Unplanned drilling events can occur when actual well conditions differ from what was expected during planning. Events such as losses, influxes, differential sticking and wellbore instability may increase operational complexity, particularly in wells with narrow operating margins.
This article looks at how these events are typically managed and where MPD can support through pressure management and operational response.
Many unplanned drilling events are related to the operating window. If the Bottom Hole Pressure (BHP) becomes too high, the formation may fracture and lead to losses. If the BHP becomes too low, the well may take an influx or experience wellbore instability. In wells with narrow pressure margins, small pressure variations can create operational challenges, particularly during transitions between static and dynamic conditions.
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 can help reduce the likelihood of losses by allowing the BHP to be adjusted 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 enable 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.
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Influx Detection and Management
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 conventional well control situation, unless using an MPD system with a riser closure device for influx management. Conventional well control situations are time consuming and can be challenging. This is particularly relevant in deepwater operations, where narrow pressure margins and friction effects in long choke lines can complicate influx management.
Early and reliable kick detection is essential for minimizing the severity of an influx. Early influx detection can help reduce the severity of the event and support earlier operational response. 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 can help reduce the likelihood of an influx occurring. An MPD system may allow the influx to be managed and circulated out within the operational limits of the system, potentially avoiding conventional well control procedures. 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.
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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, the BHP can be adjusted by changing the riser fluid level rather than displacing the full well volume and release the stuck pipe faster.
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 can reduce pressure fluctuations associated with transitions between static and dynamic conditions when operating with a CBHP approach. With CML MPD, tripping in and out is faster and safer, reducing the time the open hole is left exposed.
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Many unplanned events are pressure related. If the pressure in the well becomes too high or too low, issues like losses, influxes, or wellbore instability may occur. Actual well conditions are not always as estimated, due to depletion, formation variability or other subsurface uncertainties.
Being able to adjust the BHP according to actual well conditions, reduce unnecessary pressure fluctuations and detect changes in the well early can support faster operational response and improved pressure management during drilling operations.