Centralizing in the underream

Under-reaming of wells aids the running of both casings and liners into the borehole. This could be done for a variety of reasons, however, in many ways, it can make centralizing tubulars difficult. Cliff Berry, Sales and Marketing Manager at Centek Limited, examines centralization issues generally and the problems that emerge when things go wrong.

“Everyday someone, somewhere, is pulling casing and leaving debris in the hole for the simple reason that the wrong type of centralizer was used”
-Cliff Berry, Centek

Many fields, such as in the North Sea, are depleted because of years of production, and the removal of hydrocarbons has weakened the formation. Nowadays, when drilling through these old reservoirs, rig teams need to be aware of the pressures produced by circulating fluids while drilling, and operators are increasingly looking at underreaming to try and reduce drilling pressures and risk.

Another reason for underreaming is it improves the flow and pressure of the annular fluids, but by increasing the fluid flow you run the risk of eroding the surrounding formation especially if the casing is not centered and it’s sitting to one side.

A fundamental problem with underreamed wells is getting effective casing centralization in the underreamed section. Most bowspring centralizers will break on passage through a smaller casing bore. Once through this tight restriction units still have to expand into the now enlarged open hole which is of a greater diameter than the casing they have just passed through. A new problem now occurs as oversized bows will now create an insertion, running and accumulated re-start force. It is critical that these forces be measured exactly, especially as today’s wells are highly complex in profile and run to ever increasing lengths.

Conventional bowspring centralizers are not designed to fit the underreamed section with any accuracy as they will most often get damaged when they have to pass 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 underreamed hole - a condition known as permanent set. Ordinary bowspring centralizers, while perfectly adequate in straightforward vertical applications, are not designed to be severely compressed and passed down thousands of feet of smaller bore pipe for hours, to then emerge and expand to their design diameter in the under-reamed hole. The end result is junk left in the hole.

To understand centralizer failure, the technology needs to be addressed along with some history. Centralizers since the early 1960’s have been of a multipart design and construction, being either welded or interlocked and having hinges and pins to hold them together. That design has remained largely unchanged to this day. When wells were vertical this type was ideal as the string is in tension and no radial loads are being applied. However, these units are oversized to the borehole, have high start and high drag forces and are weak as far as restoring force is concerned. These centralizer types are still a regular choice in the Industry but they are also the single largest contributor to downtime and overspend.

All this gave bowspring centralizers a bad name in the more arduous applications. As a result the industry veered towards strength before all. The strength before all solution to the underreamed centralizer problem is the solid or rigid centralizer. Produced from a piece of solid steel, zinc, aluminium or plastic, they are certainly strong, though often brittle, and completely inflexible. The solid centralizer has a problem when it emerges into the underreamed section, because with its fixed diameter it is undersized to the previous casing let alone for the underreamed hole. This is exacerbated in highly-deviated underreamed wells, as in the open hole the centralizer is too small to provide effective centralization, and will lie on the low side producing a much less effective cement job. This type would be the least effective solution for such wells.

Problems caused by ineffective centralization and actual failure should not be dismissed lightly. Annual downtime during drilling, or running casing, is only marginally better than it was 20 years ago. Estimates from North Sea operators suggest drilling efficiency is still only around the 50 percent mark, considered across all rig activities from spud to completion, including running casing tubing.

Downtime attributed to centralization problems is reaching sums in excess of $0.5 billion per year, and the industry needs to examine how things can be improved. Spiraling rig costs, of all types, including $1 million per day drill ships, mean downtime is becoming prohibitively expensive. Centralizers in real terms are cheap, but when they fail due to damage, breakages or simply getting stuck in hole due to fitting insufficient centralizers at the correct intervals - or even none as sometimes happens - then they assume a consequential cost out of all proportion to their price. Centralizer failure costs a fortune.

Most centralizer failures are due to choosing an incorrect unit for the job. An alarming fact is that everyday someone, somewhere, is pulling casing and leaving debris in the hole for the simple reason that the wrong type of centralizer was used. A second and equally disturbing fact is that the Industry always seems to reward failure. If unit failures cause massive overspend, then a ‘so what?’ attitude becomes common, and the easy answer is to buy some more of the same and hope they work better next time!

As the oil industry developed and moved away from vertical wells, other  solutions had to be found as well trajectories and types changed. Some wells now have extremely close tolerance casings, some are under-reamed and many are now horizontal. Along with this, the technical aspects also changed as now radial and axial forces come into play.

Reducing risk when running casings is of paramount importance: getting to bottom first time and achieving a good cement job are vital priorities. In this case why is so little importance put on the selection of the right centralizer to achieve these aims and eliminate the risk?

Centek, through rigorous engineering and testing bring solutions to the Industry. Key characteristics such as zero start force and zero running force reduce drag significantly. Reducing torque ensures that casings can be rotated without wear, in both cased and open holes, at a deeper level than before. The low profile of the single-piece centralizer improves the flow by area. Robustness of the centralizer is a paramount design and manufacturing criterion. Flexibility too is vital as units must be able to pass well tight spots, squeeze down, and then expand back to the original OH (Outer Hole) size.

Pack-off can be greatly reduced due to the increased flow by area of modern low profile centralizers. Units must have a high restoring force to allow for radial compression, weight of pipe and well geometry, yet get to bottom and give the stand-off required to achieve the best possible cement job. All these criteria have been successfully met by Centek’s single-part centralizers proven across a range of applications, worldwide since 2002.

Centek centralizers are manufactured from a single piece of steel which is fully heat-treated to give a hardened 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.

The Centek S2 UR centralizer, designed, tested and engineered on a per well basis, is robust yet flexible and is designed to the gauge of the underreamed section. Despite being fully compressed during passage through the casings it offers exceptional restoring force once in the open hole. The units are of a low profile design, so the ECD (Equivalent Circulating Density) signature is low allowing the operator if necessary to pump at a slightly higher rate. This improves well cleaning, and the low torque aids rotation and minimises stall-out, all of which contribute to improved cementation.

The Centek S2 UR centralizer is oversized for the casing it has to pass through but despite this it requires only a low start and running force, and is robust enough to withstand compression when passing the casings. Typically it can pass through the internal diameter of a 95/8 pipe (approximately 8½ inches) to pop out and expand to a 9½ inch underream. If drilling is carried out with an 8½ inch bit and then underreaming to 9½, 10 or even 11 inches. Centek can produce a centralizer that will compress and then expand to fit the largest holes created by an underreamed tool or bi-centre bit.

Centek products have been used successfully in thousands of applications worldwide without a single centralizer failure. Centek can justifiably claim to have reinvented the bowspring centralizer with unequalled flexibility and strength. In an underreamed hole, the Centek S2 UR gives the best likelihood of cementing a tubular in place and getting the optimum amount of cement around it.

For an engineered solution for your well that reduces torque and drag, gets to bottom and offers good zonal isolation it is time to investigate change. Call or email www.sales@centekltd.co.uk and start looking at where saving money protects better than wasting it. The choice, as they say, is yours!

Web: www.centekltd.co.uk.

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 centralizer 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.