Schlumberger

Case Study: First North Sea One-Trip, Dual-String Exit for Thru-Tubing Sidetrack

Full-gauge, window and 9.5-ft rathole completed in 8 hours

Challenge: Mill a full-gauge window through 4 1/2-in tubing and 7-in liner, and drill a rathole to allow entry of a directional sidetracking assembly.

Solution: Run a 4 1/2-in TrackMaster TT thru-tubing whipstock system and 3.8-in OD bi-mill.

Result: Completing the window and 9.5-ft rathole in 8 h marked the North Sea’s first successful one-trip dual-string exit through production tubing.

Sidetrack planned through production tubing and liner

An operator decided to sidetrack a mature well in the North Sea to access additional reserves. Thru-tubing rotary drilling (TTRD) technology was selected as a cost-effective alternative to pulling the completion tubing. A thru-tubing whipstock system was required to create the exit through 4 1/2-in, 12.6-lbm/ft tubing cemented inside 7-in, 29-lbm/ft liner. The window had to enable a 3.7-in X 4.125-in bicenter bit to be run on a 3 1/8-in OD directional assembly with a positive displacement motor (PDM) set at 1.8° bend angle.

TrackMaster TT whipstock system and bi-mill used to create exit

A 4 1/2-in TrackMaster TT thru-tubing whipstock system, with an OD of 3.62 in, is designed to pass through the tubing ID. Running in with the whipstock, orienting and setting it, shearing the milling assembly from the whipstock, milling the window, and drilling the rathole can all be accomplished in a single trip. 3.8-in OD bi-mill was used to create the window. The TrackMaster TT whipstock system enables a longer casing window to facilitate subsequent entry of drilling assemblies.

First North Sea dual-string exit accomplished in single trip

After milling a full-gauge in 1.5 h, a 9.5-ft rathole was drilled in 6.5 h. This enabled the sidetracking BHA to enter the rathole and drill a 9,200 ft sidetrack with an inclination of 20.67°. This operation marked the North Sea’s first successful one-trip, dual-string exit through production tubing.


Download: First North Sea One-Trip, Dual-String Exit for Thru-Tubing Sidetrack (0.64 MB PDF)

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