Innovative Mechanical Solutions for Scale Removal: Optimizing Pre Plug and Abandon Operation Through Robotic Milling Solution to Achieve Multiple Milling OD’s in One Run

Milling and wellbore cleanout operations are fundamental to successful well intervention, particularly in aging wells nearing the plug and abandonment (P&A) phase. These interventions are often challenged by scale build-up, tubing restrictions, and the need to access specific depths to install permanent barriers. Conventionally, such operations require multiple runs with separate cleanout assemblies—ranging from milling, explosives, and caliper logging runs—to first gain access and then ensure the wellbore is clean enough to deploy and seal a plug. Each additional run adds complexity, cost, rig time, and environmental footprint, making single-run solutions highly desirable. To address these inefficiencies, a single-run multi-OD milling tool was developed with the purpose of enabling operators to bypass restrictions, mill scale, and clean the tubing to drift ID—all within one continuous descent. This tool was designed through a close collaboration between field operations, design engineering, and sales teams to meet an urgent market need: a cost-effective, emissions-reducing, and rig-time-saving cleanout solution that could adapt to variable wellbore geometries and scale conditions. The tool combines a fixed lead bit with a second-stage expandable milling bit. The lead bit allows the bottom hole assembly to pass through scale-narrowed or ID restricting tubing, while the expandable bit is selectively activated once at the target depth. Surface-controlled activation enables precision depth targeting and real-time control of the milling passes. The first commercial deployment of this tool occurred in a North Sea well with known caliper-logged restrictions and heavy scale buildup. The planned operation required passing through an upper ID restriction to reach the intended plug setting depth, followed by cleaning the interval to near-drift ID to ensure effective plug sealing. Due to the sensitivity of the plug to debris and scale, high cleaning efficiency and accuracy were crucial for success. The full BHA—including a power mechanical tractor, stroker, and the single-run multi-OD milling tool—was mobilized and deployed in a single descent. The lead mill bit successfully cleared the scale restriction, allowing further descent to the scale-heavy interval. Once at the plug setting depth range, the expandable milling bit was activated and engaged in four milling passes. High and variable current readings during the first pass confirmed active engagement with scale, while diminishing current feedback on subsequent passes suggested effective cleaning. Following milling, the BHA was retrieved, and a multi-finger caliper log was run to verify internal diameter restoration. The data confirmed complete scale removal in the plug setting zone. The plug was then run to depth, set, and pressure tested with success, allowing the P&A sequence to continue. This operation validated the tool’s design and confirmed the success of the single-run approach. Notably, the lessons learned included the realization that current monitoring during activation provides sufficient feedback to verify scale removal—eliminating the need for caliper logs in future operations. Historical operations attempting similar objectives have required as many as seven separate runs over two days; this new approach achieved the same goal in a single run, reducing rig time by more than 24 hours and delivering substantial cost and emissions savings. Additionally, tool performance helped validate new arm designs and highlighted the potential of different material configurations for improved milling efficiency across a range of scale conditions. This paper outlines the full development process, including design iterations and bench testing, and describes the operational execution in detail. It shares comprehensive results and key takeaways from the deployment. The introduction of the single-run multi-OD milling tool marks a significant step forward in efficient wellbore cleanout and intervention methodology. It not only simplifies operations but also reduces environmental impact and total cost, paving the way for broader implementation in complex well interventions and P&A projects globally.