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%

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 - Pegasus Vertex, Inc.

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?