Central importance

Cliff Berry

Of all the components involved in running casing it is probably the centralizer that goes most unnoticed, that is until it fails or a centralization problem occurs.

Centralizers just centre up the casing in the wellbore to allow cement to circulate freely around the tubular and produce a robust cement seal, ensuring zonal isolation. Simple enough, I hear you say. But, if the centralizer isn't strong enough to centre the pipe, or if it breaks, the consequences can be very expensive. If it breaks in a deviated well, centralization is usually completely lost rendering effective cementation impossible added to which the centralizer 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 centralizer. A conservative estimate is that annually, 400 wells worldwide are affected by centralizer problems, at an average cost per well of around $1.5M producing a total annual loss of $0.6B. This loss is largely preventable.

Mud displacement is very important for 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 making it very difficult to circulate the cement around the pipe and achieve a uniform cement sheath. Poor centralization can also impair the cement bond by causing channeling, when the cement flows in channels on only some sides of the casing or annulus producing an inadequate seal. In addition, if the annular clearance is restricted in some sections, back pressure can result necessitating pumping at a very slow flow rate during cementation in order to avoid fracturing the formation with consequent fluid loss.

Under-reamed wells are among the toughest challenges a centralizer has to face. Under-reaming is a widely used drilling technique for enlarging the diameter of a borehole in order to increase the size of tubular that you can put down.

A major problem with under-reamed wells is getting effective casing centralization in the under-reamed section. Conventional bow-spring centralizers that are wide enough to fit the under-reamed section accurately often get damaged in passing though narrower casings. The bows get compressed to such an extent that they lose their elasticity and can't expand to the correct diameter of the under-reamed hole - a condition known as permanent set.

One response to bow spring centralizer failure in under-reamed holes was the introduction of the solid or rigid centralizer. The solid centralizer however has a problem when it emerges into the under-reamed section, because with its fixed diameter it is undersized for the under-reamed hole. This is made worse in highly-deviated under-reamed wells, as in the open hole the centralizer is too small to provide effective centralization, and will lie on the low side producing much less effective cementation than a precisely fitting bow spring designed for that wellbore.

The latest generation of centralizers from Centek is manufactured from a single piece of heat-treated steel giving a hardened and tempered surface that results in greatly reduced torque and drag losses, so abrasive wear caused by running to depth and rotating the tubular is virtually eliminated. These centralizers offer exceptionally high fatigue strength for axial forces and radial side loads on bows during tubular rotation. Reducing torque ensures that casings can be rotated without wear, in both cased and open holes, at a deeper level than before.

Despite being fully compressed during passage through the casings, these one piece construction centralizers offer exceptional restoring force with a very high stand-off ratio once in the open hole. As low profile units they takes up less annular space, so the ECD (Equivalent Circulating Density) signature is low allowing the operator to pump at a slightly higher rate improving well clean out.

When running casing, losses can result from not getting to bottom due to friction or mechanical interference, getting packed off or stuck, while extra trips and side-tracks, all add to well costs. In addition poor cementation due to inadequate stand-off causes improper cement placement resulting in production delays and early water entry directly attributable to incorrect tubular placement in the well-bore. All these problems can be resolved by ensuring correct centralization.

Two typical examples of centralizer failure:

• Germany, land rig, September 2008

7-in. liner, 5400m horizontal well - liner length 1900m. The centralizers collapsed due to axial load.

Result: 30 days downtime.

Cost: $14m plus. Solution: Fitting centralizers that offer flexibility with improved stand off-and high axial loading capability that will ensure couplings are kept off the wellbore wall.

• UK North Sea, Southern Sector, October 2007

7-in. liner, 80 degree inclination, liner length 1100 ft. The liner was stuck in hole, the centralizer packed off and the web broke. The liner was pulled.

Result: Fishing trip and 7 days downtime.

Cost: $6m.

Solution: Improve flow by area, improve stand off, and improve flexibility. Run simulations and dynamics. Fit appropriate centralizers.

These examples demonstrate a failure to identify and assess hazards, and then lessen the risk by running the correct centralizers.

Before choosing a centralizer careful consideration, including the use of modeling software, must be given to the flow by area, desired stand-off, well strength and geometry, the zonal isolation required, and the degree of centralizer flexibility needed to traverse known formations, as well as an evaluation of start and running forces.

Centek wins the Queen's Award for Innovation 2009

Centek Limited, the UK-based market leader in oil and gas casing centralizers, has won the Queen's Award for Enterprise 2009 for continuous product innovation and development sustained over a five year period. In 2008 Centek also received the Queen's Award for outstanding achievement in International Trade. Centek manufactures casing centralizers, which are used in oil and gas drilling to position pipes down the drilled hole prior to cementation. 

"This is a great success story and we are thrilled to receive the UK's most prestigious innovation award," said Tony Cutmore, Managing Director of Centek. "Over an eight year period we have taken the product we originally designed and improved it to such an extent that it is now the best casing centralizer in the World. We've put in place a new, original and highly effective joining process that further increases the already substantial strength of our centralizers, which combined with our proprietary heat treatment produces a product of unequalled flexibility and robustness."

Innovation is at the heart of Centek's approach and success, based on an original idea of the company founders back in 2001. Centek centralizers have no welds or moving parts and are made from a single piece of heat-treated steel that reduces friction and gives great strength combined with flexibility, allowing the centralizer to compress, without breaking, as it goes down the drill hole, and to expand to its correct diameter centring the oil pipe in the hole. This allows cement to be pumped evenly around the pipe to secure it at depths as great as 15,000 feet.

"The Queen's Award recognizes the innovative designs and technologies that have enabled Centek to double its export sales since 2004," continued Tony Cutmore. "It comes as a result of years of dedication and hard work, and I congratulate and thank every member of the team for contributing to this commitment to excellence."

Centek employs 49 people in Newton Abbot, Devon, exporting to 32 countries with patents held in 16 countries. Centek emphasises product quality and customer support, and 80 per cent of its sales in the company year ending 31^st October 2008 were repeat orders from existing customers. Centek has achieved ISO 9001-2000 and API SPEC 10 D - 0022 certification, both essential competencies in the oil industry.

The Queen's Awards for Enterprise are the UK's most prominent awards for business performance, and are awarded for achievements in three categories: International Trade, Innovation and Sustainable Development.

BIO

Cliff Berry's oilfield career started in 1977 with Halliburton in Brunei, Malaysia and Sarawak as a cementer and tool operator. He also worked offshore in the North Sea and Persian Gulf with Halliburton. He then worked for Diamond B (UK) Limited, a leading centraliser manufacturer in the mid 1980's. Cliff joined BJ Tubular Services as European Operations Manager working from the German office and successfully introduced BJ Tubular Services into Denmark, Hungary, and Holland in addition to growth in Germany. He joined Centek in 2001 as Sales and Marketing Manager responsible for worldwide sales. He can be reached at cliff.berry@centekltd.co.uk.