Casing Centralizers: Are We Using Too Many or Too Few?
Are we using too many or too few casing centralizers? Are we using the right type of centralizers and placing the right amount at the optimum position on the casing? The following white paper shares information on the theory behind standoff calculations, centralizer placement, methods for determining how many centralizers is the right number, and more!
Casing centralizer is a mechanical device secured around the casing at various locations to keep casing from contacting the wellbore walls. As a result of casing centralization, a continuous annular clearance around casing allows cement to completely seal the casing to the borehole wall.
Casing centralization is one of the key elements to ensure the quality of a cementing job by preventing mud channeling and poor zonal isolation. Centralizers can also assist in the running of the casing and the prevention of differential sticking. Its usage is extensive! It is estimated that 10 million centralizers are manufactured and used every year globally.
Centralizer manufacturers likely want to increase the demand for centralizers. However, operators on the other hand, may wonder: “Should we use that many?”
While centralizers are used extensively, well problems continue to arise due to poor cementing job. Centralizer properties and placements directly or indirectly affect the quality of cementing job.
The challenges that both operators and service companies face is to choose the right type of centralizers and place the right amount at the optimum positions on the casing to achieve a good standoff profile.
There are 4 types of centralizers: bow-spring, rigid, semi-rigid, and mold-on, each with its own pros and cons.
Figure 1: Types of Centralizers
Since the bow springs are slightly larger than the wellbore, they can provide complete centralization in vertical or slightly deviated wells. Due to the flexibility of bows, they can pass through narrow hole sections and expand in the targeted location.
The shape and stiffness of bows determines the restoring force, which is defined as the resistance force when a bow is compressed 1/3 of its uncompressed height. The effectiveness of this type of centralizer is heavily dependent on the restoring force. When the casing is heavy and/or the wellbore is highly deviated, they may not support the casing very well. For example, on a riser tieback casing string, helically buckled casing could create a side force 50,000 to 100,000 lbf [222 to 445 kN], well beyond the capabilities of spring-bow centralizer. A solid centralizer would be able to meet the requirements.
Rigid centralizers are built out of solid steel bar or cast iron, with a fixed blade height and are sized to fit a specific casing or hole size. This type is rugged and works well even in deviated wellbores, regardless of the side force. They provide guaranteed standoff and function as bearings during pipe rotation. But, since the centralizers are smaller than the wellbore, they will not provide a good centralization as bow-spring type does in vertical wells.
Semi-rigid centralizers are made of double crested bows, which provide desirable features found in both spring bow and rigid centralizers. The spring characteristic of the bows allows the semi-rigid centralizers to compress in order to get through tight spots and severe doglegs. The double-crested bow provides restoring forces that exceed those standards set forth in API specification and therefore exhibits certain features normally associated with rigid centralizers.
The mold-on centralizer blades, made of carbon fiber ceramic material, can be applied directly to casing surface. The blade length, angle and spacing can be designed to fit specific well applications, especially for close tolerance annulus. The non-metallic composite can also reduce the friction in extended reach laterals to prevent casing buckling.
The term standoff (SO) describes the extent to which the pipe is centered (Fig. 2). If a casing is perfectly centered, the standoff is 100%. A 0% standoff means the pipe touches the wellbore. Regardless of the centralizer type, the goal is to provide positive standoff, preferably above 67%, throughout casing string.
Figure 2: Definition of Standoff
Casing deflection between centralizers obeys the laws of physics. Engineering analysis can help both operators and service companies arrive at optimized number and placement of centralizers for a particular well.
Casing standoff depends on the following conditions:
• Well path and hole size
• Casing OD and weight
• Centralizer properties
• Mud and cement slurry position and densities (buoyance)
IV. Casing Deflection
Between centralizers, casing string sags or deflects. To study the casing deflection, one should study the force balance for a pipe segment.
Figure 3: Force Balance
2 types of forces act on casing:
• Gravitational force on the pipe body, pulling the casing downward
• Axial tension force at the end, pushing the casing upward
Depending on the weight and tension, the net side force is either upward or downward.
To obtain side force, we start the analysis from the bottom and perform calculations for each element. Step by step, we move upward and obtain the side force profile, as shown below.
Figure 4: Side Force Calculation and Profile
In the profile, red lines indicate that the side force is acting upward and that the casing touches the upper side of the well. Blue lines indicate that the side force is acting downward and that the casing touches the lower side of the well.
In a typical wellbore (build-and-drop), the standoff profile of casing without centralizer looks like the one shown in the Figure 5.
Figure 5: Standoff Profile without Centralizers
Any fluids present in the wellbore create an up-lifting force (buoyancy) on casing, making the force acting on the wellbore less. During cementing job, when heavy cement slurry is inside casing, and drilling mud is outside the casing, casing is at its “heaviest”. As cement slurry turns corner and light displacement fluid occupies the casing interior, casing is at its “lightest”. Centralizer design considers the "lightest" casing condition. Fig. 6 illustrates the buoyancy conditions at various stages of cementing job.
Figure 6: Casing Deflection between Centralizers
A good centralizer design requires top of cement (TOC), cement slurry densities, and mud weight, etc. Larger density difference between cement slurry and mud would improve the standoff profile.
The puzzle of centralizer selection and centralizer placement can be best solved by using computer models. Over the past 20 years, a variety of models have been developed - some simply utilizing Excel spreadsheets, others implementing t as part of the cementing software. These efforts help engineers to understand the importance of casing centralizers and placement.
Since 2000, PVI has been working with both operators and centralizer manufacturers and been developing CentraDesign, the advanced engineering software that geared toward centralizer placement analysis.
There are 2 methods to model casing deflection between centralizers: hinged-ends model (Lee, Smith and Tighe) and fixed-ends model (Juvkam-wold and Jiang Wu).
Hinged-ends model assumes that a casing string transmits no bending moment across centralizers. This assumption results in the excessively high casing deflection. The hinged-ends model was replaced by the more advanced fixed-ends model, which is used to calculate the deflection between the centralizers. Anyway, casing string is a continuous beam in the wellbore. In this more sophisticated model, the casing deflection between centralizers in a 3D wellbore no longer occurs solely in the vertical plane or solely in the dogleg plane, instead it occurs as a spatial deflection in 2 planes: one in the dogleg plane and the other in the plane perpendicular to the dogleg plane. The resulting deflection is the vector summation of these 2 deflections, caused by the axial tension and the casing weight.
CentraDesign uses this latest model to predict casing deflection in a 3D well, which also consider the contribution from the changes of the azimuth angle . For bow spring centralizers, the compression of the bows themselves caused by the side force on the centralizer also must be considered as well in the standoff calculation.
3 methods are used to design the placement of centralizers.
|Specify Spacing||Users specify the spacing. Software checks the standoff.|
|Specify Standoff||Users specify the standoff at the mid-span, between the centralizers. Software calculates the spacing.|
|Optimum Spacing||Users specify the standoff and the spacing increments. Software calculates the spacing.|
In the first approach, spacing is specified utilizing the users’ experience; software then checks for satisfactory standoff at the centralizers and at the middle of the span. This mode offers simple-to-install centralizer placement because of its constant spacing. This method, however, may compromise the quality of standoff or quantity of centralizers, because the side force changes as the wellbore deviates.
For users without significant experience, or who prefer that the software calculates the spacing, the second approach - specify standoff - can be used. Simply specify the required standoff at the middle span, and the program uses a numerical method to obtain the centralizer placing, such that the standoff at the middle point between centralizers is as specified. The “specify standoff” mode ensures the minimum standoff of casing between centralizers, while yielding a difficult-to-follow placement program.
To benefit from the best elements of both approaches, we have developed an optimum placement solution, the third method in the diagram. In this approach, users specify the standoff with an incremental spacing requirement. This ensures the standoff requirements, yet results in a not-difficult-to-follow placement program. For high impact operations such as deep water and use of inline bow spring centralizers, these methods can be used once a casing schematic is available to optimize the exact placement of each centralizer.
With the help of computer modeling, centralizer placement optimization becomes easy to perform for all types of wells. Ideally, this kind of optimization should be done before each casing job. Here is an example of optimization.
Figure 7: Example Well
The example well shown in Figure 7 has a kick-off point of 2,000 ft. The previous casing (ID = 8.535”) was set at the same depth. Our goal is to centralize the 12,345 ft of 4 1/2” casing, deviated from 0o to 90o. The centralizer considered is bow spring type with restoring force of 800 lbf.
40 feet is used for the centralizer spacing (1 centralizer per joint). Figure 8 shows the resulting standoff profile. The blue line is the standoff at the centralizer, while the red line is the standoff at the middle point between centralizers, which is always lower than that at the centralizers. Because bow spring centralizers are used, the standoff at the middle point between centralizers is the summation of casing sagging between centralizers and the bow spring compression at the centralizers. For this approach, the number of centralizer required is 309.
Figure 8: Standoff Profile (Specified Spacing = 40ft)
From 2,000ft to 7,000 ft (inclination from 0o to 30o), the standoff at mid-span is between 100% and 70%, which meets industry standard of 67%. From 7,000ft to 12,345 ft (inclination from 30o to 90o), the standoff drops from 60%, which is problematic: poor standoff profile at this section may cause potential cementing problem.
Now try 2 centralizers per joint (spacing of 20 ft). Figure 9 shows the resulting standoff profile. The number of centralizers needed is 617.
Figure 9: Standoff Profile (Specified Spacing = 20ft)
The standoff at the mid-span is very good, at more than 90%. This new placement may be too conservative leaving doubts in engineer’s mind: “Am I using too many centralizers?”
Alternatively, the required standoff can be specified by the user, while the software instructs the user on how to space centralizers.
With the required 70% standoff throughout 4 1/2” casing, CentraDesign displays the following spacing necessary to achieve the specified standoff. The total number of centralizers used is 230, a significant reduction from previous approaches.
Figure 10: Calculated spacing required to achieve 70% standoff
Logically, as the well builds up from 0o to 90o inclination angle, the spacing decreases: casing needs more support in more deviated or horizontal section. But, putting centralizers strictly following the placement required by Fig. 10 is somewhat impractical.
To get the best elements from both approaches, we have designed the optimum placement solution, which is specifying standoff (70%) with incremental spacing requirement (20 ft). The resulting standoff profile and spacing required are displaced in Fig. 11 and Fig. 12, respectively.
Figure 11: Optimum Placement - Standoff Profile
Figure 12: Optimum Placement - Spacing
This method meets the standoff requirement and gives easy-to-follow spacing. The total number of centralizers resulted is 360.
The results of the three placement modes illustrated above are summarized in Table 1. The optimum placement gives satisfactory standoff, ease of field installation, and good economics.
Table 1: Centralizer Placement Comparison
Our industry is blessed with many talented and experienced engineers. We also have centralizer vendors producing top quality products.
It is critical that we maximize engineering potential while selecting the proper type of centralizer, and placement. Software like CentraDesign should be an integral part of the total approach of centralizer placement optimization.
Figure 13: Total approach of casing centralization
When optimizing centralizer placement, consider the following:
• Each well is different. Our past experience may not apply to the next well.
• Operators aim to obtain satisfactory standoff with fewer centralizers.
• Centralizer vendors similarly aim to obtain satisfactory standoff and hopefully sell more units.
• CentraDesign optimizes centralizer placement and usage, reduces risk and cost.
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