Coiled Tubing (CT) is a commonly used well intervention technique for sand/debris cleanout, milling, fishing, cementing, and gas lifting applications. Performing cement plug operations with CT in high-pressure live wells can present a relatively high level of risk of getting stuck, improper placement of cement plugs, and poor quality cement plugs. In this scenario, an offshore well in Vietnam was left with CT held only on the surface by the Blow Out Preventers (BOP) after getting stuck downhole more than a year previously. Retrieval of the CT was required without any downhole barrier in place and circumstances that made the conventional killing of the well difficult. This paper will discuss the equipment necessary, factors to consider in the job design, sealant design with lab testing, and describe the technique used to safely retrieve the fish from the well.
During the design phase of the solution, risk assessments were carried out to cover various scenarios such as:
Poor condition of one or more of the CT string, BOP, dual ball kelly cock valve, double flapper check valve.
Inability of the Organic Crosslinked Polymer sealant to hold well pressure.
Difficulty in latching the surface CT stump, and the potential requirement for freezing to establish a surface barrier.
Along with the specially designed sealant treatment, multiple yard tests for dressing the sheared CT and latching were performed to represent actual conditions during the operation and allow risk mitigation plans to be put in place.
Firstly, the barrier verification process was performed to monitor any well pressure build-up from the downhole/surface pressure reading to confirm a barrier was in place, allowing surface equipment to be rigged up. After this was confirmed, the equipment was rigged up, and a fluid circulation test followed by the inflow test of the double flapper check valve in the bottom hole assembly was performed to verify CT integrity. The first of the organic crosslinked polymer was then pumped to plug inside the CT before electric line (E-line) was run into the CT to cut at the free-point above the stuck location. The second organic crosslinked polymer was then placed to plug between the CT and annulus, forming a barrier allowing the makeup of the spoolable connector. Finally, retrieval of the 1,700 meters of CT string took place without any loss of well control.
Retrieving the CT from this high pressure well presented a lot of challenges. Achieving a safe and successful operation showed that with proper planning, design, and risk mitigation plans, a potential well control situation can be prevented with the well being secured and successfully returned to production. This paper can now serve as a guideline for future operations with similar circumstances requiring retrieval of stuck CT from high-pressure wells where it has been held on surface for an extended period of time.