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Casing Centralizers Types & Spacing

Objective: The target of this article is to identify the different casing centralizers types & to understand how to do casing centralizer spacing while you are in the designing phase of the casing (Check Our Guide For Casing Design). The focus will be on is on the below items:

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  1. The objective of casing centralization.
  2.  Casing Centralizers different Types.
  3.  Formulas For Casing Centralizers Spacing.
  4.  The installation Patterns For Casing Centralizers.
  5.  Related Casing Centralizers Case Study

What is the major function of casing centralizers ?

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 the casing 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 Are The Divergent Casing Centralizers Types?

  • Bow-spring Centralizers
  • Rigid Centralizers
  • Semi-rigid Centralizers.
  • Mold-on Centralizers.

Bow Spring Casing Centralizer Type

Drilling Engineers use Bow Springs Centralizers 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

Rigid Centralizer

Rigid Casing Centralizer

Solid, welded blade casing centralizers types offer high strength in horizontal and extended reach wells as they provide a guaranteed standoff and function as bearings during the pipe rotation. This rigid bars are welded to the centralizer collars, giving the design strength and durability. Rigid centralizers come in a variety of configurations, including hinged and slip-on, with both straight blade and spiral designs.

Rigid centralizers are also available in composite materials which can minimize drag, enabling conventional casing runs in long laterals regardless of the side force. Set screws are used in some configurations to lock the centralizer in place on the casing joint, providing extra holding force. But since the centralizers are smaller than the wellbore, they will not provide a good centralization as the bow-spring type centralizers in vertical wells

Semi-Rigid Centralizers

Semi-rigid casing centralizers types 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.

Mold-On Type Casing Centralizers Type

The mold-on casing centralizer blades type, 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 Spacing

Any type of casing centralizers 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.

equations for casing centralizers spacing

Where: 
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
ρmiand ρmo= the mud weights in and out of the casing, respectively
DLS = dog leg severity
do= outer casing diameter
di= inner casing diameter

The selection of the proper Casing Centralizers Types installation pattern for mechanical cementing aids i.e. centralizers, scratchers, turbolisers, etc. is essential to optimize casing running and cementing results.

Case Studies For Casing Centralizers