First Coiled Tubing Operation in Alaska Requiring 15,000-psi-Rated Well Control Equipment: A Case History

Abstract

Drilling activity in remote, complex environments has increased in Alaska as operators seek to combat falling production in existing wells and shift to commercialization of natural gas and condensates. The completion of these wells often requires coiled tubing (CT) intervention, whereby CT is used for various applications, including wellbore cleanout, milling, fishing, and acidizing operations. To complete multiple high-pressure wells in a remote field, an operator required a CT contingency to shift formation isolation valves. A new collaborative approach was implemented in which the operator and CT service provider closely worked together on the technical job design from the onset of the project to optimize planning and execution. The planned intervention was part of a large project in which a single company provided most of the services. This allowed the CT service provider to work closely with the operator and third-party providers, such as the fluid supplier and completion equipment supplier, to complete key technical design elements, including CT string design, fluid design, and downhole tools selection. In this way, an integrated, fit-for-purpose solution was delivered to the operator. Many ke y challenges were associated with this intervention. The biggest challenge was the absence of previous such experience in a well in Alaska where maximum allowable surface pressure (MASP) exceeded 8,500 psi. The intervention would require well control equipment and other pieces of treating equipment and downhole tools rated for 15,000 psi that were not readily available in Alaska. In addition to the well's high MASP (8,564 psi), other key challenges included being ready to perform a CT milling operation of a formation isolation valve in large casing (7⅝ in.) in an environment with 30-ppm H2S and 4.55% CO2 where ambient temperatures could drop as low as−50° F. A 2-in. CT string with a length of 19,000 ft was designed to provide sufficient weight on bit and overpull to complete all required contingency CT operations. A fluid system was designed to not only control the high pressure in the well but also be pumped through the CT string at circulating pressures that did not exceed the limits of the pipe. Furthermore, a test was completed prior to the mobilization of equipment to location to determine the optimal design for milling the formation isolation valve with CT. This paper presents the job design and preparation processes completed for the first planned CT intervention contingency in Alaska, in addition to lessons learned that can be applied to future high-pressure CT operations requiring well control equipment rated to 15,000 psi.