Over the last years there has been a change of reason for using Controlled Mud Level (CML). Initially, CML was used to enable drilling. It added cost to the operation but made it possible to drill the wells.
Today, the reason for using CML has shifted and enabling drilling is no longer the main driver. CML is now used to optimize well design, improve operational efficiency, and in many cases, maximize savings. This article will highlight the key value drivers behind the modern use of CML.
Dual Gradient Effect: Improving well architecture
When the fluid level in the riser is lowered, a Dual Gradient Effect is generated. The second “gradient” being the column of air above the fluid. This effect creates a unique, curved static pressure profile with lower pressure gradient at shallower depths.
This profile follows common operating windows and enables drilling with negative margins – sometimes even without having to make pressure adjustments between static and dynamic conditions. This can by itself be a significant operational advantage.
The Dual Gradient Effect is actively used to combine sections and eliminate a casing/liner. The direct savings are easily calculated, for example through having one casing/liner less and one cement job less. On top of that, there are also notable indirect cost savings through a simpler well architecture allowing:
- More standard bit and casing sizes
- Less requirements for reaming
- Larger hole size in the reservoir
The Dual Gradient Effect is illustrated in the graph below, where no pressure adjustments are made between static and dynamic conditions (the large hole size in the upper part of the well does not generate high ECD).
In this example, the pressure profiles remain well below the Fracture Gradient of 1.33SG (11.08ppg) at the Shoe, while staying well above the Pore Pressure of 1.43SG (11.91ppg) at sectional TD. Connections are faster since no pressure adjustments are required.
The Green curve is the static profile, and the Orange Curve is the ECD.
Check out: What to Consider When Implementing MPD in Deepwater
Optimization as an MPD tool
When it is necessary to use MPD to enable drilling, CML will optimize those drilling operations. The flat time associated with MPD can be high in narrow windows:
After drilling a section, the well must first be displaced to a heavier tripping fluid before pulling out. The casing must then be run in slowly in the high-density tripping fluid, without exceeding the upper boundary of the well. The well is then displaced a second time to a cementing fluid. This displacement can be painfully slow because of the narrow annulus between the casing and the open hole – and the heavy tripping fluid being displaced out.
All this flat time can be removed with CML. No displacements are needed as the fluid level is adjusted according to ongoing operation. Tripping can be sped up by facilitating a better margin towards the pressure boundaries in the well.
Using the same example in the previous section, the two graphs below show tripping out with the BHA (left) and then running in hole with the casing (right). The fluid level is first increased to move the static pressure profile away from the pore pressure before tripping out. Before running in with the casing, the fluid level is lowered, moving the static pressure profile away from the fracture gradient. These adjustments enable faster tripping without pressure adjustments or displacements.
The Green Curves are static profiles, and the Orange Curves are the resulting pressure profile caused by the swab and surge respectively.
Instead of displacements, CML can move the pressure profile in the well from the Pore Pressure Gradient all the way to the Fracture Gradient simply by adjusting the fluid level in the riser.
The ensuing cement job is done by lowering the riser level in steps as the heavier more viscous cement goes up the annulus.
Additional reading: What Are Undrillable Wells in 2025 and How to Make Them Drillable
Boosting Completion Performance
CML is just as valuable during completions – especially for Open Hole Gravel Pack (OHGP).
In OHGP, CML achieves a full screen-out without going on losses. This is done by adjusting the riser level as the gravel-laden fluid backfills (Beta wave) towards the heel of the reservoir section.
CML also supports the use of drill-in fluids for reservoir sections, reducing skin damage and enabling higher production rates. It expands the range of densities available for drilling and completion fluids, which can be a major operational advantage.
Completion with CML is, savings wise, the main driver for this MPD method. The savings in this case come as an earning through increased production. This is also the least quantifiable savings before a project commences. There are historical cases where offset wells can be compared with the production rates from wells completed with CML. There is also a case where a sidetrack was completed with CML for direct comparison with the original completion.
Although predicting production gains upfront is difficult, a practical approach is to involve the subsurface team early. Their input is essential in evaluating the benefits.
Takeaway
CML is still used to drill wells otherwise considered undrillable. But the industry mindset has evolved.
Now, cost savings and optimization are often the primary reasons for choosing CML. Whether it’s simplifying the well architecture, speeding up operations, or increasing production – CML is delivering value across the well lifecycle.
The question is no longer “Can we afford to use CML?”
It’s “Can we afford not to?”
