In Which Drilling Environments Are CML Most Applicable?

The Controlled Mud Level (CML) method is used in most drilling environments for a variety of challenges and cost saving measures. In some of these environments CML is used more frequently where its unique benefits stand more out.

This article describes the two most natural habitats, so to speak, of CML – deepwater drilling and mature, depleded fields – and explains why CML belongs there.

Check out this case study: EC-Drill® Dual Gradient in Gulf of Mexico Deepwater

Deepwater Operations

CML has become an established approach for deepwater drilling, where multiple sections often require precise pressure management.

Key advantages of CML in deepwater include:

• Handling large hole sections with high ROPs and flow rates, efficiently returning cuttings to surface without limitations
• Dual Gradient Effect, which allows upper sections to be combined and drilled in one run, now a standard in deepwater well planning
• Faster drilling connections, since the unique pressure profile created can avoid the need for pressure adjustments during connections.
• Optimized tripping conditions, with surge and swab mitigated by adjusting the riser fluid level
• No need for fluid displacements before tripping or cementing—avoiding time-consuming operations with large fluid volumes and reducing shipping, storage, and treatment requirements
  
  

CML is currently used in water depths as shallow as 300ft. Shallow depths limits the ability for pressure adjustments, and in deeper waters the Subsea Pump Module (SPM) can be placed deeper maximizing the capabilities of the system.

Additional reading: What to Consider When Implementing MPD in Deepwater

Mature and depleted fields

By many considered the “Bread and Butter” of CML, mature field operations have been a success story for over a decade. In depleted reservoirs, CML enables drilling where conventional methods fall short. 

Benefits in mature fields include:

• Lowering BHP in line with depletion levels, enabling continuation of drilling activities
• Drilling longer reservoir sections, unlocking additional reserves and extending field life
• Hydrostatic pressures below seawater density when drilling with Water-Based Mud (WBM), achieved by lowering the riser level
• Flexible pressure adjustment, allowing operators to start above virgin pressure in case of isolated pockets and reduce pressure as drilling progresses

CML also supports completion operations, particularly Managed Pressure Gravel Packing (MPGP). By adjusting the riser level, CML enables a full screen-out without inducing losses, directly improving well productivity. Documented results show significant increases in production rates compared to offset wells.

Watch webinar: Deepwater MPD | Introducing EC-Drill® Dual MPD

Conclusion

CML is applicable across a wide range of offshore drilling challenges, including HPHT wells, Extended Reach Drilling (ERD), and total loss zones. It is not suited for land operations or wells with surface BOP stacks, but beyond those limitations, it offers clear advantages.

While versatile, CML delivers its greatest impact in deepwater wells and depleted reservoirs. In these environments, the unique pressure profiles and dynamic adjustment capabilities of CML clearly set it apart from other methods.