This is one of the types of the casing with connections that can be run through the conventional casing (or other expandable casing) then expanded to a larger diameter than a conventional casing run through that same size pipe. While this is a relatively new technology, it has seen some good success in numerous applications. However, it is not necessarily a panacea, as there are drawbacks, too.
Expandable Casing Is a Good Solution
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Two problems with casing sizes sometimes arise in the drilling of wells, both of which can increase the well costs (check also drilling cost per foot) and possibly prevent a well from reaching its objective:
- Unanticipated conditions that require an additional casing string of casing after the well has been started.
- Known conditions that require multiple casing strings for a well before it has been started.
In the first case, the sizes and depths already are selected through casing design, and one or more strings may be set before the need for an additional string arises. Unexpected bore-hole stability or pressure problems may require an additional string that was not originally planned for in the casing setting depth design. Another problem of similar nature is the possibility that a planned casing string may stick (pipe sticking problem) before reaching its planned depth and thereby necessitate an additional string. In these cases, an additional casing string or casing liner must be set and the final casing string at total depth will be smaller than desired unless some contingency was included in the original plan to allow for such an event.
The second case is becoming more common in some areas, especially where depleted zones may be present. Typically, we think in terms of a surface string, an intermediate string, and a production string, possibly with a liner somewhere in that mix, but basically three or four strings. Occasionally, we may even find it necessary to run five or six strings, counting liners. In recent times, however, we are seeing wells that require 7 or even as many as 10 strings of the casing to reach an objective. A conventional approach to this problem requires some very large wellbores and casing to reach the total depth with a final casing string size that allows for adequate production.
In each of these cases, size and clearance become serious problems. One answer to these problems is the expandable casing.
Expandable Pipe Then Casing
The expandable casing is not your typical casing product. First of all, it must be ductile enough that it can be expanded without rupturing and still have sufficient strength to function properly. You can read more about plastic material behavior, so we do not recap that here, but this is exactly the type of behavior that takes place with this type of material.
Consequently, it does not come in standard API Casing grades, weights, and so forth. Likewise, there are no published standards of performance properties but rather those are set by the manufacturer. The most expandable pipe is not a seamless pipe, since the wall thickness has to be much more uniform than the most seamless pipe. It is manufactured from flat plate steel that has been precisely rolled to within close tolerances.
Now, the seamless pipe can be used and is being used in some expandable applications other than casing, but the wall thickness must be very carefully determined to within close tolerances. You can imagine the results of the expansion process if the wall thickness is not uniform before the expansion; that is, most of the expansion takes place in the portions where the wall thickness already is the thinnest.
Additionally, the connections must be expanded, since they must be run as individual joints. When you consider the amount of expansion of the casing pipe body and the threaded connections, you come to appreciate the technology of the process, in that it is not nearly as simple as it might first appear. Obviously, for the performance properties of the expanded pipe to be reasonable, the expansion process must be uniform.
Two basic processes are used for expanding pipe, and they essentially are the same two processes that have been around for more than 40 years, since the first internal casing patches were introduced. Of course, they have seen considerable improvement since that introduction.
- One process involves a swaging operation in that an internal swage mandrel is run with the expandable casing, and it expands the pipe from the bottom up as it is pushed or pulled through the tube. This typically is a hydraulic process.
- The other process employs a roller-type device that expands the casing from the top down, using a tapered device with rollers that expand the casing as the device is rotated with a work string.
One thing that must be kept in mind, though, is elastic unloading (elastic-plastic behavior). If we expand a tube plastically, it always exhibits some amount of elastic shrinking from its plastic state. This means that it has to be expanded to a slightly larger diameter than its final diameter to account for the elastic unloading once the expansion tool is removed.
Swaging Process For Expandable Casing
The swaging process uses a mandrel with a circular cross-section that is in the expandable casing as it is run. The mandrel can be either a solid piece or of a type that allows for retraction and retrieval through a smaller diameter. In its expanded position, it is pumped or pulled through the casing from the bottom upward and expands the casing as it moves upward.
This Expandable action of the casing may be accomplished hydraulically or with a mechanical pulling force, as long as the casing is not allowed to move as it is being expanded. It is a positive type of expansion, in that, for the mandrel to pass through, the casing must expand to the diameter of the mandrel.
The advantages of this process are that it imparts true hoop stress to the casing being expanded. If the wall thickness of the casing is uniform and the material is isotropic, then the hoop stress is uniform. The expanded tube should be round.
The disadvantages are that the swaging process induces axial stress in the pipe as it is being pumped or drawn through and may require a special coating internally to reduce the friction. If the mandrel is a one-piece device and for some reason, it cannot be pulled through the entire expandable casing, then it cannot be removed unless it can be milled up. This may not be a problem with retractable-type mandrels, though they lack the simplicity of a single-piece mandrel. Figure 1 shows a typical swaging expansion process.
The Roller Type Process
The roller-type process was in use long before the expandable casing patch was introduced, over 40 years ago. It historically was used to try to restore partially collapsed casing. The roller process is simple, in that the process starts from the top and expands the casing as it is rotated downward into the expandable or collapsed pipe.
It has the advantage that it can be removed at any time, replaced, and resume the operation where it stopped. The historic problem with rollers is that they do not work very well, at least in the fixed version. A roller device does not induce true uniform hoop stress in a tube, because it contacts the casing at only a finite number of points, usually three or four. The old roller-type casing patches typically failed because they never were round in cross-section once expanded, because the rollers had only three contact points. Expanders with four rollers were introduced and had better success than three rollers but still never were as successful as the swage-type process.
The use of expandable casing rollers to restore partially collapsed casing historically enjoyed limited success primarily because of the point contact with the casing wall and the elastic unloading between contact points.
The Main Applications Of Expandable Casing In Drilling
At the beginning, we mentioned the unanticipated well problem as a possible application for the expandable casing. For someone who has been involved in drilling operations for a period of time, this usually is the first application that comes to mind. The expandable casing could be used in such an application as a temporary means of getting past some troublesome zone.
Originally, the availability and lead time required for the expandable pipe to be a readily available solution for this type of problem was limited. The expandable casing had to be ordered and available as a backup for a particular well before the actual need arose. This changed in time and is seldom a limitation now. Another drawback to the expandable casing as an unplanned contingency string is the cementing issue. If the expandable string is to be reliably cemented, then the hole in which it is to be placed must be either underreamed (Drilling Reamers Types) to a larger diameter than the oilfield drilling bit that will pass through the casing above it or it must be drilled initially to a larger diameter as with a bi-center bit. These are not necessarily amenable to unanticipated situations that may arise and require an additional casing string. As it currently stands, the expandable casing is a planned part of the casing program and for that, it has proven quite successful.
The cementing process in regard to expandable casing is a bit different from conventional casing cementing. The usual procedure is to displace the cement prior to expanding the casing. This requires that the casing expansion be completed before the cement begins to harden.
The expandable casing can be reciprocated and even rotated during the displacement process, so in that respect, it is no less effective than a conventional liner cementing job. The biggest differences may be the cement near the top of the liner (liner running & cementing procedure) and whether or not one wants cement in the annulus above the liner before the expansion process begins. As the casing is expanded, the mud and cement in the annulus must be displaced somewhere, and it goes into the annulus between the running string and the previously set casing. If cement actually is displaced into this space above the expandable liner, then there is a considerable discomfort factor until the expansion is complete and this cement can be circulated out of the wellbore.
For the most part though, since the expandable casing is used for a temporary drilling liner, it is not critical to have cement all the way to the top of the liner. The process has been successful in numerous applications, but it is a cause for concern. The most expandable casing is run as a liner and the final part of the expansion process is the expansion of the overlap in which some type of elastomer seals on the outside of the expandable casing seal against the casing through which it has been run. Once that seal has been established there is no way to displace cement into the annulus short of perforating and cement squeezing.
The collapse rating of the expandable casing is usually less than what one is accustomed to in similar sizes of the conventional casing. This is mostly due to the thinner wall of the expanded tubes as compared to API tubes. The thinner wall is the tradeoff we accept for the larger internal diameter which in turn is the primary reason we choose the expandable tube. In the discussion on plasticity, we mentioned that a material that strain hardens in plastic tension may gain yield strength in that direction, but in the process, it loses yield strength in compression. Also, if the casing wall does not expand uniformly, then the casing collapse strength is less than if it had expanded uniformly. With conventional casing, we can inspect its wall thickness and eccentricity before it is run in the hole.
With expandable casing, there is no way to know with certainty the final wall thickness and eccentricity, if any, until after it is in the hole and expanded. It can be seen in some of the commercial videos that show the expansion process on the surface that the final expanded tube has some amount of curvature in it. The causes of this curvature could be attributed to variations in wall thickness, residual stress, or anisotropic hardening.
This is not said to denigrate the expandable casing but to point out that one must understand that expandable casing is not the same thing as conventional casing. It has different properties, one of which is a reduced collapse strength, and that must be considered in any particular application and casing design.
Consequently, the most obvious practical application for the expandable casing is as an intermediate string or liner to be utilized during the drilling of a well that eventually will be cased with conventional casing. In those applications, it can be invaluable. For instance, Figure 2 shows a conventional casing program for a particular application.
By utilizing expandable casing in the same well the program can be modified as seen in Figure 3. The advantage is readily apparent, in that the total depth now can be reached with the same size conventional casing and smaller casing at shallow depths, if expandable casing strings are used in the well plan. There are several possible variations. While this may not be applicable for the most common wells drilled in the world, it represents a considerable advantage in those costly wells that do fall outside the common category.