Mechanical Failure of a Bottom Hole Assembly During Composite Plug Milling Operations: A Field Case Study

This paper presents a field case study of a mechanical failure that occurred in the bottom-hole assembly (BHA) during composite plug milling after hydraulic fracturing operations. The failure sequence was reconstructed using field hook load and torque records, operational documentation, and inspection of the damaged components recovered from the borehole. The results indicate that the critical condition developed progressively and was associated with increasing resistance to drill string movement, insufficient hole cleaning, and repeated attempts to continue milling and release the partially immobilized assembly. The observed damage pattern, together with the presence of residual cuttings and metallic debris in the borehole, supports the conclusion that the loss of the BHA section at the hydraulic safety sub resulted from the interaction of several adverse operational factors acting simultaneously, particularly the combined action of pull-up force and rotation under deteriorating borehole conditions. A supporting strength assessment of the hydraulic safety sub was used to relate characteristic operating points to the admissible working range of the connector. The study shows that hook load and torque data provide the greatest practical value when interpreted jointly and in their operational context rather than as isolated peak values. The findings support safer planning and execution of plug-milling and stuck-pipe remediation operations in highly deviated wells.