What Does Well Planning Look Like With CML

Controlled Mud Level (CML) is a method where the pressure in the well is controlled and adjusted by changing the height of the hydrostatic column. This is done with a Subsea Pump Module (SPM) placed subsea with an inlet from the Riser. The SPM controls the pressure in the well by adjusting the fluid level in the riser. 

For well planning purposes, this method goes far beyond holding a Constant Bottom Hole Pressure (CBHP) to enable drilling in narrow windows. Significant changes to the well architecture in the upper parts of deepwater wells have become standard together with the ability to change drilling and completion fluid, 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 a couple considerations where CML can improve and simplify the well design.

Contingency Liner

Contingency Liners serve as a plan B for sections where there is a notable risk for not reaching the desired sectional total depth (TD). In some cases sections are cut short as a safety precaution in fields where previous drilling has been difficult. MPD goes a long way in eliminating most of the expected drilling challenges with a CBHP approach. CML can also quickly increase or decrease the pressure according to actual well conditions and reduce the likelihood of needing the contingency liner.

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

Complex Well Architecture

Complex well architecture applies especially for deepwater wells where narrow windows are common for all sections. Together with the mismatch between the hydrostatic column starting from the rig and the operating window starting from the seabed, it leads to frequent casings and liners.

This is problematic for several reasons. The most obvious is the added cost with drilling and cementing multiple sections, additional tripping – now with narrow clearance - and using non-standard casings and bit sizes. The carbon footprint also increases as a result of this.

Other challenges include finding sizes for any contingency liners, even when using Reamers, which often results in a slim hole design in the reservoir. A small hole size in the reservoir can limit the length of the section because of the high ECD, even with MPD, and result in less of the targeted production zone being covered.

The wells can still be drilled and completed using MPD to address the narrow windows however the complexity of the operations remain and drilling time and cost is high. This is where CML is a significant game changer. Besides holding a CBHP to enable drilling in general, the use of the Dual Gradient Effect in the upper parts of the well allows for sections to be combined and drilled as one. By combining upper sections, the complex well architecture disappears and the well design becomes simple to plan and execute. 

An example of the Dual Gradient Effect is illustrated below.

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

Combining sections in the upper part of the well is now the default approach with 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 give access to more of the reservoir. ECD management is critical when planning for Extended Reach Drilling (ERD) or simply staying within the operating window. When drilling in areas with unknown degree of depletion, having the ability of going in above virgin pressure and then adjust down to actual well conditions, will help enable the reservoir section to reach desired TD, even in mature fields with high degree of depletion. 

CML is used for CBHP approach in the reservoir and is ideal for depleted zones with its ability to lower the Bottom Hole Pressure by reducing the height of the hydrostatic column. Highly depleted fields have been drilled with a hydrostatic pressure even less than water by using unweighted WBM 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/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 reason for drilling production wells is to, well.. produce. The higher the production rate the better. As previously mentioned reaching desired target and not damaging the reservoir when drilling sets the condition for a good producing well. The final step is ensuring the completion gives as high a production rate as possible. 

CML gives the operator the choice of selecting between different completion methods otherwise not selectable. There are case examples with CML enabling switching from Standalone Screens to Open Hole Gravel Pack, and by lowering the riser level enabled the use of a brine with an effective downhole density less than water. Planning completion by combining Gravel Packing with CML has yielded exceptional results in ensuring a full screen out is achieved. Without CML, gravel packing often ends when taking losses. The difference in production rates between comparable wells have been significant.

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

 

Conclusion

Well planning with MPD commonly consist of using a CBHP approach for sections with narrow windows. CML provides additional options for well planning and it is recommendable not to limit it solely for CBHP.  The figure below shows the development in a field where CML has been used consistently. 

Over time the well architecture was simplified, different types of fluids could be selected and an improved completion method was utilized.

When planning with CML considering the Dual Gradient Effect and the different pressure profile this method creates is probably the most important thing. Bringing in the subsurface team to evaluate how the completion can be planned with CML is recommended.