In order to optimize production from existing wells, it is often desirable to re-enter the wells and, after milling a window in the well casing, sidetrack the well into better reservoir rock. Combining these re-entry techniques with the industry's growing interest in underbalanced and coiled tubing drilling necessitated the development of an improved window milling system for use on coiled tubing.

An evaluation was completed on the conventional window milling tools available. The evaluation revealed a number of obstacles for coiled tubing drilling applications. It was decided that a fresh approach to window milling was required and the development of tools to be run specifically on coiled tubing was initiated.

This paper will discuss the successful development of coiled tubing window milling tools for use in 114.3 mm and 139.7 mm casing. It will also discuss bench tests and field trials of the window milling system which demonstrate the viability of the tools and methodology.


Re-entering existing wells to drill a side track out of the original wellbore is often an effective technique for revitalizing older producing wells and fields. Frequently, the side-tracked well may be drilled in an underbalanced condition to minimize the impact of the drilling operation on partially depleted or sensitive reservoirs. The use of coiled tubing, rather than conventional drilling or servicing rigs to drill the underbalanced section of the re-entry, has several advantages which have been well documented in other papers.1,2,3

Even where it is desirable to use a coiled tubing unit to drill the underbalanced section, however, it has been common practice to use a conventional rig to set a whipstock and mill a window in the well casing for the reentry. The potential to use the coiled tubing drilling rig to do all of the workover operations on a re-entry well is quite attractive, particularly where the remoteness of the well site would require a large expenditure to move on a conventional service rig solely to cut the window.

A common approach to window milling with a conventional rotary drilling or servicing rig is to first set a whipstock anchor packer, orient and land the whipstock on the packer and mill a window by running a sequence of casing mills. The milling sequence will begin with a tapered starter mill which kicks off the whipstock to initiate an opening in the casing. The starter mill is then followed with a window opening mill and possibly a reamer mill to polish the window opening. This process includes up to four runs in and out of the well.

The design of the CT window milling system included plans to restrict the number of trips into the well. On the first trip, the whipstock is carried and seated in the packer with the starter mill, on the second trip, a window opening mill is run in combination with a reaming mill.

Another variable that differentiates rotary and coiled tubing drilling is the torque available at the bit in the casing sizes of 114.3 and 139.7 mm casings.

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