Centraliser design has not changed fundamentally since the early 1950s. Most centralisers consist of bows welded, or attached by a mechanical interlock device, to the end-band. As drilling and well design has changed so has well geometry. While it might seem obvious that centraliser design also had to change, for the most part this has failed to happen and serious downtime and loss of wells due to centraliser mechanical failure has become commonplace.
Solid body or rigid centralisers were introduced in response to the failings of bow spring types but while axially strong, solid body centralisers cannot centralise adequately. In an under-reamed section, performance is even more limited because units are undersized to the casing let alone the enlarged diameter of the open hole.
It became obvious to the founders of Centek that a radically new approach was needed.
Engineering and design
Engineering and design were the key parameters that needed addressing, with eradicating damage and breakages as the goal. Optimising standoff was a major engineering concern as both start force and running force needed to be reduced or better still eliminated, yet standoff must be maximised. The Centek S2 centraliser, introduced in 2001, is a single piece unit with an integral bow to end-band design; manufactured from a single piece of heat-treated steel, it achieves these goals. With zero start and running forces being achieved drag was reduced while still allowing pipe rotation when reaming is necessary, or once the string has landed to aid cementation. The S2 was initially run successfully in the North Sea and is now being used worldwide.
Key operational requirements such as flexibility are addressed. The S2 is designed to compress virtually to pipe OD with no permanent set occurring, and then spring back to its design diameter. This is a significant change from the old centraliser designs, which are suitable for use in vertical wells, but are susceptible to mechanical failure in deviated, horizontal or ERD wells, for which the S2 was designed and where it is now being run successfully on a daily basis.
Abrasive wear on centralisers constructed of ordinary steels can reduce standoff performance. The S2 is made from a special steel alloy which when heat-treated has a Rockwell hardness of around 42-43, resisting abrasion and maintaining correct standoff, ensuring that should windows milling become necessary there will be enough annular clearance.
Another area of concern was flow by area, because the larger rigid centralisers by their very size in the bore considerably reduce the annular space, this increases the pressure drop over them. The S2 centralisers are low profile units with an annular encroachment of only 28 percent when compared to a coupling, which has around 40 percent encroachment. Other centraliser types show greater increases, some as high as 46 percent.
Robustness and strength
Robustness and strength was perhaps the greatest challenge, as rig-teams were apprehensive about using standard bow centralisers due to the delays and costs of failures experienced with conventional bowsprings. Retrieving casings was fraught with difficulty as centralisers were often left in the well. This fear led, naturally enough, to increased adoption of the rigid centraliser. Centek carried out a study of the majority of centralisers available to the Industry. From this, key performance criteria emerged and were engineered into the Centek centraliser, including getting to bottom first time, providing sufficient standoff once on bottom and pipe rotation. It became apparent that a bow centraliser was the ideal choice, yet it must be one designed and manufactured to a sufficiently high standard to withstand the axial and lateral loadings encountered when running downhole.
Examine a Centek S2 and the difference is immediately apparent – no welds or mechanical interlocks between blades and end rings and no multipart construction. This simple yet highly effective design and manufacturing process has permitted significant changes to be introduced in other areas of drilling and cementation, changes hitherto considered difficult to achieve.
Underreamed sections are an area in which conventional bowspring centralisers are prone to breakages, and rigid centralisers provide inadequate centralisation in the open hole. Each S2 centraliser must compress down virtually flat to casing, traverse 1000’s of feet of previously set casing, yet once in the open hole it regains its original size and shape and provides the standoff required to achieve effective centralisation.
Deep water wells
Another critical application is with close tolerance casings in deepwater wells. Until recently the choice was limited to a single type of centraliser, the centraliser sub. Centraliser subs suffer from all the drawbacks of ordinary bowspring centralisers with the additional disadvantage of being very expensive to fit. The Centek CT centraliser for close tolerance wells meets the tightest annular tolerances, removes the need for an inline casing centraliser sub, with no welds, no mechanical interlock devices, no pup joint cost, no threading cost and improved performance downhole with reduced drag. Centek claims that the CT provides a reduction of up to two thirds the cost of using centraliser subs for the same operation.
Centek has also introduced a centraliser for unconsolidated formations (UCF) that greatly reduces the risk of damage and delays. Standard centralisers exert a high point pressure against the bore wall but in a UCF section the centraliser can dig into the bore wall causing standoff and rotational problems. The S2 UCF increases the bow surface area at the point of contact with a wider, load-bearing pad, which can reduce bow loadings to as little as half the compressive strength of the bore wall while still providing effective centralisation.
Centek believes that it has made a contribution of real value to drilling engineering with these new centralisers, a belief borne out by the experience of our customers in a variety of applications worldwide. No longer need a broken or damaged centraliser be an industry ‘standard’. No longer should costly downtime be acceptable, or fishing for a centraliser be a regular undertaking. The innovative S2 centraliser saves time, money and raises performance criteria in all applications of well design. Previously centralisers were rated by the incidence of failures. This does not apply to the S2, where reliability is the guaranteed standard and the incidence of failure is zero.