# Casing Wear Series – 1: Causes

During the drilling phase, the most costly component is the casing. On top of the expensive casing materials and the costs likely to be encountered in cutting, pulling and replacing a worn or damaged string, casing wear creates more serious problems for operators due to its potential catastrophic incidents such as oil spills, blow outs or loss of the well.

To analyze the forces behind casing wear, we need to study the torque and drag (T&D) of the drill pipe during drilling operations. The basic mathematical and physical model of T&D has not changed significantly since Johancsik et al. published their paper on T&D prediction. Pipe movements such as drilling ahead or tripping create drag, while rotation produces torque. The magnitude of T&D is determined by the combination of these two movements.

Since the so-called vertical well virtually does not exist (the whirring action of the bit always creates a micro-helical shape of the well path), the contact of the drill pipe and its tool joint with the casing ID is unavoidable. The gravitational force acting on the drill pipe is always trying to pull the pipe to the lower side of the wellbore, while the axial tension on the drill pipe (in a build-up section) tends to push the pipe to the upper side of the wellbore. Depending on the pipe weight, dogleg severity, and axial force along the pipe, the drill pipe either touches the upper or lower side of the wellbore.

Typical T&D analysis starts by dividing the pipe into small elements. Calculation begins from the bottom element of the pipe, where weight on bit (WOB) and torque on bit (TOB) are expected. For each element, force and torque are balanced and the T&D at the top of the element are calculated. From bottom to top, calculations are performed for each pipe element, until it reaches the rig floor. This step-by-step calculation also determines the direction and magnitude of the side force, which pushes the drill pipe against the wellbore as shown in Figure 1.

Figure 1. Snapshot of Side Force along a Drill Pipe

Under this side force, the rotating tool joint on the drill pipe against the casing inside, gradually removes steel from the casing wall and forms a crescent-shaped wear on the casing as shown in Figure 2.

Figure 2. Rotating Tool Joint Wears Crescent Grooves in Casing

The seriousness of friction between two contacting surfaces is dependent on the nature of the rubbing surfaces and the mud.

The tool joint coating plays a bigger role here compared to the casing wall. The industry has seen tool joint coating evolve from “casing killer” (rough tungsten carbide) to “casing friendly” as shown by many high-tech hardbanding materials.

Tungsten carbide is applied on the tool joint. While it is a very good protector of the tool joints, it aggressively wears the casing so much that the mud type and its additive will not help much in reducing casing wear if rough tungsten carbide is present.

Once a casing friendly tool joint coating has been selected, the mud type and its additives play an intermediate role in casing wear. Water-based mud causes twice as much casing wear as the oil-based alternative. Lubricant reduces friction and severity of the wear.

Generally speaking, high dogleg will create a high side force and severe casing wear. The wear profile resembles the shape of dogleg severity. Higher RPM and lower ROP make more rotation time between the tool joint and casing and will cause aggressive wear.

The following conditions contribute to casing wear:

• Well path and dogleg
• Drill pipe weight
• Tool joint coating
• Mud and additives
• RPM and ROP

# 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.

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

Yes!

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).

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.

# Flying Among The Clouds

Louis D. Brandies once said:

“Most of the things worth doing in the world had been declared impossible before they were done. Impossible means that you haven’t found the solution yet.”

A little over 100 years ago there were things that were considered impossible to do and that there was no way they could ever be achieved. For instance, to be able to fly among the clouds, but was it really an impossibility? Time proved that it wasn’t.

Just like flying among the clouds was impossible to do once, there are many things that thanks to the advancement of technology now are possible. For instance, a few decades ago horizontal or extended-reach drilling was considered impossible as well as casing wear prediction. In these environments, casing design is critical to a safe and successful drilling operations and well production, and unexpected casing wear can result in significant costs or even the loss of the well itself. This is the problem that drilling companies want to prevent.

So the question is: Is there any tool or software to calculate and predict casing wear severity? Yes there is! It’s called CWPRO.

This casing wear model uses the number of drill string rotations and contact force between the drill pipe and casing to calculate wear. The contact force is calculated using the dogleg severity inside the well. The maximum dogleg severity frequently determines the location and extent of the most severe casing wear. CWPRO helps operators and service companies identify, control and prevent potential problems. In overall the goal of CWPRO is to more accurately quantify casing wear risks and to ensure that the integrity of the casing is maintained during drilling operations.

Like mentioned before, there are many things that were considered an impossibility not too long ago like for instance, flying among the clouds. Likewise, thanks to software like CWPRO, predicting casing wear is no longer impossible; it is a fact.

# Fear of the Unknown

“I can’t believe what I just ate.” Those were my words when I had my last bite of … Jellyfish. I didn’t want to eat it at first, but I succumbed too quickly to peer pressure and I tried it, and before I knew it, I had eaten it all. That’s why I said the words at the beginning of the blog, because I would have never imagined that I would eat such a thing as jellyfish and I loved it.

As much as I love food, I have never been the kind of person that is into eating uncommon foods, especially the ones that were once alive. I guess it is all due to cultural ideas. In some cultures they eat anything and everything, while in other cultures they limit themselves to specific kinds of foods. Having the privilege to work with people from another culture has given me the opportunity to expand my limitations by trying out different kinds of foods, in this case, Jellyfish.

The problem with many of us in America is that when we see something we are not used to seeing or in this case, eating, we give it too much thought on whether or not we should try it and the majority of the time we simply decide not to try it. The reason: fear of the unknown. We simply feel safer trying what we know, what is common to us, but when facing a situation where we have to try something completely new to us, we hesitate because we are uncertain of the results.

In the beginning of this blog I mentioned that I got to eat the jellyfish due to succumbing to peer pressure. I was having lunch with my workmates and they kept on telling me that I should try it, that I shouldn’t say that I don’t like something if I haven’t tried it yet, and that made me realize that what they were saying was completely true. How can I assure that I will not like something if I haven’t even given myself the opportunity to at least try it? I was a little hesitant at first, but the result was that I ate the whole plate.

This is a lesson for everything in life, but in this case, it’s a lesson for drilling engineers. Technology has evolved in such an unimaginable way that drilling operations can have beyond compare results. Technology has given drilling engineers the opportunity to explore the unknown with cutting edge software that make their work easier in every possible way. This technology can be new to most engineers, but wouldn’t it be the best option for them once they give it a try? They might be a little hesitant, because since they have not tried it they might question the essence of such software, (the fear of the unknown) but the reality is: if they don’t try it, they won’t find out how incredible this software is.

PVI is equipped with such an exquisite technology that once drilling engineers try it, they will not regret it, just like I don’t regret eating Jellyfish which is now one of my favorite dishes.

# I Never Felt So Good When Taking Off My Shoes

During Thanksgiving weekend, our company staff and families went to Lake Tahoe to have a retreat. Skiing was one of the activities. Quite a few of us were first time skiers. Here are some of our children expressing their experience on wearing the ski boots.

Rachel: "I felt short when taking off the shoes."

Nathan: "I never felt so good when taking off my shoes."

Nowadays, almost all our activities are enriched with high-tech equipment. Skiing is no exception. A pair of ski boots can easily weight a few pounds. Walking with them is no fun. But once the shoes are attached to ski and ski is on the slope of snow, they make us run faster on snow than on road. Technology makes wonders. Besides specific skills and physical training, more sportsmen rely heavily on gears to enhance their performance. To swimmers, it is swimming suit; to tennis players, it is racket and strings and also as the running shoes to runners. The pairs can go on.

These enabling technologies also play big role in our drilling industry, making drilling operations more cost effective and safe. Among them is drilling engineering software. Even though the drilling software does not weight as much as ski boots, it carries the significant results of research and engineering in the areas of pipe mechanics, hydraulics, casing wear, etc.

One might think the drilling software as one more package to install in computer, one more burden to carry. However, the benefits of running drilling software far out weight the cost or trouble of using it.

Download and installation of our MUDPRO (mud reporting software) may take 15 minutes. Once mud engineers start to use MUDPRO, they could easily save hours of work every day, not to mention the much better quality of report and end-of-well recap.

Sometimes, a little bit trouble, such as wearing heavy ski boots, brings tremendous convenience; sometimes, a little bit spending is rewarded with big saving.

Just as Nathan described his feeling on ski boots, our MUDPRO user might say: "I never felt so relieved when making daily mud report."