TECHNOLOGY Expandable tube is novel tool for difficult completions, drilling

Daljit S. Gill, Wim C.M. Lohbeck, R. Bruce Stewart
Shell International Exploration & Production B.V.
The Hague

A novel expandable slotted tube (EST), recently field tested, offers substantial technical and financial benefits for both drilling and completion operations.

The EST has been field-proven for two distinctly separate applications. During drilling operations, the EST, combined with fiber-cement, can maintain hole diameter while eliminating the need for running intermediate casing across troublesome zones. In well completions, the EST can expand a screen against perforated casing to provide effective sand control without gravel packing.

EST description

The EST is essentially a pipe with a series of staggered, overlapping axial slots cut into it. This slot pattern allows the pipe to expand more easily than a solid tube.

The EST can be expanded by several times its initial diameter, depending on the slot dimensions (Fig. 1 [26897 bytes]).

Expansion of the EST's diameter causes a shrinkage of its length. The reduction in the EST's length is small in comparison to the increase in diameter. Generally, an EST expansion by 50% results in less than 1% axial contraction.

Also, a correct combination of slot geometry, cone angle, and cone-base diameter results in the EST actually expanding to a diameter greater than that of the expansion mandrel itself, typically by 10%. A relatively low force is required to expand the pipe.

Applications that have been identified can be categorized as follows:

• Well design improvements-EST expansion directly against the borehole or casing eliminates wasteful annular space and would reduce the telescopic nature of casing and/or completion designs. Drilling costs could be reduced.

• Installation of completion equipment-The EST's expansion could allow installation of ancillary equipment, such as sand screens, directly against the casing or borehole, improving diameter efficiency.

• Open hole (horizontal wells) completion device-The EST could stabilize horizontal well bores by providing additional support to the formation. An EST would also minimize or eliminate the liner-borehole annulus, allowing selective treatment of the formation and improving production log interpretation.

  Despite the large size of the expanded slots, the EST alone might also provide sand control by stressing the formation.

Fig. 2 [29395 bytes] illustrates how an EST can be applied in a horizontal well to install screens for sand control. Typically, for open hole applications, a sand screen could be expanded by about 50% with an EST. In the future for through-tubing applications, higher expansions are planned.

Various hangers can be provided for the EST. One hanger type is removable after expansion of the EST. Other hangers can include packers.

Because the EST shrinks to its original size when pulled from one end, it can be removed from a well with a fishing spear.

Drilling

Fig. 3 [26181 bytes] illustrates the application for isolating a problem zone by lining the well bore below a 95/8-in. casing string with a steel EST reinforced fiber-cement. The object was to maintain the same 81/2-in. drift as in the 95/8-in. casing through this section.

Specific applications were envisaged for high-pressure, high-temperature wells.

The procedure involved installing an unexpanded EST into an underreamed hole, displacing the well with fiber-cement, and expanding the EST before the fiber-cement set. A 103/4-in. casing joint run on the bottom of a 95/8-in. casing string allowed the EST to be tied back to the casing.

Once set, the fiber-cement inside the EST is drilled out leaving a borehole lined with a steel reinforced fiber-cement sheath with the same drift, 81/2 in., as the casing (Fig. 3 [26181 bytes]).

The feasibility of lining a borehole with the EST fiber cement was proven through a series of yard trials. The technique was then tested to prove its practicality in a series of field trials to confirm the ability to place fiber-cement effectively and achieve the desired pressure integrity.

The expansion of the EST downhole was determined by means of an ultrasonic (UBI) log (Fig. 4 [29757 bytes]). The minimum radius, shown on Track 4, is the signal recorded when the caliper sees the inside wall of the EST. The radius recorded confirmed that the EST expanded as designed and uniformly. The maximum radius is obtained when the signal reflects off the cement in any incompletely filled slot. The small difference between the minimum and maximum radii show that the fiber-cement filled all slots.

Table 1 [26294 bytes] summarizes the three field trials conducted.

The laboratory, yard, and field trials of the EST fiber-cement system confirmed that the technology can isolate a borehole against a minimum pressure differential of 1,000 psi in either radial direction.

The EST/fiber-cement system has a wide range of applications such as sloughing shales, depleted or over-pressured reservoir, or even a slimmer casing/well design.

The EST can temporarily isolate problem zones without the requirement of setting intermediate casing. Moreover, problem zones encountered subsequently can be isolated in sequence without compromising well bore diameter.

Once the target has been reached, conventional casing would be cemented inside the EST to provide total isolation of the formation. The EST can be manufactured and/or expanded with different size mandrels to suit any casing size requirements.

Typically, the EST is manufactured in 12-m lengths. During the field trials, the EST joints were welded together, but now an expandable threaded connector is available.

Underreaming can be eliminated if a minor telescopic effect in the well can be tolerated.

Sand control

An EST tool was developed to expand a screen across a short, 9-m interval in a shallow, vertical, cased well in Oman. This was the first step towards developing the EST as a high-expansion sand screen system offering selectivity and sand control without the need for gravel packing in horizontal wells.

The EST screen in this particular application replaced an internal gravel pack. It was also decided, at the risk of reduced productivity if formation sand filled the perforations, not to prepack the perforations to avoid complications in this operation and clean-up.

The first installation and expansion of an EST screen in a producing well was operationally successful (Fig. 5 [24297 bytes]). Moreover, since the well was restored to production in August 1995, the EST's performance has surpassed expectations in terms of sand control and productivity.

The trial confirmed that EST technology will offer benefits to the industry by providing sand control without the need for gravel packing as well as offering selectivity in horizontal wells that is not available with slotted liner or wire-wrapped or prepacked screen completions.

Acknowledgment

The authors thank Shell for its permission to publish this information.

The Authors