Casing Centralizer Series – 1: Types of Centralizers

Casing centralizer is a mechanical device secured around the casing at various locations to keep the casing from contacting the wellbore walls. As a result of casing centralization, a continuous annular clearance around the casing allows cement to completely seal the casing to the borehole wall.

Casing centralization is one of the key elements to ensure the quality of a cementing job by preventing mud channeling and poor zonal isolation. Centralizers can also assist in the running of the casing and the prevention of differential sticking.

Centralizer’s usage is extensive! It is estimated that 10 million centralizers are manufactured and used every year globally. Centralizer manufacturers likely want to increase the demand for centralizers. However, operators on the other hand, may wonder: “Should we use that many?”

While centralizers are used extensively, wellbore problems continue to arise due to poor cementing jobs. Centralizer properties and placements directly or indirectly affect the quality of the cementing job.

The challenge that both operators and service companies face is to choose the right type of centralizers and place the right amount at the optimum positions on the casing to achieve a good standoff profile.

There are 4 types of centralizers (Fig. 1): bow-spring, rigid, semi-rigid, and mold-on; each with its own pros and cons.

Types of Centralizers | Illustration from Pegasus Vertex, Inc. - Drilling Software

Fig. 1. Types of centralizers

1. Bow-Spring

Since the bow springs are slightly larger than the wellbore, they can provide complete centralization in vertical or slightly deviated wells. Due to the flexibility of bows, they can pass through narrow hole sections and expand in the targeted locations.

The shape and stiffness of the bows determine the restoring force, which is defined as the resistance force when a bow is compressed by 1/3 of its uncompressed height. The effectiveness of this type of centralizer is heavily dependent on the restoring force.

When the casing is heavy and/or the wellbore is highly deviated, they may not support the casing very well. For example, on a riser tieback casing string, a helically buckled casing could create a side force of 50,000 to 100,000 lbf (222 to 445 kN), well beyond the capabilities of the spring-bow centralizer. A solid centralizer would be able to meet the requirements.

2. Rigid

Rigid centralizers are built out of solid steel bars or cast iron, with a fixed blade height and are sized to fit a specific casing or hole size. This type is rugged and works well even in deviated wellbores, regardless of the side force. They provide a guaranteed standoff and function as bearings during the pipe rotation, 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.

3. Semi-Rigid

Semi-rigid centralizers are made of double crested bows, which provide desirable features found in both the bow-spring and the rigid centralizers. 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.

4. Mold-On
The 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.

An Illustration and A Twisted Finger

Let’s illustrate in an easier way what torque and drag is:

Hold your index finger tightly in the fist of your other hand.

Now twist your finger.

Twist Your Finger and You Feel The Pain







Do you feel how your finger does not want to twist?


You know why?

Because it's not built to be twisted.

The pain you feel is because of the torque you are putting on the joints.  - That's torque.

Do you feel how your finger resists the pull because you have a good grip on it with your fist? - That's drag.

Torque and drag can have a dramatic increase in horizontal and extended-reach wells and can become the limiting factor in determining the horizontal length or extended-reach of a well. For this reason, precise calculations of torque and drag are necessary for drilling operations. Torque and drag are the results of friction caused by a moving pipe inside the wellbore: torque occurs when rotating the pipe along the wellbore and drag occurs when moving the pipe.

When drilling horizontal or extended-reach wells, excessive torque and drag may become troublesome both in the drilling operations and later in the completion operations. Estimating torque and drag is very important, but the calculation of drag in the build section of a well is complicated by the effect of the axial force (tensile or compressive) on the lateral contact force which produces the sliding drag and in turn causes changes to the axial force itself. The axial force has a great effect on the torque and drag calculations in the build section. When the axial force (tension or compression) becomes large enough to let the pipe contact only one side of the wellbore, the torque and drag in the build section will increase proportionally with the increase in the axial force.

The most common way to calculate approximate torque and drag values in the build section involves monotonous numerical calculations: dividing the build section into many small pieces, assuming the axial force remains constant in those small pieces, calculating the friction factor for each of the pieces, and then summing these values to get the total drag over the entire build section. This process is both time-consuming and difficult for field engineers.

The analysis of torque and drag is made easier by today’s technology. There is a comprehensive torque and drag software in the market that removes many of the risks during the drilling process. This software was developed by PVI and it’s called TADPRO (Torque and Drag).

TADPRO - torque and drag

This software comes with features that help users to:

  • Calculate hookload and surface torque
  • Identify potential buckling
  • Perform sensitivity analysis
  • Determine side force
  • Analyze forces downhole
  • User-friendliness and graphical outputs

Illustrations have always been a great learning method and today we have learned two things:

  1. Fingers were not made to be twisted.
  2. Likewise a pipe is not built to be twisted, but the torque and drag inevitably occurs during horizontal drilling, but with the help of TADPRO, torque and drag can be calculated and predicted, therefore the risks are reduced.