Since 1991, BP Exploration (Alaska) has installed several coiled tubing (CT) completion designs on the North Slope. From the first remedial completion to the most recent primary completion, valuable lessons have been learned. The purpose of this paper is to share these lessons so that other CT completion programs may benefit. A brief review of the program evolution is provided followed by a summary of the main completion types run. The remaining focus is on the lessons learned in the areas of surface equipment, downhole completion tools, design and planning, personnel, and safety.
When beginning a new program such as CT completions, there are two primary options for obtaining the technology needed:
transfer knowledge from the industry if available, or
develop the technology independently, learning from performance along the way.
Since little industry experience was available when North Slope completions began, operators largely pursued the latter option. The intent of this paper is to transfer to others the knowledge that has been gained on North Slope CT completions.
Coiled Tubing is used for numerous completion applications. These applications range from patching an existing completion to using CT as initial completion tubulars Installing a CT completion in a well is not a trivial task. Generally the CT used in these completions is of larger diameter and wall thickness than the standard CT used in routine well service operations. This requires thorough evaluation of the equipment and components needed, which in turn impacts completion design, procedures, safety, arid training requirements.
CT completion operations by BP Exploration (Alaska) in the Western Operating Area of the Prudhoe Bay field have gone through several stages of evolution. The early uses of CT as a completion tool were predominantly in a reactive mode. Most applications were on wells that developed tubular integrity problems and could not justify the expense of a rig work over (RWO) or fell low in the rig priority level. CT was used to straddle the bad section of tubing allowing the well to be put back on production. In this stage, CT sizes ranged from 1- 1/2" to 2-3/8".
The next stage of development was to straddle the entire tubing string and retain the ability to gas lift the well by using 2-3/8" CT and spoolable gas lift valves (Fig. 1). The bottom section of the CT string was isolated with a seal assembly stabbed into a polished bore receptacle (PBR) that was hung inside the existing tailpipe (Fig. 2). The top section of the CT string was hung from a CT hanger with energized seals.
With the successful use of CT in these early applications, a switch to a proactive mode was made by using CT in initial completions. The first initial completions utilized 3-1/2" CT with standard 3-1/2" gas lift mandrels (GLMs) and sub-surface safety valves (SSSV). These completions were installed in the Endicott field, 18 miles east of Prudhoe Bay. The next group of completions was at the Milne Point field, 30 miles west of Prudhoe Bay, where 2-7/8" CT was used to run electric submersible pumps (ESPs).
During these development stages of the CT completion program the goal was to start as simple as possible arid increase the complexity as experience increased. This allowed lessons from previous operations to be captured and transferred to future job designs.
Remedial CT Completions. Remedial CT completions in the Prudhoe Bay Field consist of:
Tubing straddles to cover corroded or damaged pipe,
Liner perforation straddles for gas or water shut-offs (GSO, WSO),