Sidetracking a cased wellbore presents numerous challenges because the operators have to plan ahead to select the sidetracking depth and then ensure that all the objectives are met from a well authority for expenditure and geological target perspective. The quality of the geological target window is of great concern to operators because it ensures that the subsequent bottomhole assembly (BHA) will pass through the window without any problems. Sidetracking the wellbore when the milling assembly has to cut two strings of casing is an additional unique challenge. The centralizer and casing collar locator along the length of the wellbore at and in the region of the kickoff point are significant because they can be additional risks, which can lead to costly multiple trips to ensure that the window is in good-quality form. An operator was faced with potential geological losses at the kickoff point in a wellbore while attempting to sidetrack an existing wellbore containing 9.625-in., 43.5-lbm/ft L80 buttress thread casing, and 13.375-in., 68-lbm/ft K55 buttress thread casing, dual strings at the kickoff point.

The BHA for this challenging application was modeled using a finite-element analysis (FEA)-based modeling system to select the optimum BHA to sidetrack the wellbore with the least amount of vibrations. The feasibility of using a bi-mill vs. tri-mill BHA was evaluated. As a result, a parameter road map, taking into consideration the dimensions of the whipstock slide and mill position during the milling operations, was finalized. The placement of the centralizers and casing collars along the length of the casing at the kickoff point was considered. The exact locations where the lead mill would initiate the cut at each casing string were analyzed to determine the whipstock setting depths.

A corrosion and collar locator tool was used to identify the collar location along the 13.375-in. casing because standard casing collar locator logging tools were not be able to identify the location of the casing collars along the length of the 13.375-in. casing string.

Whipstock simulation software was used to check the bending moments and stresses for the pass-through BHA because the dogleg through the window can create additional issues and challenges. The program calculates the dogleg severity for a liner or BHA pass-through in addition to forces and stresses on the liner or BHA. The total quantity of cuttings that would be generated from the milling operation while cutting two strings of casing was also analyzed.

This methodical planning resulted in a successful dual-casing exit operation. The success is the result of a proactive planned initiative to mitigate BHA shock loading, which included real-time monitoring using a predictive compressive-strength analysis system. The proactive plan also increased confidence in the FEA-based modeling system's ability to accurately identify the root cause of damaging vibrations while sidetracking through carbonates in a dual-casing Kuwait well.

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