An Engineering Approach to Successful Cement Milling in a 3,777 m Long Interval Using Coiled Tubing in the Mahakam Field

Abstract

Cement milling over long intervals presents significant operational challenges, particularly in highly deviated wells with extended measured depths. In the Peciko Offshore area of the Mahakam Field, a cement milling operation was required across a continuous 3,700 m interval in a well with approximately 70° deviation and 3-1/2″ completion. This scope exceeded typical coiled tubing (CT) milling application limits due to high frictional losses, limited weight-on-bit transfer, torque constraints, extended operating duration, and increased risks of CT buckling and fatigue. This paper presents an engineering-based approach that enabled the successful execution of this operation using coiled tubing. The scope of this study includes detailed pre-job engineering, operational planning, and real-time execution. The methodology comprised comprehensive CT mechanical and hydraulic modeling to define operating envelopes and establish safe limits for axial load, torque, and circulation pressure. Bottomhole assembly (BHA) design was optimized through appropriate selection of milling tools, motor characteristics, and stabilizers to improve milling efficiency while minimizing mechanical risk. Circulation rates and differential pressure were carefully managed to maintain effective milling performance throughout the extended interval. Real-time monitoring and adaptive parameter adjustments were applied to address changing downhole conditions and mitigate risks related to CT fatigue, lock-up, and stuck pipe. The operation was completed successfully, achieving full cement removal across the entire 3,777 m interval without major non-productive time or equipment failure. Milling efficiency, tool integrity, and post-job wellbore conditions met the acceptance criteria, allowing subsequent well-intervention objectives to proceed as planned. The results demonstrate that long-interval cement milling using coiled tubing in highly deviated wells is technically feasible when supported by robust engineering design, disciplined operational control, and real-time decision-making. This paper provides practical insights and lessons learned on extending the application envelope of CT cement milling. The proposed workflow offers a replicable engineering framework for addressing similar long-interval intervention challenges in mature offshore fields under tight operational and mechanical constraints.