Integrated Low-Weight High-Temperature Water-Based Mud Solution for Complex Carbonate-Clastic-Basement Drilling with Total Loss Risk: Field Results and Lessons from an Exploration Well in Offshore East Java

<jats:title>Abstract</jats:title> <jats:p>The drilling-fluid engineering approach incorporated a comprehensive thermal-stability evaluation program, including hot-roll aging, rheological degradation monitoring, filtration-performance assessment, and contingency planning under simulated downhole environments. Operational execution emphasized proactive fluid conditioning, real-time hydraulic optimization, and continuous surveillance of equivalent circulating density (ECD), low-gravity solids, and gas behavior to maintain drilling efficiency and well integrity throughout the interval. The integration of engineered bridging materials and thermally resilient rheology modifiers enabled the HTWBM system to maintain suspension capability and filtration control despite prolonged static exposure during formation-evaluation operations. The successful implementation demonstrated that appropriately engineered HTWBM systems can provide a technically and environmentally viable alternative to synthetic oil-based mud systems for HP/HT exploration drilling campaigns in loss-prone carbonate environments.</jats:p> <jats:p>This paper presents the successful application of a low-weight High-Temperature Water-Based Mud (HTWBM) system to drill 8-1/2″ hole section with karstified carbonate, clastic, and basement formations in the exploration well which is located in Offshore East Java, Indonesia. The objective was to maintain wellbore stability, minimize formation damage, and mitigate the extreme risk of total fluid losses in the carbonate, all under high bottomhole temperatures exceeding 300°F.</jats:p> <jats:p>A low-weight HTWBM system was engineered with advanced high-temperature polymers, bridging agents, and optimized inhibition properties to address the distinct challenges of each formation. Mud properties were dynamically monitored as drilling progressed: enhanced inhibition &amp; thermally stable rheology with high-temperature viscosifiers as well as advanced bridging &amp; filtration control for the carbonate to minimize losses. Comprehensive contingency planning enabled seamless transition to Pressurized Mud Cap Drilling (PMCD) if uncontrollable losses occurred.</jats:p> <jats:p>The HTWBM system maintained stable properties throughout the 8-1/2″ section, with mud weight starting from 9.5 ppg. This was critical in mitigating severe to total loss events in the carbonate, preserving borehole integrity in the clastic interval, and ensuring operational continuity in the basement. No non-productive time was attributed to mud-related issues, and the section was drilled to an open hole length of approximately 580 meter. The system demonstrated excellent cuttings transport and effective hole cleaning, as validated by hydraulic simulations and field returns. The operation confirmed that a single, environmentally responsible water-based mud system can address the combined challenges of carbonate, clastic, and basement drilling at high temperatures and extreme loss risk. The methodologies and lessons learned from this well establish a new benchmark for future developments in similarly complex and high-risk drilling environments worldwide.</jats:p> <jats:p>This is the first documented field case of a low-weight HTWBM in Indonesia, enabling safe, efficient drilling through a high-risk carbonate, clastic, and basement sequence in one interval at temperatures exceeding 300°F. The approach sets a new industry benchmark for environmentally responsible, technically robust drilling fluid solutions in complex geological settings. The methodologies and lessons learned from this exploration well are directly transferable to similar high-risk wells worldwide, supporting the industry's transition toward sustainable drilling practices.</jats:p>