The cost of remedial work on marginal gas wells suffering from water unloading problems can be prohibitive, especially in an offshore environment. The expected financial return often does not justify the rig costs associated with pulling and running a new completion, while the formation damage caused by many well-killing methods can reduce the production potential of already marginal wells.
A Coiled Tubing velocity string can often prove a quick and cost-effective method of assisting in water unloading. The ability to work in live well conditions avoids damaging the formation, making it an ideal solution in many cases. However, the limited lifespan of carbon steel strings in corrosive environments calls for a different solution. The next option is often to run and hang off a chrome tubing string with a snubbing unit, which makes running corrosion-resistant tubing in live well conditions possible. However, the higher costs and increased time associated with a snubbing unit reduce its attractiveness.
The unconventional operational procedure of running corrosion-resistant jointed tubing with Coiled Tubing equipment has been used on few occasions to combine the benefits of Coiled Tubing and snubbing interventions. Although generally restricted to relatively short tailpipes, this method has on occasion been extended to running full velocity strings. The limited tensile load capacity of the externally flush threads has limited the length of the string in some cases.
A solution to this problem where a complex string is run in two independent sections has been applied in the field to increase the total velocity string length to 4115m.
The paper discusses the design and execution of the operation where Coiled Tubing and jointed tubing were used as a complex velocity string in order to restore production on a gas well, while retaining the full functionality of the downhole safety valve. Particular attention will be paid to the design of the string, which had to be tailored to remain within the operating envelope of the externally flush thread.