The Plug and Abandonment (P&A) requirement stipulates a permanent barrier to be placed for restoring the cap rock during well abandonment. For a single casing, section milling has been successfully implemented and widely used for a number of years. For a dual casing string, this becomes particularly challenging when both casing strings are cemented. Conventional techniques require milling the entire inner casing from the top of cement followed by section milling the outer casing. This could require milling up thousands of feet on of the inner string and is not a cost-efficient solution. The service company has been heavily involved in a number of P&A campaigns, where the requirement was to come up with a solution for dual casing section milling. This paper discusses the design, technology, field runs and best practices developed to overcome this operational and economic challenge and save rig time in P&A operations. The challenge was to design a robust section mill that can drift through the inner casing restriction and expand to a high ratio to mill the outer casing. It was equally critical to manage shock and vibrations during the milling operation, ensure stability and, competitive ROP without incurring tool damage.

To solve the operational and economic challenges, a unique system was developed to reduce the rig time. The system is a combination of the newly engineered high-ratio hydraulic section mill, with a 180% expansion ratio and a precisely oriented hydraulic stabilizer below it. This creates a unique 6-point stabilization system that helps to maintain the dynamic loads and vibrations to a manageable level. The system allows for a dual casing section window in few trips as compared to conventional techniques. In the first run, a window in the inner casing is milled by a section mill. During the second run, the High-Expansion Ratio Section Mill (HRSM) is run through the restriction, and mill the entire casing. A high ratio under reamer can be included in the bottom hole assembly to clean the section and expose the formation prior to the cementing operation. This completes achieving a rock-to-rock barrier in a dual cemented casing application.

The new HRSM system has so far been 100% successful on the five challenging jobs completed in Europe, Asia and the Middle East, resulted in significant rig-time savings along with the added benefits of fewer trips and less swarf at the surface.

Two sizes have been developed and tested for 7"x9-5/8" and 9-5/8"x13-3/8" applications. The success during the early jobs is largely due to the robustness of the mill design, proper pre-job planning, meticulous execution, and implementation of key learnings from in-house and prior field tests.

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