Staying centred

Cliff Berry outlines the importance of good casing centralisation.

Correct centralisation of oil casings is vital to the success of a well, as poor centralisation can result in channelling and faulty cementation. In extreme cases the centraliser can fail causing distortion or splitting of casings necessitating the withdrawal of the damaged casings, at great expense and production downtime. Centralisers assist the passage of a tubular into the well bore to the required depth by keeping the tubular and its protruding connections off the borehole wall to minimise mechanical interference and differential sticking in the open hole. At final depth centralisers need to counteract the forces exerted by the tubular and centralise it within the borehole to allow optimum annular passage of fluids and displacing cements.

Traditionally bow spring centralisers were widely used, but their construction of welded or interlocking, multi-part components required that they be made substantially oversize in order to withstand the forces exerted by the tubular. This created considerable 'start' and 'running' forces. Multi-part centralisers are also prone to component failure when down hole. Once the failings of traditional bow spring centralisers became apparent, solid centralisers were adopted whose main virtue was robustness. However fixed dimension centralisers need to be undersise so as to pass through any previously installed tubular, with the result that they do not centralise efficiently once out in the open borehole. With the recent introduction of 'under-reaming' i.e. passing a drilling head through a previously installed tubular to create an open borehole greater in diameter then the tubular bore, fixed dimensional centralisers have proved even more inadequate.
 
Complex
Modern wells are increasingly complex in profile and run to ever increasing lengths. The preload of traditional bow spring centralisers, created by being oversize, results in a cumulative resistance that can prevent the passage of the tubular to final depth. In addition, their multipart or welded construction produces centralisers of inadequate strength for the more complex wells and catastrophic failures can occur, especially in under-reamed wells. The cost implications for the industry are considerable. If the centraliser isn’t strong enough to centre the pipe, annular circulation of the cement is impaired, an inadequate cement seal results and the life of the well can be shortened. If the centraliser breaks the consequences can be very expensive. If it breaks in a deviated or horizontal well, the casing will lie along the bottom of the borehole.

Centralizstion is usually completely lost, making effective cementation impossible, added to which the centraliser may jam the pipe down hole. Stuck drill strings are one of the major contributors to drilling downtime, and a common cause of sticking down hole is a failed centraliser. A conservative estimate is that every year 400 wells worldwide are affected by centraliser problems. Assuming an average daily cost per well of US$250,000, and a typical downtime of six days, the cost per well is around US$1.5 million, with an overall annual loss of US$600 million. These losses are largely preventable.

UK based-Centek went back to basics and concluded that the only way to ensure consistent centralser performance, particularly with the extreme demands of under-reamed bores, was to radically redesign the bow spring type of centraliser to remove its deficiencies and incorporate the ruggedness of fixed dimension units. Centek believes that its family of centralisers achieves this. Centek products have been used successfully since 2002, in thousands of applications worldwide without a single centralizer failure.

• Centek centralisers are manufactured from a single piece of steel with a hardened surface that greatly reduces torque and drag losses.
• The construction strength is such that failure will not occur in even the most exacting well bore profiles.
• Reactive force is achieved without being oversize, and performance is substantially in excess of industry standard ANSI/API 10D/ISO 10427-1&2.
• Complete compression of the bow-spring’s outer diameter allows the centraliser to pass through narrow tubulars and then expand fully to its design diameter enabling it to centralise effectively in an under-reamed bore.

Cliff Berry is Sales and Marketing Manager for worldwide sales at Centek Ltd. His oilfield career started in 1977 with Halliburton in Brunei, Malaysia and Sarawak as a cementer and tool operator. He has also worked for Diamond B (UK) Ltd., and was European Operations Manager for BJ Tubular Services.