This paper discusses challenges in section milling operation for three strings of casing configuration across the cap rock, to enable a zonal isolation for a well that was drilled in year 1980. A high ratio section milling (HRSM) technology was successfully implemented to deal with this complex well that had all the casings cemented to the surface, together with centralizers for each joint to prevent a simple cut and recovery of casings. The conventional operation on the other hand would require the cutting and pulling of the inner casing, followed by casing milling of the long string of inner casing.

The new HRSM technology eliminated excessive casing milling operation which is conventionally required to access two or three strings of cemented casings prior to place a well barrier. The HRSM technology has a high expansion ratio to enter small restrictions coupled with 6-point active stabilization to counter dynamic loads and vibration when section milling. The operations challenges were optimized using the standard section mill assembly, a high ratio underreamer designed with casing scraper arms and the newly developed HRSM with new arms for the 3-string configuration.

As part of the planning phase, identifying the depths for inner and outer milling were considered along with the placement of couplings and centralizers which is crucial for milling operation. Once the window plan was completed, a thorough BHA vibrations analysis was carried out followed by a comprehensive hydraulic analysis to identify key parameters for milling and mud rheology for the entire operation.

One of the key challenges during the execution phase was the milling the centralizers on each joint of 7-inch and 9.625-inch casing. The operation was initiated by milling in the inner 7-inch casing in two stages. First the cut out of 7-inch casing was initiated followed by flush knives to mill the casing and creating a 34.7m window to access the 9.625-inch casing. A high ratio underreamer was used to clean the cement sheath in 9.625-inch casing before the new HRSM technology was run to section mill a window of 24.15 meters in 9.625-inch casing. The new HRSM was specifically designed with new cut out and flush knives to mill the 9.625-inch casing inside 13.3.75-inch casing. The final clean out of 13.375-inch casing was completed by newly designed scraper arms for the high ratio underreamer thus providing a solution of restoring the caprock across 3 strings of casing.

The unique design of HRSM technology led to the completion of first of its kind application by restoring caprock across 3 strings. This niche job has been able to save the client significant rig time for its plug land abandonment operations.

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