# Casing Centralizer Series – 3: Modeling

We are going to study on the 5 parameters affecting casing standoff.

1. Well trajectory

Well trajectory is expressed in terms of survey data, consisting measured depth, inclination and azimuth angles. It defines the shape of the well path and thus has great impact on the direction and magnitude of the side forces that pull the casing string to the wellbore. Fig. 5 shows the magnitude and direction of the side force distribution on a casing in a horizontal well.

Fig. 1. Side Force Profile

For a casing section in a build-up or horizontal section of wellbore, the weight of pipe pulls the casing toward to the lower side of hole. The blue lines indicate that the casing touches the lower side of wellbore. The upper section of the casing string has to sustain the weight of lower casing sections. This creates tension force along the casing string.  Wellbore doglegs cause the resultant force to pull the casing toward the upper side of the hole, as indicated by the red lines. Therefore, casing string in a deviated or horizontal well always touches the wellbore, upper or lower side.

Fig. 2. Side Forces with Casing Positions

Generally speaking, horizontal or extended reach wells require more support from centralizers to maintain a good standoff profile.

2. Casing size and weight

Casing weight determines the gravitational force which pulls the pipe toward the lower side of the borehole. The heavier the casing string is, more or stronger centralizers are required.

3. Fluids inside casing and in annulus

The buoyancy force calculation is further complicated by the multi-fluid configuration during a cementing job. When heavy cement slurries are inside the casing and light drilling mud in the annulus, the effective weight of casing is at its greatest. On the other hand, when cement slurries are in place and displacement fluid inside the casing, the buoyancy is at its peak and the effective weight of the pipe is the least.

When we design the centralizer placement for the scenario of cement slurries in place, it favors us to have less effective casing weight, pulling the casing string downward; but when the cement slurries are inside the casing during the displacement, the lower standoff could cause mud channeling problems. It is better to study standoff for all the situations.

4. Centralizer properties

Not all centralizers are created equal.  Centralizer manufacturers are striving to improve the performances of their products.

For solid centralizers including mold-on type, the blade OD is the key parameter as far as the casing centralization is concerned.

For bow-spring centralizers, the restoring force is the measurement of the strength of a centralizer. It is defined as the side force to deflect the bow by 1/3 or its original height.

5. Centralizer placement

Once the well is planned, casing designed, cementing procedure prepared and centralizer type selected, we do not have many options other than placing the centralizers strategically to achieve the desired standoff. However, this is also a great leverage.

Poor spacing will result in poor standoff even with the best centralizers in the market.

# People Should Fall in Love with Their Eyes Closed

Advertising does have impact on people’s purchase decision. Recently, I saw quite a few TV commercials on Perrier bottled water, which shows a scene in a hot summer day. Everything is melting and a lady catches the last bottle of Perrier water before it falls into a swimming pool.

On Saturday, I bought 4 big bottles of Perrier water because the store was selling 4 bottles for \$5. This year’s Labor Day was on Monday. It was so hot that I was totally soaked in sweat only after a dozens of minutes doing yard work. It was refreshing to enjoy the big bottle of Perrier water, sitting in the shade. While feeling the tiny bubbles bursting on my tongue, I studied the top and bottom labels, just to make the time in my hands useful.

Suddenly, I noticed a hand-written type of sentence on the back side of the bottom label, which read the title of this article. I was delighted. The pleasure of finding this secrecy and reading this sentence was more than that of drinking the water. How nice it is to have thirst quenched and emotion excited at the same time?! Maybe that is exactly what the bottling company designs. I was wondering.

I admire the idea. We, as drilling engineering software developers, often do something similar on the graphic user interface (GUI) of our software. For example, users can use “Well Path” functions in our Dr. DE (drilling engineering toolbox software) to design different well trajectory.  For example, user can easily create a build-and-drop shape. Dr. DE performs the calculation and displays the results in one click.

The tabulated numbers may be too abstracted to many users. So, we have this window of explanation.

It is not quite insightful as the title of this article, but it visualizes the table, just as the sentence elevates the sensation.

We are making our software sophisticated yet simple to use. By doing so, not only do we liberate drilling engineers from problems, but also bring smiles to their faces.