Milling Through Barriers: How Active Stabilization Is Driving Down Subsea P&A Costs

By Tejas Ghegadmal

When Conventional Section Mills Aren’t Enough

Plug and Abandonment (P&A) of subsea wells is a critical step toward responsible energy production—and it's one of the most expensive and technically demanding operations in the life of a well. For wells with depleting production, regulations often require a rock-to-rock cement barrier, meaning operators must remove multiple casing strings before placing and testing a competent cement plug.

In many cases, cutting and pulling those casings is simply not feasible. The fallback? Section milling. But here’s the challenge: Conventional section mills struggle in restricted environments like those found in subsea wells. Their large gauge prevents passage through smaller tubulars—like production risers—forcing operators to either remove infrastructure or abandon the milling plan altogether.

The Cost of Conventional Milling

Traditional section mills have stabilization features that are passive and sized nearly equal to the casing ID they’re meant to remove. While this allows for stable milling, it creates a paradox: the tool can’t reach the casing if it has to pass through narrower diameter components first.

The workarounds are costly:

Removing risers or BOPs to create access.
Performing multiple trips to ensure sufficient milling.
Accepting increased vibration, tool wear, and HSE exposure.

Each trip and intervention can add days of rig time and millions in cost to the abandonment operation.

Rethinking Section Milling: A Smarter, Leaner Design

To overcome these limitations, a new section mill with active stabilization was developed. The design allows the tool to pass through restricted diameters and still mill larger casing sections below—without sacrificing performance, stability, or safety.

Here’s how it works:

Undersized mill body: Fits through narrow tubulars like production risers or BOPs.
Expandable stabilizer arms: Deployed first to centralize the mill before cutting begins, reducing lateral vibration.
Controlled knife expansion: Knives gradually open to achieve full cut diameter only after centralization, preserving the integrity of the casing wall and extending tool life.

Field-Proven Success in the Gulf of Mexico

In a challenging Gulf of Mexico subsea P&A operation, this actively stabilized section mill:

Passed through a restricted 9-3/4" riser,
Milled a 70-ft. section of 11-3/4" casing,
Saved the operator 3 days of rig time and over USD 1 million, and
Delivered the performance of a full-sized mill—without the associated logistical complexity.

This success highlighted the potential of the design to streamline casing removal, especially when traditional section milling would have been unfeasible or cost-prohibitive.

Bottom Hole Assembly Used in Gulf of Mexico

Why This Matters

This isn’t just a mechanical improvement—it’s a step-change in how we approach the economics and logistics of plugging and abandoning offshore wells.

Benefits at a glance:

✅ Fewer trips into the wellbore
✅ No need to remove surface equipment
✅ Lower tool wear and vibration
✅ Reduced HSE exposure
✅ Full compliance with “rock-to-rock” barrier requirements

For operators focused on cost control, sustainability, and regulatory compliance, this technology presents a practical path to safer, more efficient well retirement.

Want to Dive Deeper?

For detailed technical insights, tool design principles, and field data, download the full SPE paper:

👉 Download the Paper – IADC/SPE-178827-MS: “Reduce P&A Cost with Section Milling Technology”

Have questions about downhole cutting & milling, or want to collaborate on plug & abandonment tool development? Let’s talk. Reach out via E-mail or connect on LinkedIn.

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