With re-entry drilling systems available for hole sizes as small as 3-7/8", slimhole re-entry horizontal drilling offers operators an opportunity to improve field recovery and revitalize wells suffering from gas or water coning by infill drilling. In particular, the potential for coiled tubing drilling has created significant industry interest over the last 18 months. Offshore where there is no derrick in place, use of a coiled tubing drilling system makes it possible to redrill old wells and perform horizontal re-entries.

To date, coiled tubing re-entry wells have used a conventional rig to cut the window, adding unnecessary costs and time to the program. It was recognized that a coiled tubing conveyed window cutting system will enhance the capabilities of coiled tubing drilling by eliminated the need to cut the window with a conventional rig.

The problems of slimhole window cutting include controlling weight on bit while starting the window, and reducing torque while milling to prevent drillpipe failure.

In the coiled tubing application, rotation is supplied by a downhole motor, which presents additional problems with respect to controlling weight on bit.

A milling system that combines downhole motors with hydraulic thrusters to supply constant weight on bit has been bench-tested and field proven. While the prototype system required three milling runs after setting and orienting the packer, subsequent systems are in testing to reduce the number of milling runs to one.

A description of the applications and requirements of the window cutting system is provided, along with bench test and field test results.

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