A major operating company (operator) demonstrated interest in the development of high-strength (>125 ksi yield) coiled tubing (HST-CT) for use in high-pressure/high-temperature (HP/HT) fields in the central North Sea. The operator's objective was to perforate underbalanced (in base oil) with coiled tubing (CT) using up to 1,700 ft of guns in one run. The combination of the low hydrostatic pressure of the perforation fluid and high bottomhole-pressure results in high wellhead pressures. Therefore, the CT had to be strong enough to avoid collapsing while pulling the weight of the bottomhole assembly (BHA) and the string inhole from the working depth of a live well. Because of this, the CT needed to be carefully designed to minimize its weight without compromising its strength. Computer simulations were run for string designs made from the currently available materials but none were found suitable for the operator's objective, so the operator pursued the development of HST-CT.

A major CT manufacturer working in conjunction with a material supplier developed the product and, allied with a service company, proved the product suited the needs defined by the operator.

It was decided to run the HST-CT in a newly drilled well in the same area but in a shallower reservoir with sub HP/HT conditions as a test bed to determine any potential issues with the use of HST-CT in reverse-deployment perforating operations. This job was performed with a shorter BHA and a lower wellhead pressure than true HP/HT conditions. Some of the operational testing normally conducted in Houston, Texas was performed in Aberdeen, Scotland because of job-time restrictions.

The test results, operational details of the CT job, and observations and conclusions derived from the use of the HST-CT developed are discussed in this paper.

This project involved the first offshore application and the first commercial application of HST-CT worldwide. The importance of this project is highlighted by the increased number of unconventional reservoirs being perforated, thus increasing the need and demand for stronger CT.

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