The oilfield industry is facing more challenges as wells have become more complex. Companies are designing casing programs that utilize larger casing sizes and setting them deeper than ever before. For cased hole sidetracking projects, this means sidetracks with increasing levels of difficulty such as thicker casing walls, double casing exits, and extended ratholes. These new challenges also represent issues involving premature cutting structure damage, ringouts, coreouts, twistoff, and undergauge windows and ratholes that require unplanned or additional runs—both of which result in nonproductive time to the operator. The ability to analyze and predict the performance of milling tools is critical, and reliability performance curves provide value to the remedial operation planning phase of the sidetrack project. These curves can also be used as a decision-making tool while on a job.

These are the first known reliability and performance curves for sidetrack milling tools. The curves are built using 10 years of historical data, including mill type, size, and casing grade. The parameters were selected based on acceptable gauge criteria, among others. Up to six casing grades can be analyzed and compared for median hours of milling, average ROP, percentage of cutting structure wear, and milling tool reliability.

These curves establish a baseline of performance for mills in their current and future designed state. Any future design changes can then be referenced against these curves to monitor improvement of the tools over time. Engineers may also utilize the curves to conduct operational risk assessments and estimate the performance of mills, especially in challenging environments such as hard formations or thick or double casing string applications. Educated decisions can then be made to prevent or reduce unexpected time loss.

Reliability curves are being generated to improve planning for sidetracking operations as well as making informed decisions on when to pull a mill and replace with the backup. This helps to maintain a full-gauge window or rathole, avoiding tool failures, unplanned trips, and nonproductive time. This decision-making tool can save valuable rig time, which is increasingly important with larger spread rates and deeper wells.

A real scenario was analyzed from a previous job and demonstrated that using the curves as a decisionmaking tool could have saved 30% of the actual downtime.

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