Industry's First Sidetrack in 12-1/4-in Casing Enables Operator to Re-Establish Production in GOM


Tom Emelander


OTC - Offshore Technology Conference

Publication Date

August 9, 2021


Offshore Technology Conference, August 16–19, 2021

Paper ID



In a Gulf of Mexico (GOM) ultradeepwater well, liner integrity issues forced an operator to consider milling a conventional casing exit to sidetrack as deep as possible to re-establish production. Milling a window in 12 -1/4-in. heavy wall casing above the liner hanger had never been achieved before because of the thickness and grade of casing. A successful installation would require significant preplanning and testing to prove capability before real-world application.

The service provider recommended an off-the-shelf solution to accommodate a 12 -1/4-in. casing exit. It was determined that the best fit for the application would be the standard equipment used for exits out of 10 -3/4-in. casing, given the similar internal diameters (IDs). Despite never having performed an installation in this casing size, the provider had a successful run history for exits in heavy weight casing strings. Job challenges included avoiding cutting a casing connection, managing swarf, milling through a centralizer, and achieving a low dogleg for production packers. Additional challenges included torque limitations, mill gauge, and the limestone formation.

An 8-in. outside diameter (OD) system with mechanical anchor and 9 -7/8-in. OD mills was sent to a test well designed to replicate the target section of the offshore well. Based on determinations made in the planning phase, milling of the window and rathole would be staged in two trips. Additionally, a replica drilling bottom hole assembly (BHA), 8 -5/8-in. casing, and a replica production packer would need to pass through the window to ensure both window quality and low enough dogleg. Dogleg data was acquired through multiple logging runs during the 10-day operation. The installation went as planned, along with an additional custom window elongation run to decrease the dogleg severity to approximately 4.5°/100 -ft (30m). Having successfully validated the equipment for the application, the operator and servicer prover were comfortable moving onto the GOM well. Considering the test results, the team planned to mill the window and rathole in one trip. They achieved the 22 -ft long window and 15 -ft rathole in one run that lasted 26 hours. This installation is the first sidetrack conducted with a whipstock in 12 -1/4-in. casing.

This paper shows that a safe, reliable casing exit installation is possible in difficult applications, such as uniquely heavy wall casing, even though it may previously have been considered impossible. This successful application provides the industry with contingency options in similar scenarios.