Drilling wells are increasingly challenging. Many of the easiest and most accessible wells have already been developed. Today, operators are drilling deeper wells with higher temperatures, greater pressures and more complex subsurface conditions. In many cases, these wells are located in deepwater environments.
These factors often narrow the drilling pressure window and increase operational risk. In some cases, wells may appear almost impossible to drill using conventional methods.
In practice, however, most of these wells are not truly undrillable. They require improved pressure management and new drilling approaches.
In this article, we look at some of the conditions that can make wells appear “undrillable” and how modern pressure management methods such as Managed Pressure Drilling (MPD) can help operators drill them safely and efficiently.
In the article covers common challenges including:
Deepwater wells and narrow pressure windows
High-pressure drilling environments
Geological uncertainty in exploration wells
Depleted reservoirs and loss zones
Deepwater wells often present some of the most complex drilling conditions.
A common challenge is the narrow drilling pressure window, where the difference between pore pressure and fracture gradient is small. This means that even minor pressure fluctuations can lead to losses or influx.
In deepwater operations, the long hydrostatic column from the rotary kelly bushing (RKB) down to the seabed also complicates well design. Because the pressure window often begins below the seabed, wells frequently require multiple casing strings and liners to maintain pressure control.
Managed Pressure Drilling (MPD) can help address these challenges by enabling near Constant Bottom Hole Pressure (CBHP). This is particularly valuable in the reservoir section where Equivalent Circulating Density (ECD) may become too high during circulation.
In the upper sections of the well, Controlled Mud Level (CML) MPD can create a Dual Gradient effect. By adjusting the fluid level in the riser, operators can modify the pressure profile and reduce hydrostatic pressure in the well.
This can allow two or more sections to be drilled as one. Eliminating a casing or liner can significantly simplify the well design and reduce overall drilling costs.
The pressure profile generated by CML is particularly useful in the upper sections of the well, where large hole sizes and high rates of penetration can otherwise make pressure management more difficult.
Conventional well control can also become challenging in ultra-deepwater wells because of friction losses in long choke lines. MPD can improve influx management in these situations by using a riser closure device to maintain pressure control.
Check out: 11 Things All Drilling and Completion Engineers Need to Know About MPD
New drilling campaigns are increasingly targeting reservoirs with very high pressures and temperatures. To support these operations, 20K rigs have entered the market.
These rigs are designed to operate in environments requiring pressure control up to 20,000 psi.
These environments, typically deeper wells with high pressure and temperature, encounter the same tight operating margins as deepwater wells due to high Equivalent Circulating Density (ECD).
There is an ongoing campaign in a 20K field previously deemed too difficult to drill, where Controlled Mud Level (CML) MPD has been required to enable drilling.
In this field, CML is applied across all sections. This includes:
leveraging the Dual Gradient effect in the upper sections
enabling managed pressure cementing
creating better margins for tripping
controlling Bottom Hole Pressure in the reservoir section
A Dual MPD system is used to provide improved influx management.
Discover: EC-Drill® Dual MPD
Exploration wells often involve significant geological uncertainty.
Estimating pore pressure and fracture gradients can be difficult before drilling begins. Subsurface conditions may differ significantly from predictions made during the planning phase.
This requires continuous evaluation of real-time data and adjustments while drilling ahead.
For these wells, Controlled Mud Level (CML) MPD, or a Dual MPD system combining CML with a riser closure device, provides the flexibility to adjust according to actual well conditions and improves influx management.
In addition to operating with Constant Bottom Hole Pressure (CBHP), CML MPD allows the pressure setpoint to be adjusted by increasing or reducing the fluid level in the riser.
The Early Kick and Loss Detection System, equipped with pressure sensors positioned subsea inside the well, provides near-instant feedback on gains or losses in the well.
Keeping the fluid level below the telescopic joint reduces measurement uncertainty caused by rig motion and allows the riser to function similarly to a trip tank for direct well monitoring.
Combined with the ability to instantly adjust Bottom Hole Pressure, the system is well suited for managing uncertainty.
Explore: How to Perform Influx Management With EC-Drill® Dual MPD
Mature fields often present a different type of challenge.
When drilling new phases of development wells in depleted reservoirs, pressure conditions may vary significantly across the field. Some zones may retain near-virgin pressure, while others may be heavily depleted.
A common approach is to drill with Bottom Hole Pressure above the expected pore pressure and attempt to manage losses that occur in depleted zones.
However, in highly depleted reservoirs, even a hydrostatic column of unweighted drilling fluid may cause losses.
Operating in Constant Bottom Hole Pressure mode can help maintain the correct pressure balance and reduce the likelihood of severe losses. This can also allow longer reservoir sections to be drilled.
In these environments, the ability to reduce Bottom Hole Pressure when losses occur becomes critical.
CML MPD provides this capability by adjusting the riser fluid level to lower hydrostatic pressure.
In highly depleted fields or when drilling into total loss zones such as karstified formations, operators may plan to drill using an unweighted mud system with a reduced riser fluid level. This creates hydrostatic pressure lower than seawater.
If total losses occur, the operation can transition to Controlled Mud Cap Drilling (CMCD).
In CMCD mode:
Seawater is pumped down the drillstring as sacrificial fluid
The annulus is monitored and kept overbalanced using the Subsea Pump Module (SPM)
Annulus volume is maintained using a top-fill pump and boost line
The SPM maintains the desired fluid level in the riser
This allows drilling to continue even in formations where conventional circulation is not possible.
Read more: How Does CML MPD Work
Many wells that once appeared impossible to drill are now technically feasible.
Deepwater wells, high-pressure environments, exploration wells with uncertain geology and mature fields with depleted reservoirs all present significant drilling challenges. In many cases, these challenges are related to narrow pressure windows and the difficulty of maintaining stable Bottom Hole Pressure.
Advanced pressure management methods such as Managed Pressure Drilling, Controlled Mud Level MPD and Dual MPD systems provide new ways to manage these conditions.
By improving pressure control, enabling faster detection of gains and losses and allowing operators to adjust the pressure profile during drilling, these technologies make it possible to drill wells that were previously considered undrillable.