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.

Like The Barriers on The Road

This is a story about a town located at the bottom of a deep valley.

The Vally

The only road into town was steep and windy, barely hanging on to the steep mountain-side surrounding the town. Very often, cars traveling into town would get too close to the edge of the road and tumble into the valley. Over time, the town spent hundreds of thousands of dollars responding to wrecks and they got tired of going out to respond to the same thing over and over. The town’s people rendezvoused to discuss the matter and how to solve it. Some folks thought they should ignore what was happening and let people fend for themselves. Others thought they should continue to help the people who went off the road, but should charge them for the costs involved. A few suggested that they should just close the road so those strangers wouldn't cause them any more problems. The majority of the people quickly acknowledged that the road posed a risk for strangers but also for friends, family and the townspeople themselves. Since nobody agreed to a specific solution, one person suggested they put up a barrier on the edge where cars most often went off the road and everybody agreed and so they did. Over the years, the barrier cost the community far less than all the rescuing they had been doing for so many years. A simple barrier was the solution.

It's a simple story, but a great metaphor for prevention. Like the barriers on the road, for more than 6 decades, casing centralization has been established as being essential to efficient mud removal and therefore to a successful cementing job. Prior to a production it is very common for field engineers to spend time improving casing centralization using software, particularly for highly deviated wells. However today, while long lateral wells are being drilled, they become more challenging in getting the casing to bottom and achieving good zonal isolation. That is why casing centralizers play a key role in achieving these objectives and should be evaluated differently than they have been in the past.

Predicting casing standoff is essential since not knowing where to locate the centralizers and how many are needed can cause several problems. The main reason for centralization is to ensure a uniform distribution of cement around the casing. No centralization or poor centralization will cause channeling of the cement and therefore produce poor cement adherence.

What do centralizers prevent?

  • When running a casing, the adequate use of centralizers reduces the chance of wall sticking.
  • In deviated wells because of the increased amount of support, the casing requires to stay in the center of the hole – especially in build-up sections - a more dense distribution of centralizers is required than in straight holes.

There are two main types of centralizer:

  1. Spring (Bow) Centralizers
  2. Rigid Centralizers

Spring (bow) centralizers are often used for vertical and deviated wellbores and rigid centralizers are used for horizontal wellbores. The method of installation for both of these depends entirely upon the centralizer design. However, care must be taken to ensure the quality of the cementing job. Centralizer placement is synonym of prevention.

For this PVI developed CentraDesign software that optimizes the centralizer placement, predicts casing standoff and torque and drag for extended reach drilling and deviated wellbores.

CentraDesign (Centralizer Placement)

CentraDesign also determines the number and placement of centralizers, hence providing both service companies and operators with a very sophisticated yet easy to use tool that will help prevent problems during the cementing process.

Prevention is like the barrier put up to keep cars from going over the edge and it works to keep unwanted things from happening in the first place.

The Best is Yet to Come

The following is told by an American lady:

“My grandmother always used to tell us, “keep your forks.” when the main dishes were being cleared from the table. It was my favorite part of dinner, because I knew that something wonderful was coming… like a velvety chocolate cake or a deep-dish apple pie.”

A similar expression would be “You have not seen anything yet.” or as my 8-year old daughter told me, “I am not done yet!” when I gave her an applause after she sang the song from the movie “Frozen”.

Life is a simple and normal routine. Everyone has the same number of hours in a day and the same number of days in a year. It is up to us to make our daily routines more interesting, to fill our time doing things that will make us grow in every aspect of our lives, to make the most of our time. We cherish the hope that our present situation is not our final destination. The best is yet to come.

For us software developers, we are continuously enhancing the drilling software that is being developed. We certainly can’t add more hours to our day, but we can make our development more efficient. We will probably spend the same amount of time as we did last year, but we will have better products in 2014.

The latest release of our cementing software CEMPRO+ is a milestone. Why? Because this is the first time we address the displacement efficiency during the multi-fluid displacement operations such as cementing or wellbore cleanup. Before CEMPRO+ we used to assume that the drilling mud was completely displaced by the cement slurry with the use of a piston. This convenient assumption makes the hydraulics relatively easy, but it fails to predict the mud channeling which occurs due to many factors such as, differences in the cement properties and mud flow rates, and the casing centralization. A typical illustration of mud channeling is shown here:

Mud channel left on the narrow side of the annulus

Mud channel left on the narrow side of the annulus (Macondo incident-Chief Counsel’s report, 2011)

CEMPRO+ can help predict the mud fractions in the annulus during a cementing job. The following picture is a snap shot from the program. It looks neat. Graphics are more significant, because they show what really happens in the wide and narrow sides of the annulus.

Displacement Efficiency Illustration in CEMPRO+

Displacement Efficiency Illustration in CEMPRO+

If our CEMPRO has been on your software menu, keep your forks, because the best is yet to come.