Advance Milling and Cutting Structure Development and Implementation to Reduce Well Abandonment Cost for Major Operators
Authors: Khan Muhammad Zafar , Swadi Shantanu , Solorzano Efrain 
Published: May 2023

Decommissioning of end-of-life wells is an essential operation in the lifecycle of a well and requires thorough planning and execution. The plug and abandonment of the wells contribute over 45% of the overall decommission costs and hence there is a clear mandate from the operators and service companies to offer new technologies and solutions to reduce the overall decommissioning costs.

The service company has been working with several operators to provide casing cutting and casing milling operations. The aim is to provide the most efficient and cost-effective method to perform the plug and abandon operations for a given well especially in the casing removal and recovery operations which in turn provides rig time savings. Some of the major operations are involved in section milling the casing, removal of large casing conductors or triple casing cuts by the pipe cutter, and subsea wellhead removals. The service company has designed an advanced cutting structure technology to facilitate the milling and casing cutting operations and provide a reliable, durable, and efficient milling solution for well abandonment. The new cutting structure provides reduced cut times and/or a longer useful cutting life to cut large dual casing conductors such as 20″x30″ or 22″x36″ during the conductor recovery phase or the wellhead recovery phase. Similarly, the advanced cutting structure is designed to efficiently mill extended casing sections, especially for high grade casings with higher ROP and lower downtime (rate of penetration) during the milling operation.

The development efforts started with an evaluation of the current cutter designs and shortcomings. After an assessment of the field performance and dull characteristics, it was evident that high shock and vibration loading during the downhole operation results in excessive and premature impact damage leading to sub-optimal cutter geometry for cutting steel. Likewise, bird-nesting of swarf was also a common source of NPT due to the interruptions in operations for breaking up and clearing the swarf periodically, before milling commenced again.

Several competing concepts for insert shapes were considered and analyzed. Improving the edge strength was considered as a key attribute. As well as the ability to break-up swarf into smaller segments for efficient transportation. Modelling and simulation, and physical testing helped narrow down to a few concepts for full scale lab tests, and eventually to select the most promising concept for field tests.

The new advanced cutting structure has been 100% successful in multiple challenging applications of casing cutting and milling operations in the North Sea and is being implemented in Middle East and Asia. Improvements in the conductor cutting time has resulted in record recovery of the subsea well heads. Likewise, for section milling applications – record ROPs and longer intervals have been achieved for high grade casing such as P110.

The technology demonstrates how the material behaves in downhole cutting operation and what further development can be made to further enhanced the efficiency, reduce rig time and wells decommissioning cost.