Slimhole re-entry horizontal drilling offers operators an opportunity to improve field recovery by infill drilling, and revitalize wells suffering from gas or water coning. Re-entry drilling systems are available for hole sizes down to 3-7/8". The potential for coiled tubing drilling has created significant industry interest over the last 18 months. In offshore applications 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.

Coiled tubing re-entry wells completed so far have used a conventional rig to cut the window. This adds unnecessary costs and time to the program. A coiled tubing conveyed window cutting system will dramatically reduce the overall wells costs, and thereby expand the market for coiled tubing re-entries.

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 using downhole motors with hydraulic thrusters to supply constant weight on bit has been bench-tested and field proven. The prototype system required three milling runs after setting and orienting the packer. Subsequent systems will be tested 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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