Dual Casing Section Milling Using High Ratio Section Milling Technology to Achieve Rock to Rock Zonal Isolation
Authors: Carasco Anant , Khan Muhammad , Zolhaili Azzril , Yntema Michiel , Leuranguer Chris , Alexander Gregg 
Published: February 2023

The plug and abandon (P&A) challenges of each well are known to be different. This paper narrates unique challenges faced during the abandonment of a land well which intersected multiple over-pressured reservoirs containing high concentration of H₂S and CO₂. Because zonal isolation was paramount for this project, section milling was selected to enable a rock-to-rock cement plug to restore 5 critical caprocks. Remediation of annular cement was complex because all production and intermediate casings were cemented to surface with 2 or 3 casings across the caprocks. Conventional methods would entail pilot milling the 7 inch production casing, exposing the A-annulus to enable section milling of the 9.625 inch intermediate casing for a cement plug across the caprock. This technique is time consuming and uncertain, which adds to the cost and complexity of the P&A operations.

In response to these challenges, the operation was optimized utilizing both a standard section mill and a new High-Ratio Section Milling (HRSM) technology, which allows for milling windows through two casing strings. The HRSM is a combination of a high-ratio hydraulic section mill, achieving a 180% expansion ratio, and an expandable stabilizer. The orientation of the stabilizer is set to enable 6-point contact stabilization in the outer casing and helps to reduce dynamic shocks and vibrations. The HRSM is deployed after the inner 7-inch casing window has been milled for a length of approximately 140 ft. The expandable stabilizer in the system ensures that the section milling assembly can efficiently mill a 110 ft casing window in the 9.625 inch casing, through a 7 inch casing window. A high-ratio underreamer is utilized to clean the formation and enlarge the diameter to 13.5 inch to enable a rock-to-rock seal through two casing strings, without pilot milling the inner string from surface.

Milling two casing strings is done in 5 stages. The inner casing window is initiated with a dedicated run using rapid cutout knives. This allows for deployment of "flush knives" while milling the inner window, reducing the risk of skimming the outer casing and enabling a single run of 139 ft casing milled. Following a clean out run with an under reamer, the new HRSM technology was then run and completed a 111 ft of 9.625-inch casing in one run, which was followed by 100 ft of hole enlargement by a high ratio underreamer to open the hole to 13.5 inches. All of the above stages were carried out in a single run and with good ROP. The cement job was completed and the objective of restoring the cap rock seal across two strings of casing was achieved, saving rig time and cost for the plug and abandonment operation.

The development and successful deployment of HRSM technology provides a reliable solution to achieve a rock-to-rock cement plug in a dual-casing environment. During the execution phase various lessons were learnt and implemented as best practice, this included design changes of the HRSM technology and the bottom hole assembly. The combination resulted in setting new benchmarks for HRSM technology and enabled savings of 30 days rig time when compared to the conventional method of pilot milling the inner casing.