Oil & Gas News
Wednesday, 15 October 2008
Centek Centralisers: Underreaming - the challenge for centralisers by Cliff Berry
Underreaming of wells brings extraction benefits but places heavy demands on casing centralisers. Cliff Berry, Sales and Marketing Manger at Centek Limited, discusses some of the issues.
Underreaming is a widely used drilling technique for enlarging the diameter of a borehole at some point below the surface. Underreamed sections are generally drilled in order to maximise the size of tubular that you can put down. A conventional well is drilled in a telescopic fashion, but as you telescope down you end up being limited by the final casing size, which can be as small as five or six inches.
An important benefit of 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 as the fluid usually has drilling particles in it and is highly abrasive. You are also often limited to how central you can get the pipe, which depends on the borehole shape. If you can increase the diameter of the annulus you improve the potential for cleaning out on the narrow side, so if a pipe is sitting to one side having a bigger annulus should help increase the flow without risk of damage to the formation.
A fundamental problem with underreamed wells is getting effective casing centralisation in the underreamed section. Ordinary bow-spring centralisers can be damaged when passing through previously set casing. In order to try and ensure centralisers are not damaged once in the underream, some suppliers make them oversize so as to improve stand-off down hole, but this results in a new penalty as the tightness of these centralisers in the casing requires a huge prestart force to get them moving. Modern wells are highly complex in profile and run to ever increasing lengths. The preload of an oversize bow-spring centralizer results in an accumulative resistance that can prevent passage of the tubular to final depth.
In addition, conventional bow-spring centralisers that are wide enough to fit the underreamed section with any accuracy 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 bow-spring centralisers, while perfectly adequate in straightforward 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.
All of this gave bowspring centralisers 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 centraliser problem is the solid or rigid centraliser. Produced from a piece of solid steel, zinc, aluminium or plastic, they are certainly strong, though often brittle, and completely inflexible. The solid centraliser has a problem when it emerges into the underreamed section, because with its fixed diameter it is undersized for the previously set casing let alone the underreamed hole. This is exacerbated in highly-deviated underreamed wells, as in the open hole the centraliser is too small to provide effective centralisation, and will lie on the low side producing a much less effective cement job than does a well fitting bowspring designed for the purpose.
This leaves the drilling engineer with an unsatisfactory dilemma between centraliser failure and inadequate cementation. Devon-based Centek believes it has the answer in its family of bow-spring centralisers that combine flexibility with unparalleled strength.
Centek centralisers 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 centralisers offer exceptionally high fatigue strength for axial forces and radial side loads on bows during tubular rotation.
Despite being fully compressed during passage through the casings, the centraliser offers exceptional restoring force with a very high stand-off ratio once in the open hole. As a low profile unit it takes up less annular space, so its ECD (Equivalent Circulating Density) signature is low allowing the operator if required to pump at a slightly higher rate. This improves well cleaning, and the low torque aids part rotation and minimises stall-out all of which contribute to improved cementation.
The Centek S2 UR centraliser is oversized for the casing it has to pass through, but despite this it requires only a low start force and running force once the unit has been inserted into the previous casing, and it is robust enough to withstand compression when passing through 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 centraliser that will compress and then expand to fit the largest hole the customer requires, within reason. Underreamed centralisers are not manufactured to API standards as there is no API standard for underreamed applications.
Centek’s range of under-reamed products have been used successfully in thousands of applications worldwide without a single centraliser failure. Instances where tubulars have had to be pulled out of the borehole for reasons unrelated to centraliser performance, have revealed Centek centralisers in full working order and suitable for re-running. These overall results and design strengths have allowed Centek to successfully challenge the view that bow spring centralisers should not be used in extended reach or highly deviated wells - you just need to ensure the correct bow spring is specified. In an underreamed hole, the Centek S2 UR gives a better likelihood of cementing a tubular in place and getting the optimum amount of cement around it.12:38 0 Comments