Casing Centralizer Series – 5: Are We Using Too Many or Too Few?

Our industry is blessed with many talented and experienced engineers. We also have centralizer vendors producing the very best and top quality products. It is critical that we maximize the engineering potential while selecting the proper types of centralizers and placements. A software package like CentraDesign should be an integral part of the total approach of the centralizer placement optimization.

Theories and equations determining the casing deflection between centralizers are well established, even though a hand calculation for a deviated well is impractical.

Experience plus software technology enable both centralizer vendors and operators to conduct centralizer optimization prior to field execution.

Fig. 1. Total Approach of Centralizer Placement

Fig. 1. Total Approach of Centralizer Placement

When optimizing the centralizer placement, consider the following:

  • Each well is different. Our previous experience may not apply to the next well.
  • Operators aim to obtain a satisfactory standoff with less centralizers.
  • Similarly centralizer vendors aim to obtain a satisfactory standoff to sell more units.
  • Software like CentraDesign optimizes the centralizer placement and usage.
  • Computer modeling reduces risks and costs.

Centralizer placement can make or break a good cementing job. Computer modeling is not only an easy but also a necessary step to achieve optimization of centralizer usage.  So, if you ask me the question: “Are we using too many or too few centralizers?” I would say: “If we all use readily available software to check the standoff profile for a specified spacing and optimize the placement, then we would probably use the correct number of centralizers.”

Merchant of Venice and Hook Load

I visited Venice after this year’s SPE/IADC Conference in Amsterdam in March. During that week in Venice, I stayed in a quiet world: no automobile, no motor cycle or train. You can simply add “water” in front of the names of our ordinary transportation to describe the local means of moving around: water-bus, water-taxi, and water-gondola.


While enjoying the Italian cuisine and limoncello, I suddenly thought of one of the famous play of Shakespeare’s: Merchant of Venice. The story with Venice as the venue goes like this.

A Venetian merchant Antonio has a friend Bassanio, who is desperately in need of money to court Portia, a wealthy heiress who lives in another city. Without enough money, Bassanio and Antonio finally secure the loan from Shylock, a Jewish moneylender, with Antonio as the loan’s guarantor. Shylock hates Antonio, but acts agreeably and offers to lend Bassanio three thousand ducats with no interest. Shylock adds, however, that should the loan go unpaid, shylock will be entitled to a pound of Antonio’s own flesh, to which Antonio agrees.

Later in story, news comes that Antonio has lost his ships, and that he has forfeited his bond to shylock. Shylock ignores the many pleas to spare Antonio’s life, and a trial is called to decide the matter. Portia, his friend’s fiancée, disguises as a young man of law and asks Shylock to show mercy, but he remains inflexible and insists the pound of flesh is rightfully his. Portia examines the contract and, finding it legally binding, declaring that Shylock is entitled to the merchant’s flesh. Shylock ecstatically praises her wisdom, but as he is on the verge of collecting his due, Portia reminds him that he must do so without causing Antonio to bleed, as the contract does not entitled him to any blood. Trapped by this logic, Shylock hastily agrees to take Bassanio’s money instead. The ending of the story has more drama.


We live in a world of measurements. Measurement is a cornerstone of engineering and science. For some reason, the story in my memory is that Portia reminds Shylock that he has to cut exactly one pound of Antonio’s flesh, not even one ounce more or less.

In drilling industry, we have mud weight (ppg) to measure the density of drilling fluid, pipe weight(lb/ft) to represent the thickness of pipe with given OD. The weight indicator - hook load is the total force acting (pulling down) on the hook on the rig.

Hook load is one of the few important readable operation parameters on rig floor. It’s basically the total force includes those of traveling assembly weight, buoyant pipe weight in a deviated well plus or minus the frictional drag caused by pipe movements inside borehole. Torque and drag (T&D) software such as TADPRO serves the purpose of predicting hook load, surface torque and other variables for drilling and tripping operations.

We can calculate hook load and surface torque precisely if we know all the details downhole. However, the uncertainties downhole, such as open hole sites, survey accuracy, make the prediction exactly match rig floor reading unlikely. This does not discount the importance of hookload prediction, because the significance of T&D calculation is its trend.

The following picture shows the hook load changes as we drill to TD. The hook load is increasing at beginning (due to longer pipe into the hole) and decrease later as we drill into build-up and horizontal sections (more drag).

Hook Load Predicted By TADPRO | Pegasus Vertex, Inc.

Hook Load Predicted by TADPRO

Eventually, the hook load is approaching zero. This indicates that the pipe weight in the vertical section will not be enough to overcome the frictional drag resulted from the horizontal section: a problem associated with horizontal and extend-reach well drilling.

A pound more or less on this curve is not as important as the trend, which signals us what will happen according to our planning.

Friction: Drilling Engineers’ Friend and Foe

Friction, the resistance force between two rubbing surfaces, the very drag consumes our energy while we walk, run and drive, also prevents us from falling, colliding and accident.

Friction is everywhere. We can see it painting on roads when car makes sudden stop. We can hear it screaming when wheels stop rotating while car keeps the momentum. We can smell its anxiety when Native Indians rotate drive in wood making fire. These are some appearances of friendly sides of friction. And sometimes, we need to amplify its power by the means like the following.Friction: Drilling Engineers’ Friend and Foe


For downside, we have seen plenty in our daily lines. Our shoe wears out (so our feet do not). We dress smart phone up by putting screen or cover so that scratch (a form of friction) only damages the dress, not the body.

This is friction: we hate, love and cannot get rid of it. And we had better use it toward our advantage.

In drilling and completion practices, we encounter friction whenever we move tubular inside wellbore. When moving pipe downward such as in drilling and casing running, friction slows us down. If we move pipe upward such as in trip-out, friction is pulling pipe down. Yes, friction always acts in the opposite direction of the moving object.

In torque and drag analysis, one graph is worth noting: the hookload for trip-in and trip-out operations, as shown here.

Drilling Software and Friction Pegasus Vertex Inc


The green line is the hookload when pipe is stationary inside wellbore at various depths.

The blue line shows the hookload when pipe is moving downward. The red line is the hookload when pipe is moving upward.

Note that during trip-in, the frictional drag is against the gravitational force, so the hookload is smaller than the hookload during trip-out, when the frictional drag acts with the gravitational force.

In other words, during trip-in, friction helps hook to hold the pipe weight, while during trip-out, the friction is on the same side of gravitation to make hookload higher.

For coiled-tubing (CT) operation, since CT has relatively thin wall (could be as small as 0.125 inch), the tensile limit of CT may not withstand the pulling of drag and weight during trip-out, as shown in the following graph.

Drilling Software And Friction Pegasus Vertex Inc.


We can run the CT into a deviated well. However, we may not be able to pull the same CT out of the hole!

Friend or foe, friction shows up in various fashions. We just need to know his characteristics and dance with him.