# 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.

# Can You Afford Not To Use Drilling Software?

We decide to buy things based on the benefits those things may bring to us. Those benefits can be either tangible or intangible. If the tangible benefits are greater than the price, the decision process is easy, or if the benefits give us a perception of peace then we will most likely make the purchase.

Drilling software, in particular, is a product packed with advanced engineering calculations. One can say it is a condensed result of research, an interactive digital toolkit or an expert who never gets tired. It normally takes years of development by a well-trained team.

Setting prices for software packages is challenging because there are many uncertainties involved, such as market size, other similar products, etc. One thing is certain in any drilling software, if successfully used in pre-drilling analysis, it will most likely bring more benefits than the money spent on purchasing it. The cost of drilling an oil and gas well is so high that any non-productive time prevention (NPT) is well worth the spending.

Drilling software provides a good way of identifying potential problems in a drilling design and making good recommendations.

Take an example of casing centralizer placements, the purchase of centralizers is to provide a good casing standoff (>70%) to be better prepared for a cementing job. The standoff profile of a casing in a directional well depends on many parameters such as well path, casing weight, fluid densities, top of cement (TOC), centralizer properties and placement. Our past work experiences can help us select the proper types of centralizers and placement, but for a given well condition, it is best to use computer model to make recommendations for the centralizer usage. The following picture shows the resulting standoff profile with a designed centralizer spacing.

Standoff vs Measured Depth - CentraDesign

Thomas Edison once said: “I shall make electricity so cheap that only the rich can afford to burn candles.”

Nowadays, drilling software has become commonplace. Applying the latest drilling technology includes using the available solutions. Drilling software is like the electricity to light our understanding and design of drilling operations. Can you afford not to use it?