Reliable, low cost coiled tubing window milling technology has become the cornerstone of ARCO Alaska's coiled tubing sidetrack program at Prudhoe Bay. ARCO began to investigate coiled tubing drilling technology in 1989. The program grew into an aggressive field application of this technology, with over thirty wells drilled in the last 2 1/2 years. The program's first window was cut below production tubing with a coiled tubing bottom hole assembly (BHA) in the fall of 1994. To date, 14 wells have been sidetracked using this technique.

The windows are cut off of a specifically formulated cement plug without the use of a whip stock. Reliability, always high, has continued to improve, while the cost and time required continue to come down. ARCO has been able to reduce sidetrack costs by an average of 40% compared to conventional sidetrack methods through the continued evolution of coiled tubing drilling technology, of which window milling is a part. This window milling technique should also have the potential to save time and money in slim hole and conventional drilling applications as well.


When ARCO first conceived of drilling sidetracks with coiled tubing (CT), the goal was to provide a new wellbore at a cost comparable to a large fracture stimulation treatment or a cement squeeze. Achieving that objective focused on leaving the production tubing in place during the drilling operation, thus avoiding the high cost associated with pulling tubing. While coiled tubing units (CTU's) cost less to operate than rigs, the difference is not as great as one might think, particularly when operating in Arctic environments. The significant economic advantage comes from leaving the tubing in the ground.

ARCO began investigating the feasibility of coiled tubing drilling (CTD) in the fall of 1989. The program continued to gather momentum over the following years, culminating in continuous CTD operations since the summer of 1994.

The program's first CTD wells were drilled either from the few open hole completions, or out the end of liners intentionally left in the production zone during previous drilling operations. However, the economic viability of the project depended on sidetracking through the existing liner below the production tubing.

Testing the techniques for milling off whipstocks began as early as 1991, and ARCO encouraged several tool suppliers to develop whipstock systems capable of setting in 7" liner below 4.5" production tubing. Initial results were encouraging, but building tools with the reliability needed for field application took more time and effort than originally anticipated. Window milling became critical to sustaining continuous CTD operations as potential open hole sidetracks were depleted from the candidate population. An alternate method was needed to get out of the liner until suitable whipstock systems were available.

Milling off a cement plug was originally conceived as a stop-gap measure. What was initially expected to be a risky technique barely capable of bridging the program through final whipstock development matured into a reliable, robust, and economic means of exiting the old wellbore.

Field Results

To date, 78% of the cement mill-out procedures have been successful, producing 14 usable windows in 18 attempts. The definition of failure used here is somewhat conservative. Although four window operations are considered failures, the mill failed to drill completely outside the liner only once. Three failures occurred in the first four field attempts, and the last occurred during the first attempt to mill out of 9 5/8" casing with a 3.8" bit. The cause of each of these failures and other problems will be discussed in more detail later in the paper. There have been eight successful windows cut since the last failure, making the success rate for the last half of the program 100%. In addition, the time needed for a milling operation continues to decrease as our experience level increases.

The majority of the 14 windows were cut with 3 3/4" (or 3.8") diamond speed mills turned by 2 7/8" motors.

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