A mechanical device that keeps casing from contacting the wellbore wall. A continuous 360-degree annular space around casing allows cement to completely seal the casing to the borehole wall.

There are two distinct classes of centralizers. The older and more common is a simple, low-cost bow-spring design. Since the bow springs are slightly larger than the wellbore, they can provide complete centralization in vertical or slightly deviated wells. However, they do not support the weight of the casing very well in deviated wellbores.

The second type is a rigid blade design. This type is rugged and works well even in deviated wellbores, but since the centralizers are smaller than the wellbore, they will not provide as good centralization as bow-spring type centralizers in vertical wells. Rigid-blade casing centralizers are slightly more expensive and can cause trouble downhole if the wellbore is not in excellent condition.

Below video from Weatherford explains how to choose a centralizer:

Centralizers | Weatherford International

In any wellbore, it is critical to choose the right type of centralizer and to place those centralizers at optimum position on the casing to achieve good and consistent standoff. Casing centralizers are available in many shapes and sizes. It is important to understand the differences in each type and which is best suited to each application. In this blog, we cover three types of centralizers – rigid, bow spring, and unibody bow spring. These common styles cover a wide range of applications and are suitable for achieving high centralization performance.

Centralizers are used to keep the casing from contacting the borehole, while providing a good standoff clearance around the casing to aid running and cementing operations. In horizontal wells, centralizers need to have a high restoring force to support the weight of the casing, and a low running force to reduce drag. They also need to allow pipe rotation and reciprocation to aid in the casing running process, reduce differential sticking tendencies, and improve mud flow and zonal isolation while cementing.

Key Terminology

• Restoring force – the force exerted by the centralizer to keep the casing away from the borehole wall
• Standoff – the clearance between the OD of the casing and the borehole wall, normally quantified as a percent of concentricity
• Starting force – the maximum force required to insert a centralizer into a specific wellbore or casing diameter
• Running force – the maximum force required to move a centralizer through a specific wellbore diameter

Rigid Centralizers
Solid, welded blade centralizers offer high strength in horizontal and extended reach wells where rotation is needed to get casing to bottom. 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 reduce drag, enabling conventional casing runs in long laterals. Set screws are used in some configurations to lock the centralizer in place on the casing joint, providing extra holding force.  

Bow Spring Centralizers
This design is common in casing running operations offering a good balance between wellbore performance and cost. The spring steel bows are welded to rigid end collars that supply support to the bows. They are available in both slip on and hinged configurations to aid installation. Bow spring centralizers provide good borehole standoff, have favorable restoring forces, and come in a variety of shapes and lengths for most well requirements.

 Unibody Bow Spring Centralizers

For horizontal wells with long laterals, tortuous boreholes, and severe well geometries (including high doglegs), a one piece, non-welded centralizer is the ideal choice. The unibody construction eliminates weak points. This means that the casing string can be rotated and reciprocated with more confidence when running to bottom in tight wellbore situations.

During casing rotation operations, the centralizer is designed to remain in place, while the pipe rotates inside of it. This ability to agitate the pipe during cementing operations improves cement bond and long-term cement integrity. Unibody bow springs act as a semi-rigid centralizer, designed to be slightly under gauge minimizing drag while offering both flexibility and high restoring force. As the centralizer navigates restrictions, it is designed to compress and deform to a near-flat condition. As the borehole restriction is traversed, the centralizer reverts to its original form and shape without permanent deformation or damage.