, at a deeper level than before.

The low profile of the single-piece centralizer gives a lower ECD (Equivalent Circulating Density) signature allowing the operator to pump at a slightly higher rate, which improves well cleanout, and the low torque aids rotation and minimises stall-out, all of which contribute to improved cementation. Robustness ensures both radial and axial loads are absorbed without deformation, while flexibility allows units to pass well tight spots, squeeze down, and then expand back to the original OH (Open Hole) size.

Centek’s centralisers are designed to fit gauge hole conditions, and secure stop collar technology ensures there is no centralizer movement on run in or pull out of hole. The company manufactures a range of centralizers the Centek S2 for flexibility and strength, the UROS for under-reamed wells and the UROS-CT (close tolerance) for deep water wells.

Ensuring a lasting and effective annular seal in the well bore is vital to maintaining oil production. It is important to exclude water ingress and this necessitates zonal isolation and a good cement bond. Achieving a long term annular seal is of paramount importance and this is especially true in long extended reach wells.

Vertechs Oil & Gas Technology is a major oil and gas field services supplier with headquarters in Chengdu, China. The company has been involved in drilling activities in the South Sichuan basin, where the wells have long horizontal sections and the well bore quality is often poor. These are onshore wells with a TVD (true vertical depth) of 900 metres and horizontal sections of typically 1,000 metres in length. There is a dog leg with a severity of around 12deg/30m.

Vertechs initially used locally manufactured centralisers, both rigid and conventional bow-spring types. The rigid centralizers were metal alloy and failed to get to total depth (TD). When the drill team pulled the casing out of the hole they found the rigid centralisers had broken and left fragments downhole. The rigid alloy centralisers were not strong enough to bear the side forces in horizontal wells. The welded bow spring centralisers also proved susceptible to damage and were unsatisfactory, because they could not maintain restoring force in the open hole resulting in poor casing centralisation and bad cementation.

Vertechs started using Centek 5.5” x 8.5” S2 centralisers in 2012. Centek appeared to have the best specifications, including results from Centek’s Latload simulation software in the test well, so Vertechs tried them in one well and when the performance was very good, started using them in other wells.

“The main benefit of the Centek S2s has been their performance downhole, helping casing to run to TD compared to wells with other centralisers where the casing got stuck or ran 100 - 200 metres short of TD, resulting in yields being substantially reduced compared to plan,” said Gary Zhang, general manager Vertechs Oil & Gas Technology Co., Ltd.

In this project typically just one Centek S2 centraliser was used per casing joint, although in some wells more centralisers were required. The Centek S2 is now being used in around 60 oil and gas wells across China. Vertechs has not so far used Centek’s UROS under-reamed centralizers but is considering doing so later this year.

In Chengdu the Centek S2 achieved an average reduction in liner running time of 23 per cent or 11 hours per well, compared to using the previous rigid centralizers in similar horizontal wells. The average liner running time was about two days.An exactly centred pipe has a standoff of 100 per cent. If the pipe touches the borehole wall, the standoff is 0 percent. The best mud displacement, and subsequent cementation, occurs as standoff approaches 100 percent. The restoring force defines the centralising capability.

Centralisers can also help get the pipe to bottom when properly matched to well bore conditions. The ability to rotate a pipe can also greatly assist the mud removal process and also the ability to agitate the cement. The pipe movement does not need to be fast to be effective; 6 to 10 RPM is helpful.

Before choosing a centraliser consideration needs to be given to the well geometry, stand-off required, zonal isolation, formation pressure window and well friction factors as all this will directly affect performance. Choosing a centraliser with the right attributes of strength, performance, flexibility and low friction for different well-types and configurations will go a long way to getting you to bottom first time, reducing risk and preventing many well problems.

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