ProMILLTrip-saving milling and underreaming system
The ProMILL* trip-saving milling and underreaming system combines an underreamer and a section mill in one single-trip solution, achieving rock-to-rock zonal isolation and preparing the foundation for an abandon?ment cement barrier. Performing both operations in the same trip not only saves rig time and improves ROP but ensures plug integrity for confident P&A decisions.
Compared with perf-and-squeeze methods, the ProMILL system eradicates all potential leak paths—including channeling, microannuli, and mud?cake—from the abandonment barrier, enhancing annular remediation.
Efficient section milling with rock-to-rock isolation
The ProMILL system’s section mill is a hydraulic downhole tool ideal for milling casing to set rock-to-rock well abandonment cement plugs. All cutter arms are dressed with tungsten carbide inserts (TCIs) that, along with the multiblade design, provide high ROP and maximum footage. ROP is optimized during prejob planning to ensure that swarf is produced at a rate consummate with surface swarf handling capabilities.
As pump pressure is applied, three cutter arms expand and begin the cutout. When the cutout arms are fully open, three additional cutter arms automatically expand into the milling position using a cam and ramp interface. The shorter lead arm opens slightly ahead of the longer follow arms, providing maximum force for the cutout. When all the arms are fully expanded, the cam reaches a flat interface, locking the knives open as long as a minimum pressure drop across the tool is maintained. All six cutting surfaces are then squarely sealed on top of the casing for optimal milling.
Each cutter knife incorporates chip-breaker ridges, and the continuous chip-breaker design generates steel cuttings with a size and shape that will not accumulate and block flow. These cuttings can easily be circulated out of hole, requiring a minimum of specialized mud conditioning.
Designed using the IDEAS* integrated drillbit design platform, the cutter inserts are engineered to minimize the size of milled casing shavings for optimal hydraulics without sacrificing milling speed. The IDEAS platform contains a database of casing material that catalogs the cutter performance and casing metallurgy to provide a clearer understanding of optimal milling performance and parameters.
Effective borehole enlargement fully controlled from surface
The ProMILL system’s underreamer is specially designed to meet emerging well-abandonment regula?tions. It is also ideally suited for enlargement of high-ratio boreholes in soft formations and can be used to ream short intervals in specific formations.
The underreamer features ball-drop activation, which ensures that the reamer remains inert during the entire milling operation. Once the window is milled, the BHA is repositioned with the underreamer at the top of the window. Then, a 1.5-in ball is dropped from surface to activate the tool; a pressure drop clearly indicates when the ball has raised the underreamer and has activated the tool. The section can then be underreamed, and the operator can continue to the target depth, completing the operation.
Circulating and mud conditioning
Operations are further enhanced with the WELL COMMANDER? ball-activated drilling valve, which helps boost circulation to remove cuttings at strategic points in the drillstring. It allows condition?ing or displacing wellbore fluids at high flow rates and enables bypassing sensitive downhole BHA tools with lost-circulation or wellbore-strengthening treatments with a simple, one-ball open-close functionality.
Case Study1
Reduce dedicated BHAs for each run
In a P&A campaign in the UK Continental Shelf, an operator planned to create permanent rock-to-rock barriers by placing cement across the impermeable shale formations. If the operator found poor cement behind the casing, then it would performing milling and underreaming to allow setting a cement-rock barrier. In offset wells, the milling and underreaming operations required dedicated BHAs, resulting in a 24-hour trip.
Combine multiple runs into a single trip system
To save rig time, Schlumberger recommended running the ProMILL system, which combines an underreamer and a section mill in one efficient tool. The operation began by cutting and milling the casing windows with the section mill. Then, an activation ball was dropped and pressure was applied to shear the retaining pins. This action activated the underreamer by allowing an internal piston to move down, forcing the underreamer arms outward. The open hole is then underreamed to virgin formation during the same trip.
Eliminated a trip and saved 24 hours per well
The ProMILL system enabled milling and underreaming 85-ft windows at 9,000-ft MD in three wells in one trip each. The efficient system allowed reducing openhole exposure time, manual handling, and personnel exposure on the drill floor. By eliminating the extra trips, the operator saved at least 24 hours of rig time per well.
Case Study2
Efficiently mill and underream section in offshore well
An operator was planning a P&A operation in the UK sector of the North Sea. Its objective was section milling a 150-ft window in the 9?-in casing then underreaming the open hole—all in a single trip—to enable placing a rock-to-rock abandonment cement plug against the original formation.
Use integrated, single-run solution to achieve objectives
Schlumberger collaborated with the operator on an integrated solution that would meet the P&A objectives while decreasing the cost and regulatory concerns inherent to well decommissioning. The BHA solution included the ProMILL system with new performance milling inserts that feature a concave shape. The performance milling inserts were used in combination with the conventional inserts to enhance cutter durability and improve swarf quality. Additionally, the WELL COMMANDER valve was run to boost circulation by removing cuttings at high flow rates, which eliminated a dedicated trip for conditioning the wellbore fluids for the milling program.
The ProMILL system would eradicate all potential leak paths from the abandonment barrier which is not possible to ensure when using perf-and-squeeze methods.
Saved 3.5 days and USD 600,000 with single-trip solution
The integrated P&A solution performed the following operations in a single run:
（1）displaced and conditioned the well
（2）set the 9?-in bridge plug on the taper mill
（3）milled a 150-ft section
（4）underreamed the exposed formation to prepare the wellbore for a rock-to-rock abandonment plug.
During milling, the operator achieved an ROP of 4.5 ft/h, representing a 50% increase compared with 3 ft/h, the average ROP of the operator’s jobs completed in the field. The knife wear on the ProMILL system was 30%–35%, which was lower than has been previously seen in similar operations.
The window was completed in 50 h, saving the operator 3.5 days and USD 600,000. This operation set Schlumberger records for the longest single-run window milling, section underreaming, and bridge plug setting as well as the longest single-run milled window.
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