This is a type of sleeve with connections that can pass through traditional sleeves (or other expandable sleeves) and then expand to a larger diameter than traditional sleeves that pass through pipes of the same size. Although this is a relatively new technology, it has achieved good success in many applications. Of course, this technology is not yet mature and has some limitations, which means a broader development space.

Problems with traditional casing

Typically, there are two problems with casing sizes that sometimes arise during drilling, and they both increase drilling costs and can delay drilling:

1.Unanticipated conditions that require an additional casing string of casing after the well has been started. In this case, the size and depth have been selected by casing design and one or more pipe columns can be set before additional columns are required. Unexpected borehole stability or pressure problems may require additional strings.

2.Known conditions that require multiple casing strings for a well before it has been started. Mostly, we would consider surface, intermediate and production tubulars, possibly with a liner somewhere in the mix, but essentially three or six tubulars. In recent years, however, we have seen wells requiring seven or even as many as 10 casing columns to reach the target. Conventional methods require some very large wellbores and casing to reach a total depth where the final casing column size allows for sufficient production.

In each case, size and clearance can become serious problems. However, expandable casing can be the key to solve them.

Expandable 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.

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.

Additionally, the connections must be expanded, since they must be run as individual joints. Obviously, for the performance properties of the expanded pipe to be reasonable, the expansion process must be uniform.

Expansion Process

Two basic processes are used for expanding pipe 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.

Main Applications In Drilling

At the beginning, we mentioned the unanticipated well problem as a possible application for the expandable casing.

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.

Collapse Considerations

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.

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.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.