Casing Centralizers Design
The target of this article is to know how to position the casing centralizers while you are in the designing phase of the casing. The focus will be on is on the below items:
- The objective of casing centralization.
- Casing Centralizers different Types.
- Casing Centralizers Spacing Formulas.
- Casing Centralizers Installation Patterns.
- Casing Centralizers Case Study
Why We Need To Center The Casing ?The centralizers on a casing string are used to provide clearance between the casing and the wall of the hole. The clearance is called standoff. The major function of a centralizer is to centralize the casing in the hole and to prevent it from lying against the wall, thus providing a reasonable uniform cement layer around the casing.
What Is The Divergent Types Of Casing Centralizers ?
- Bow-spring Centralizers
- Rigid Centralizers
- Semi-rigid Centralizers.
- Mold-on Centralizers.
1) Bow Spring Casing Centralizer TypeBow Springs Centralizers are used to provide centralization for casing in vertical or slightly deviated wells as its OD is more than the hole ID.Its bows are flexibility to be able to retract in narrow intervals of the upper casing and expand again in wide intervals of the drilled hole.
There is an important term called the restoring force which is the resistance force when a bow is compressed by 1/3 of its uncompressed height. This force also depend on the bow shape and stiffness. And it is so important as it is the major factor of the effectiveness of the bow casing centralizers type.
Let's assume that we have heavy weight casing and high inclination well, we shall take in our consideration that this situation may be beyond this centralizers capabilities
3) Semi-Rigid Type Casing Centralizers Type
Semi-rigid centralizers are made of double crested bows, which provide desirable features found in both the bow-spring class and the rigid centralizers class. The spring characteristic of the bows allows the semi-rigid centralizers to compress in order to get through tight spots and severe doglegs. The double-crested bow provides restoring forces that exceed those standards set forth in the API specifications and therefore exhibits certain features normally associated with rigid centralizers class.
4) Mold-On Type Casing Centralizers TypeThe mold-on centralizer blades, made of carbon fiber ceramic materials, can be applied directly to the casing surface. The blade length, angle and spacing can be designed to fit specific well applications, especially for the close tolerance annulus. The non-metallic composite can also reduce the friction in extended reach laterals to prevent casing buckling.
Casing Centralizers SpacingCentralizer spacing should be sufficiently close to keep the casing to wall clearance at some acceptable minimum distance. The casing couplings or various types of attachable stops control the vertical travel of the centralizer.
The following equations apply to all pipe in normal oilfield service.
F = force on each centralizer if spaced Cs feet apart (lbf)
W= weight of casing (lb/ft)
θ = average inclination angle (degrees) cs
T = tension in the wall of the casing (lbf)
TVD = true vertical depth
S = distance from casing shoe to the centralizer in question
ρmi and ρmo = the mud weights in and out of the casing, respectively
DLS = dog leg severity
do = outer casing diameter
di= inner casing diameter
Cementing aids and their installation pattern are a function of operational conditions and cementing objectives. Because of these variable factors, pattern philosophy is different from case to case. The Drilling Programme must specify type, quantity and installation pattern of mechanical cementing pieces, with due consideration to eventual modifications after caliper log evaluation.
Case Studies For Casing CentralizersWith the help of computer modeling, Casing centralizer placement optimization becomes easy to perform for all types of wells. Ideally, this kind of optimization should be done before every casing job. Here is an example of centralizer placement optimization using the CentraDesign software.
Occasionally casing doesn’t make it to total depth on the first run as anticipated and as planned. Today's complicated well designs and well conditions can lead to taking counter measures when running casing. Nobody considers that the centralizers may have to make 3 measured trips; trip in, pull out, and run back into the well bore. Sometimes after the trip out, typical standard welded bow spring centralizers type are not always found to be intact or in a condition to be re-run.Centek Centralizers with their unique non-weld slip-on design offer high performance with the integrity and rigidity to survive these extended casing trips and harsh well environments.
A South America customer in Ecuador in the Yuralpa Field was drilling a land well that was a re-drill design with a kick-off exit window. Centralization needed to be optimal along the string to keep pipe separated from different zones and also to get good displacement efficiency and cement sheath during Primary Cementing Procedures. The centralizers had to be of a favorable rugged design in order to pass through the exit window.
About the well: The original cased wellbore casing was 13 3/8 in. The planned well would have a depth of 9,850 ft. The open hole size at exit window would be 12 1/4 inches. The liner selected would be 9 5/8 inches.