Unplanned events are common in drilling. They can be caused by equipment failure, human error and reactive formations; however, this article focuses on the events where actual well conditions are different than expected. Typical unplanned events are losses, taking an influx, stuck pipe, and wellbore collapse.
Let’s explore how to be ready for these unplanned events in further detail.
Losses
One of the most common unplanned events is going on losses, which can range from minor seepage losses to severe losses. Losses can be caused by exceeding the fracture gradient, drilling in unconsolidated formation, or drilling into loss zones such as Karstified formations.
Most drilling operations keep Lost Circulation Material (LCM) on standby to plug smaller fractures and seal off weak zones. In some cases, Loss Prevention Material (LPM) is added to the drilling fluid as a mitigation when expecting losses.
When the fracture gradient is lower than expected, the Bottom Hole Pressure (BHP) must be reduced. This can be achieved by displacing or diluting the drilling fluid to a lower density. This approach, however, increases the risk of an influx if BHP drops below pore pressure.
Controlled Mud Level (CML) MPD offers a more dynamic solution. By maintaining a Constant Bottom Hole Pressure (CBHP) or benefiting from the dual gradient effect, CML MPD minimizes the risk of losses. Additionally, it provides the ability to rapidly change the BHP by adjusting the riser fluid level, allowing operations to resume without requiring a full well displacement.
Drilling into total loss zones such as Karstified formations require a different approach. The most common is Mud Cap Drilling (MCD). MPD offers two versions of MCD – Pressurized Mud Cap Drilling (PMCD) and Controlled Mud Cap Drilling (CMCD) – depending on which MPD method is being used. Both methods reduce the consumption of drilling fluid, and more importantly enables monitoring and control of the well.
There are two other events indicating losses – and gains: Wellbore breathing and Wellbore ballooning. Breathing is the expansion and contraction of the well due to the elasticity of the formation, whereas ballooning occurs when fluid flows in and out of the formation due to changes in effective stress and porosity. If not interpreted correctly, both events can cause considerable nuisance or challenges to drilling operations. If mistaken as actual gains or losses, adjusting drilling fluid density can worsen the situation. An MPD system can mitigate these events through precise monitoring of the well and by applying a CBHP to prevent the phenomena.
Check out: What Are Undrillable Wells in 2025 and How to Make Them Drillable
Gains
In addition to the Breathing and Ballooning events, drilling through gaseous formation can be interpreted as taking a kick. Formation gas makes the mass-flow meter readings erratic, reducing the monitoring ability while drilling.
When dropping below the pore pressure, the well will take an influx from the formation. This could be because of losses and a resulting reduction in hydrostatic pressure, leading to the BHP becoming too low – or simply if the pore pressure is higher than predicted.
Taking an influx leads to a well control situation, unless using an MPD system with a riser closure device for influx management. Well control situations are time consuming and can be challenging. This applies especially for deepwater operations where margins are tight, and the high friction generated in the long choke line.
Early and reliable kick detection is essential for minimizing the severity of an influx. Detecting an influx early prevents escalation and reduces downtime. With CML MPD, kick detection is continuous and occurs in the well, eliminating uncertainty caused by rig motion and providing near-instant feedback via subsea detection sensors. The riser functions as a trip tank and the pressure sensors are always monitoring, even when there is no circulation.
As with losses, a CBHP approach reduces the likelihood of the event occurring altogether. An MPD system can control and circulate out an influx within given limitations and avoid a conventional well control situation. This is particularly important for deepwater operations. With a Dual MPD system two influx management options are available. Both options offer improved margins, enabling larger influxes to be controlled and circulated out without breaking down the formation.
Read more: What Are the Applications of MPD
Differential sticking
Differential sticking is when the pipe is pressed against a part of the wall of the well. The part of the pipe in contact sees only the reservoir pressure, which is lower than the wellbore pressure. This pressure imbalance causes the rest of the pipe to push toward the lower pressure area, leading to a stuck pipe.
Stuck pipe caused by differential sticking can be difficult to free, as jarring is usually ineffective due to the high pressure holding it in place. The usual approach for freeing differentially stuck pipe is reducing the BHP in the well by reducing the density of fluid. However, this process is time-consuming and pose the risk of going below the pore pressure.
With Surface Back Pressure (SBP) MPD, the BHP during the displacement can be controlled by applying back-pressure. With CML MPD you don’t need to displace the well at all.
Wellbore collapse
Mechanical collapse is more common in deviated sections and occurs when the pressure inside the well is too low compared to the stress around the wall, resulting in formation breakdown.
Formation breakdown can also be caused by fatigue cycling of the wellbore – continuously changing the pressure in the well when switching between static and dynamic conditions.
Finally, leaving an open hole exposed for too long can cause it to collapse.
There are several methods for dealing with wellbore collapse, including increasing the BHP by increasing the fluid density and adding wellbore strengthening material. However, with increased fluid density there is a risk of going on losses. A CBHP approach with MPD will be required when the operating window is too narrow.
MPD also eliminates the fatigue cycling when operating with a CBHP. With CML MPD, tripping in and out is faster and safer, reducing the time the open hole is left exposed.
Discover: MPD Methods: Pros and Cons of CML vs SBP
Conclusion
Many unplanned events are pressure related. If the pressure in the well is too low or too high, issues like losses, influxes, or wellbore instability may occur. The actual well conditions are not always as estimated, due to depletion or unpredictable subsurface conditions.
Being able to precisely adjust the BHP up or down according to actual well conditions, prevent the BHP from fluctuating, and receive early notification when an undesirable event occurs will help resolve the issue faster – or avoid it altogether.
