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

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

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.

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

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%

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%

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

From The Pencil to Engineer Is Human

“Design is a way of life, a point of view. It involves the whole complex of visual communications: talent, creative ability, manual skill, and technical knowledge. Aesthetics and economics, technology and psychology are intrinsically related to the process. Design is the evolution of useful things.”

- Paul Rand, also known as the American Modernist.

When Paul Rand made this comment he was referring to two books written by Henry Petroski: The Pencil and To Engineer Is Human, in which basically he talks about how we can take everyday objects and turn them into better useful objects. For instance, how pins were turned into paper clips; how Styrofoam containers evolved; how Post-it Notes came about and even how a simple rock can be turned into something very useful. The list goes on and on, and it is easy to understand the connection between Petroski’s points of view with Rand’s ideas on invention, innovation and ingenuity.

When it comes to the evolution of useful things, the oil and gas industry has many examples, but just let’s take this moment to talk about one of them: cementing and its development process.

Cement fills and seals the annulus between the casing string and the drilled hole. It has three general purposes:

  • Zone isolation and segregation
  • Corrosion control
  • Formation stability and pipe strength improvement.

Cement forms a very strong and impermeable seal from a thin slurry. The properties of the cement slurry and its behavior depend on the components and the additives in the cement slurry.

The cement is produced from limestone and either clay or shale by being roasted at 2600 to 3000°F. This high temperature fuses the mixture into a material called clinker cement. Once the roasting step is done, the rough clinker product is ground to a size specified by the grade of the cement. The final size of the cement particles has a direct connection with how much water is required to make the slurry without producing an excess of water at the top of the cement or in pockets as the cement hardens.  However, not all cements, including those made from the same components, will have the same reaction when mixed with water. Generally, the differences are in the quality of the grind of the cement, impurities in the water and in the additives added during the cement manufacturing process.

The design and test of the slurry are essential parts of every cementing job and without an efficient lab database cementing companies can face many problems, but thanks to the evolution of technology PVI has developed the right tool for this: CEMLab (Cement Lab Data Management).

CEMLab - Cement Lab Data Management

Since its first release in the fall of 2012, CEMLab has evolved into a powerful web-integrated and highly functional software product. CEMLab formulates slurries, calculates the amount of all ingredients, generates weight-up sheets and lab reports and allows engineers to have quick access to all their slurry formulation, and testing statuses anytime from anywhere.  These are just a few of the many features that make up CEMLab. Just like from the books “The Pencil” to “To Engineer Is Human” we get to see how an object can be designed and turned into something more useful and successful, CEMLab has been turned into a useful, successful and sophisticated lab tool.