# Eccentric Annulus in a Cement Job

Finding a good picture of an eccentric annulus was challenging until Dennis (Global Sales Manager of PVI) and I walked down a street in downtown Calgary yesterday. I took a picture of this interesting sewerage cover. Dennis immediately guessed what I wanted to do with it, “Picture for your blog article about eccentricity?”

Picture 1: Eccentric Annulus

Many drilling engineering textbooks and modeling software assume concentric annulus because it is easy to model the fluid dynamics in it. Unfortunately, the most natural state of pipe in a well is almost always close to one side of the wellbore, if not touching, especially in a deviated well. Casing centralizers keep casing from contacting the wellbore wall. Even with centralizers installed, the casing between centralizers will still deform (sag) and could contact the wellbore.

An eccentric annulus has the same cross-section area as the concentric annulus. However, the flow through the eccentric annulus exhibits various forms. The following picture shows the velocity profiles in annuli with various degrees of eccentricity. The percentage represents casing standoff. A standoff of 100% means a perfectly centered pipe while a standoff of 0% represents the situation that the casing is in contact with the wellbore.

Picture 2: Velocity Profiles

The eccentric annulus has many unique characteristics such as less frictional pressure drop than that in concentric annulus. As the standoff gets lower, less energy is required to move the fluid, and mud removal becomes a problem in the narrow side.

To easily view the numerically simulated results of the fluid mixture in the annulus, we are going to unwrap the annulus into a 2D picture. In this picture, the middle represents the narrow side and the 2 edges represent the wide side.

Picture 3 shows the mud concentration with various standoffs.

Picture 3: Mud Concentration for Various Standoff

A well-centered pipe in a wellbore will lead to a more uniform axial velocity profile and shorter fluid interface length. As standoff approaches 0, the narrow side flow could even be blocked, leaving fluid not displaced.

# Casing Centralizer Series – 2: Standoff

The term standoff (SO) describes the extent to which the pipe is centered (Fig. 1).

Fig. 1. Definition of standoff

If a casing is perfectly centered, the standoff is 100%. A standoff of 0% means that the pipe touches the wellbore.  Regardless of the centralizer type, the goal is to provide a positive standoff, preferably above 67%, throughout the casing string.

The casing deflection between centralizers obeys the laws of physics. An engineering analysis can help both operators and service companies arrive at the optimized number and placement of centralizers for a particular well.

The casing standoff depends on the following factors:

• Well path and hole size
• Casing OD and weight
• Centralizer properties
• Position and densities of mud and cement slurries (buoyance)

Incomplete mud removal causes poor cement seal and non-productive time.  A good casing standoff helps reduce the mud channeling and improves the displacement efficiency. The following 2 pictures illustrate the impact of casing standoff on displacement efficiency.  The 3rd track in Figure 3 shows the mud concentration in the annulus after a cementing job with 0% casing standoff.

Fig. 2. Displacement Efficiency for Casing Standoff of 0%

You can see that there are some large red areas, which represent the high percentage of the remaining mud, in the narrow side (NS) of an eccentric annulus.

We kept everything else the same and only changed the casing standoff to 70%.  Now the displacement efficiency improved significantly, as shown in the following picture.

Fig. 3. Displacement Efficiency for Casing Standoff of 70%

# Prevention is Better than Cure

Many of us wouldn’t go to see a doctor until we get sick. This not only costs more money, but also causes more pain. Preventive medical assessments may be seemingly unnecessary and unpleasant, but it can help identify potential health problems, save lives, and reduce medical costs.

We often hear of some famous physical doctors or medical experts, who have remedied illness, when troubles surfaced. As a matter of fact, we are all doctors ourselves, in a sense that we take care of our daily lives, command our bodies to certain tasks, and avoid certain activities. We, as doctors ourselves, play a big role every day in maintaining our health. We are all CEOs of our own way of life and happiness. If we do our best to keep up good habits, take care of our bodies regularly, we will save ourselves lots of trouble and money in medical expenses.

Prevention can be beneficial to drilling engineering as well. Our experiences gained and lessons learned help us make right decisions for our next well. Drilling engineering software also serves as a great tool to identify any potential problems in our existing drilling plan or make sound engineering designs.

Take an example of cementing job design. One of the key elements of a successful cementing job is good displacement efficiency (less mud channeling). We have known that several parameters such as casing standoff, flow rate, differentials of fluid properties and hole deviation affect the effectiveness of fluid displacement. Now, with the help of computer modeling, we can see the impacts of these parameters. The following picture shows the mud concentration in the annulus at the end of the cementing job, where casing standoff is 0% throughout the wellbore (casting touches the borehole).

Displacement Efficiency for Casing Standoff 0%

You can see that there are quite large red areas in track 3, indicating poor displacement efficiency.

Now, we change the casing standoff to 70% and keep the other parameters the same. The following picture shows the much improved mud concentration after displacement:

Displacement Efficiency for Casing Standoff 70%

Time and money (not to mention headaches) could be saved if drilling engineers undergo through all the engineering assessments before problems arise. Be sure to use your know-how, skills, experiences, lessons and available drilling software. Having a complete understanding of what will happen gives you a greater sense of control, reduces risks and saves money.