MORE COLLAPSE TESTS ADD TO COILED TUBING APPLICATIONS

Eric J. Walker
BP Exploration (Alaska) Inc.
Anchorage

Doug Costall
Canadian Fracmaster Ltd.
Anchorage

The collapse limits of thicker-walled coiled tubing have been determined to ensure safe and successful workover operations.

Prudhoe Bay has been using 1.75-in. OD coiled tubing for 2 years. When BP Exploration (Alaska) Inc. initially started using this larger size coil, collapse tests were run on 0.109-in. wall thickness coil.' These tests provided a base curve by which much work has been performed in the western operating area of the Prudhoe Bay Unit.

However, use of 1.75-in. coiled tubing has been expanded to include wall thickness of 0.125, 0.134, and 0.156-in. Except for theoretical calculations, no data were available to ensure that we would know the collapsed limitations for these sizes. To fill in this gap, further collapse testing has been done.

Additionally, 2-in. OD coiled tubing was tested and is now available. Unfortunately, most of the samples provided for testing were curved to the point of uselessness for test purposes, but seven tests were completed.

BACKGROUND

Currently, seven 1.75-in. coiled tubing units are in use at Prudhoe Bay. BP has three of these on contract and is presently averaging three to four jobs per day, Work being done with these units includes:

• Cement squeezes

• Underreaming

• High-pressure well cleanouts at surface pressure of 10,000 psig

• Inflatable packer stimulations

• Well cleanouts where all material is reversed-out or brought to surface up the coiled tubing string.

Because much of our work involves reversing-out fill or frac sand from the well bore, a coiled tubings collapse pressure is critical to a safe and successful operation. Under the old guidelines, reverse-out operations were limited to 1,500 psig across the coiled tubing string.

Collapse testing performed in 19891 showed this limit could be safely increased to 3,500 psig, depending upon tension and coiled tubing condition. Since then, several hundred field operations have been performed at this higher limit with no problems.

However, these past collapse tests were done across a limited tension span and did not include other wallthickness coiled tubing sizes.

To fill in the gap and get a complete curve covering all the operating ranges, further testing has been done.

These tests show clearly that with undamaged coil one can safely operate within the limits defined by the manufacturer-supplied curves of collapse pressure vs. tension on the coil. These curves are calculated using the distortion energy theory (DET) and have been shown to be reliable for the coil tested.

Field use over 2 years support these data for 0.109-in. wall thickness coiled tubing.

Other coiled tubing sizes have not been extensively used by BP, but with increased pressure demands required by high pressure cleanouts, use of 0.125, 0.134, and 0.156-in. wall thicknesses is expected to increase. With the completion of these tests, problems with collapse are not expected.

TEST PROCEDURE

Two test procedures were followed, one using a threaded bullplug against which tension was applied to the coiled tubing sample, and one using a clamp applied to the outside of the coil against which tension was applied.

The reason for the different procedures was due to doubts expressed in the literature 2 as to whether or not rigid end seals may artificially raise collapse pressure. By using both procedures such doubt was eliminated.

TEST 1

The procedure employed for the first round of testing used a threaded bullplug in one end of the coiled tubing sample. This plug was welded to one end of the sample to allow the sample to be screwed to the base plate of the tester (Fig. la).

Once the sample was screwed to the base plate, the top end cap (with polypack sea[ assembly) was fitted around the free end of the sample and bolted to the test chamber. A 60-ton hydraulic jack was placed on the top of the end cap and a template welded to the sample.

Pressure was then applied to the outside of the coil and tension put to the sample via the jack. Tension was rapidly increased until the coil sample started stretching or collapse occurred. Stretch was noticed by a leveling off of applied tension and a decrease in outside coil pressure within the test chamber. Collapse was heard.

TEST 2

To determine if the procedure in Test 1 artificially raised a sample's collapse pressure, the procedure in Test 2 did away with the threaded bullplug. Instead, two end caps were made, each with poly-pack seal assemblies installed.

The coiled tubing sample was then fitted through both end caps and a clamp applied to one of the free ends of the coil (Fig. lb) On the other end, the jack was placed and the template welded as in Test 1. Once again, pressure was applied to the outside of the sample using water and axial tension applied with the hydraulic jack until stretch or collapse occurred.

TEST RESULTS

Results of the two test procedures are tabulated in Table 1 (1.75 in., 70,000 lb) and Table 2 (2.00 in., 70,000 lb), and plotted in Fig. 2. Each different coil OD and wall thickness is plotted separately against the theoretical curve as calculated using the distortion energy theory (abbreviated DET on the plots).

As is evident after viewing the plots, measured collapse data in all cases proved to be better than the calculated value. This was expected, as average yield strength and wall thickness is generally higher than published minimum values.

Data showed no difference between samples with the threaded bullplug and those without. This suggests that the test chamber and method employed to apply axial tension to the coil samples does not artificially raise collapse pressure.

Another point was the outside diameter of the coil samples. For the 1.75-in. coiled tubing, the OD was within close tolerance, generally ranging between 1.748 and 1.755 in. Collapse, of course, always occurred across the smallest-diameter section, which in most cases was across the longitudinal seam weld.

For the seven 2-in. OD samples, five gave results that fit a common curve. These were samples with ODs ranging from 1.97 to 2.04 in.

However, the remaining two samples showed severe drops in collapse resistance (Table 2 and Fig. 2e). These two had ODs of 1.95-2.06 in., not much different from the first five, but apparently enough to show a difference. These results verify that ovality can significantly decrease collapse resistance.

Other results are:

• For 1.75 and 2-in. OD coiled tubing, calculated curves supplied by the manufacturer, for collapse, can be safely used for field operation and job design.

• Use of a safety factor is encouraged, BP uses a 40% safety factor. This allows for unseen damage, including slight ovality changes which occur with use and other surface defects (pits, gouges, etc.) which also occur with use.

• Even small amounts of ovality will lead to severe decreases in collapse resistance.

• Previous test results' on collapse of 1.75-in. OD coiled tubing fit well with these recent tests. All tests show that the distortion energy theory adequately predicts collapse and illustrates that damage to the coil severely affects these predictions. In other words, if damaged pipe is not cut out, you risk failure while in the well.

ACKNOWLEDGMENTS

Thanks to Fracmaster for use of its facility and personnel in Calgary, and to Quality Tubing for the 70,000 psi coiled tubing samples. The techniques and conclusions expressed are those of the authoring companies. They are not necessarily shared by the other Prudhoe Bay Unit working interest owners.

REFERENCES

1. Walker, E.J., and Mason, C.M., "Collapse tests expand coiled tubing uses," OGJ Mar. 5, 1990, pp. 56-60.

2. Fowler, Klementich, and Chappell, "Analysis and Testing of Factors Affecting Collapse Performance of Casing," Journal of Energy Resources Technology, December 1983.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.