Pioneering Multistage Fracturing in Indonesia: Successful Application of Underbalanced Bridgeplug Milling for Formation Damage Mitigation in Subnormal Formation Well

Summary The primary objective is to successfully implement underbalance bridge plug milling in subnormal formation to prevent formation damage during the post-multistage fracturing process. This paper focuses on the challenges and execution of underbalance milling operations in the Semeru-01 well, particularly addressing the complexities associated with subnormal formation pressures. The Semeru-01 Multistage Fracturing well targeted Rinjani Sand, a tight sandstone formation, with an average formation pressure equivalent to only 4.1 ppg. Under these conditions, achieving underbalance with a single-phase liquid was impractical due to relatively high hydrostatic pressure. To reduce hydrostatic pressure, a precise composition of two-phase mixture of nitrogen and linear gel was employed. This approach was critical for efficient cutting transport, maintaining underbalance, and an optimal performance of the downhole motor. The specialized nature of the two-phase fluid required a carefully selected downhole motor designed for two-phase environments. The underbalanced milling operation successfully removed all 9 (nine) bridge plugs separating 8 (eight) fracturing stages. Hydrocarbon returns during circulation in each stage confirmed the establishment of underbalance conditions. Initial milling stages required meticulous adjustments to parameters such as weight on bit (WOB) and the nitrogen-liquid mixture ratio to optimize downhole motor torque and achieve efficient milling progress. For the first plug, liquid rates varied from 1.28 to 1.8 bpm at nitrogen rates of 300-500 scfm to determine the optimal combination for minimizing downhole liquid volume while maximizing motor torque and milling efficiency. The successful parameters from the first plug were used as a baseline for subsequent milling operations, although further adjustments were necessary for some plugs due to varying conditions. After milling the fourth plug, a bit replacement was required due to excessive wear, indicating that the bit was at approximately at 50% and no more bit replacement until all plugs were milled. The successful removal of all the bridge plugs in the Semeru-01 through underbalanced milling was highly contributed in lowering formation damage in the fracturing zones and also directly contributed to the ~500 BOPD initial production results. This paper enriches the discussion of best practices, experiences and challenges of performing bridge plugs milling in underbalance condition especially for subnormal formation wells requiring a two-phase milling fluid.