O f all the applications a centraliser has to handle, the underreamed hole is the toughest. A fundamental problem with underreamed wells is getting effective casing centralisation in the underreamed section.

Most bowspring centralisers will break if you leave them to locate in a hole of diameter larger than the casing they have just passed through. To try and solve this some manufacturers make them oversize to improve standoff down hole, which makes the centralisers so tight in the casing that they need a huge restart force to get them moving and which can prevent passage of the tubular to final depth.

Conventional bowspring centralisers that are wide enough to fit the underreamed section get so seriously compressed in passing though the narrower casings that the bows lose their elasticity and can't expand to the diameter of the underreamed hole. Ordinary bowspring centralisers, while adequate in straightforward applications, are not designed to be severely compressed and passed through smaller bore pipe for hours, then to emerge and expand to their design diameter in the underreamed hole. Solid centralisers do not break, but by definition they are inflexible and undersized in the open hole providing inadequate centralisation.

The CentekS2UR centraliser is made from a single piece of

heat-treated steel and can compress virtually flat to casing, traverse thousands of feet of previously set casing, yet once in the open hole it regains its original size and shape to provide the standoff required for effective centralisation.

Centek Ltd is based in Newton Abbot, Devon, UK. www.centekltd.com