The oil and gas field industries continually challenge the limitations of coiled tubing (CT) equipment. With an increasing amount of high-pressure work developing for CT, validating components for this work is a challenge. One component in particular, the stripper element, is of interest. Many manufacturers of tubing strippers rate their components for working pressures from 10,000 to 20,000 psi, but limited test data is available for element wear or resiliency at these pressures.
This paper presents results from full-scale testing of a stripper/packer element at pressures as great as 15,000 psi. Various elements were tested for longevity, internal material integrity, and safety at high to ultrahigh pressures. As a result of the evaluation,
the stripper/packer was validated to 15,000 psi,
a suitable packing element was developed and tested, and
operational guidelines for high-pressure work were developed for the field.
Stuffing boxes are used at the wellhead with other well control equipment to provide a seal for high downhole pressures. Specifically, it provides a dynamic seal so that various operations can be performed below the wellhead while safety is maintained above it.
The packer or sealing element used in a stuffing box is typically made of a polymeric material that is deformed to seal against the tubing that is being injected or withdrawn from the wellbore.
Safety is the primary reason for using a stuffing box. Without the stuffing box, CT work could not be done safely, even at low pressures. Operators are encountering increasing wellhead pressures using CT, which has created a significant safety issue. The stuffing box is the only active sealing point on the wellhead during a CT operation. If the stuffing box or any of its components catastrophically fail during the job, a blowout is inevitable. Assuming that the mechanical components of the stuffing box are adequate for the job, only the sealing elements can fail.
If a sealing element fails, the wellhead pressure must be shut off to the leaking stuffing box. Once the wellhead pressure has been isolated below the stuffing box, the sealing elements must be changed manually. Work must be performed on the stuffing box on top of the wellhead with full pressure on the well. This maintenance causes a potential safety hazard and slows the workover of the well. Therefore, a sealing element is needed that will not wear out before the job has been completed or blow out if it fails.
Standard sealing elements use a relatively soft polyurethane-based material that forms around the tubing to seal the wellbore pressure. Although these elements provide an effective seal around the CT at low to medium pressures, they cannot withstand high wellhead pressures. The tendency of these elements to wear out or blow out makes them unsuitable for high-pressure work.
Until recently, there has not been a demand to seal high-pressure wells (wellhead pressure > 7,500 psi) during CT operations. CT jobs were generally performed below 7,500 psi, and more often below 5,000 psi. With increasing demands for high-pressure work, a suitable stripper/packer element must be tested and proven.
During the search for a suitable sealing element for high-pressure wells, two critical variables were examined; the method in which the sealing element is forced around the tubing, and the material of which the sealing element is constructed.