What Does Well Planning Look Like With CML

Controlled Mud Level (CML) is a Managed Pressure Drilling (MPD) method where Bottom Hole Pressure is controlled by adjusting the height of the hydrostatic column. This is achieved using a Subsea Pump Module (SPM) installed subsea with an inlet from the riser. By adjusting the fluid level in the riser, the SPM can increase or decrease the Bottom Hole Pressure as required.

For well planning purposes, this method goes far beyond maintaining Constant Bottom Hole Pressure (CBHP) within narrow operating windows. In deepwater operations, CML has enabled significant changes to well architecture, drilling and completion fluid selection, and completion methods.

Well planning

A number of considerations go into well planning, and this short article will not cover all of them. Instead, the focus is placed on several considerations where CML can simplify well design and expand the available planning options.

Contingency Liner

Contingency Liners serve as a plan B for sections where there is a significant risk of not reaching the planned sectional Target Depth (TD). In some cases, sections are intentionally cut short as a safety precaution in fields where previous drilling has been difficult.

MPD can help address many of these challenges by maintaining Bottom Hole Pressure within the operating window. In addition, CML allows pressure to be adjusted quickly according to actual well conditions, providing greater flexibility when conditions differ from the pre-drill prognosis. This may reduce the likelihood of requiring a contingency liner and simplify the overall well design.

Check out: SBP vs CML Connection Time – Which Is More Effective?

Complex Well Architecture

Complex well architecture is particularly common in deepwater wells, where narrow operating windows can be common for several sections. Combined with the mismatch between the hydrostatic pressure profile originating at the rig and the operating window starting from the seabed, this frequently results in additional casings strings and liners being required.

This creates several challenges. The most obvious is the additional cost associated with drilling and cementing multiple sections, increased tripping activity with reduced clearance, and the use of non-standard casing and bit sizes. The overall environmental footprint also increases as a consequence of the additional operations.

Other challenges include finding sizes for any contingency liners, even when using reamers, which often results in a slim hole design in the reservoir section. A smaller reservoir hole size increases ECD, even with MPD and can limit the achievable section length, resulting in less of the target reservoir being exposed.

These wells can still be drilled and completed using MPD to manage the narrow operating windows. However, the complexity of the well design remains, and drilling time and cost can remain high.

This is where CML fundamentally change the well design. Beyond enabling pressure management within narrow operating windows, the Dual Gradient Effect allows upper well sections to be combined and drilled as a single section. By reducing the number of required casing strings and liners, the well architecture becomes significantly simpler to plan and execute.

An example of how the Dual Gradient Effect can simplify well design is illustrated below.

In this example, the pressure in the well is kept below 11.1 ppg (1.33 SG) at the casing shoe and above 11.9 ppg (1.43 SG) at sectional TD. The section is drilled conventionally in the sense that no pressure adjustments are made between dynamic (drilling) and static (connection) conditions. The large hole size in the upper part of the well results in a relatively small ECD contribution, which can be seen by comparing the static profile (green curve) with the ECD profile (orange curve).

Combining upper well sections has become a standard approach when applying CML in deepwater operations.

Additional reading: Maximizing Savings with CML

Drilling Reservoir Section

As already mentioned, avoiding a slim hole design will reduce the ECD and provide access to a larger portion of the reservoir. ECD management is critical when planning for Extended Reach Drilling (ERD) operations or simply staying within narrow pressure windows. When drilling in areas with unknown degree of depletion, the ability to start above virgin pressure and then adjust according to actual well conditions help enable the reservoir section reach desired TD, even in mature fields with significant depletion.

In reservoir sections, CML is applied to maintain Bottom Hole Pressure while also providing the ability to reduce pressure by lowering the height of the hydrostatic column. This makes the method particularly suited for depleted zones. In highly depleted fields, wells have been drilled with an effective hydrostatic pressure below that of seawater by using unweighted water-based mud and lowering the riser level.

Drill-in and Completion Fluid

When planning for the drilling fluid to be used in the reservoir section, focus is often on low-rheology oil-based fluids for ECD management. This can be a necessity in order to stay within the operating window. However, it leads to a less producing well because of additional skin damage to the formation when drilling. When planning with CML, water-based drill-in fluid can be selected using CML for ECD management instead.

Similarly, the options when selecting completion fluid widens with CML. A heavier or otherwise preferred brine better suited for the reservoir can be selected, and if for example perforating the BHP can be adjusted afterwards to reduce losses.

Completion

The ultimate goal of a production well is to maximise production from the reservoir. As previously discussed, reaching desired target and not damaging the reservoir during drilling help establish the conditions for a high-performing well. The final step is ensuring the completion supports the desired production performance.

CML gives the operator access to completion methods otherwise not selectable. There are case examples where CML has enabled a change from Standalone Screens to Open Hole Gravel Pack completions. By lowering the riser level, CML has also enabled the use of brines with an effective downhole density below seawater. Planning completion by combining Gravel Packing with CML has improved the ability to achieve a complete screen-out during the operation. Without CML, gravel packing often ends when taking losses. In documented applications, comparable wells have shown significant differences in production rates.

Read more: What Are Undrillable Wells in 2025 and How to Make Them Drillable

 

Conclusion

Well planning with MPD commonly focuses on maintaining Bottom Hole Pressure within narrow operating windows. CML provides additional options for well planning and should not be viewed solely as a method for maintaining Bottom Hole Pressure. The figure below shows the development in a field where CML has been used consistently.

Over time, the well architecture was simplified, fluid selection became more flexible, and alternative completion methods could be considered.

When planning with CML, considering the Dual Gradient Effect and the different pressure profile this method creates is one of the key planning considerations. Involving the subsurface team to evaluate how the completion can be planned with CML is also recommended 

Curious to learn more about CML? Check out this webinar: