The advent of multi-zone stimulation techniques has created an increasing need for zonal isolation devices, such as composite bridge and frac plugs. Bringing the well on to production requires that these devices be removed from the wellbore effectively and in a timely fashion. Milling composite frac plugs (CFP) using coiled tubing (CT) and positive displacement motors (PDM) is one of the most common and effective methods in the industry. Where formation properties allow, overbalanced CFP milling is routinely performed with high degrees of success. However, underbalanced milling operations that use nitrogen as a circulation aid pose additional challenges. Further, efforts to optimize the technique are often limited by the numerous parameters involved which reduce the repeatability of successful results.

This paper presents the lessons learned from over sixty underbalanced CFP milling operations performed in the Piceance Basin. The results shown are uniquely relevant due to the fact that the operations involved in this study were performed within the same field, using the same type and size of PDM, mill, CFP, and CT. Very rarely can all these variables be kept as constants. Additionally, the same core field personnel were used over a one year period. This elevated level of consistency allows the author to focus on key controllable CT variables of the operation. The paper will review the optimization and results of: two-phase flow rates, PDM stalling, milling profiles and optimum wellhead pressures.

Reviewing the field data has resulted in improved operational efficiencies by empirically developing two concepts: the "milling profile" and the "optimum milling wellhead pressure". These improvements will be discussed in the paper.

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