Integrating Managed-Pressure Drilling Into HP/HT-Well Planning
You have access to this full article to experience the outstanding content available to SPE members and JPT subscribers.
Managed-pressure drilling (MPD) is an increasingly common technique in narrow-margin high-pressure/high-temperature (HP/HT) wells. However, MPD is sometimes viewed as a bolt-on technology, only added after much of the planning work has been carried out and all other alternatives have been exhausted. The decision to use MPD should be made at the earliest stage of well planning. An early commitment to integrate MPD into an HP/HT drilling operation can make MPD more than just an enabling tool and turn it into a performance tool that offers significant operational benefits.
Project 1. For an HP/HT field redevelopment in the UK North Sea, seven sidetrack wells were drilled from a jack-up over an existing platform. MPD was used from the beginning, but the full benefits of MPD were not realized until later in the project. MPD was initially included to hold surface backpressure (SBP) across two connections to mitigate borehole-stability risk in an unstable shale. However, once the system was operational, further benefits continued to be identified and the scope of MPD operations grew significantly. At the same time, more knowledge of the subsurface was being gained, and it became clear that some of the planned wells would not be drillable without MPD because of a significantly reduced drilling window in the overburden. MPD became a key part of the design philosophy for later wells, where managed-pressure techniques were used to set and cement casings significantly deeper than would have been possible conventionally. MPD successfully mitigated classical HP/HT issues so that problems with losses and gains, handling of elevated gas levels, and mud-weight-selection issues were all virtually eliminated.
Project 2. For a standalone HP/HT exploration well drilled from a jack-up offshore the Netherlands, MPD was initially selected because of the extremely narrow margins between the pore and fracture pressures in the 8½- and 6-in. sections. Conventional drilling would not have been possible without inducing losses and spending significant time circulating to adjust mud weights, as seen in a key offset well. Taking advantage of the learnings from Project 1, MPD was selected at an early stage of the planning process and was fully integrated into the well design and operations. Mud weight, drilling parameters, and bottomhole-assembly design were all tailored to suit MPD operations. Integrated HP/HT and MPD procedures were developed on the basis of traditional HP/HT techniques but were modified to make full use of the additional functionality provided by MPD. A combined HP/HT and MPD training course was developed, and key personnel were trained in HP/HT and MPD techniques. Rig modifications were made well in advance, and MPD was implemented one hole section earlier than required to familiarize all crews with the new equipment and procedures. The result was a potentially extremely challenging well that was drilled without any of the conventional HP/HT problems observed in the offset wells.
Synergies Between MPD and HP/HT
The initial driver for MPD is typically a need to navigate narrow-margin sections, characterized by a small window between pore pressure and fracture gradient. This makes it a natural candidate for HP/HT wells, where narrow margins are the norm. However, once MPD is introduced to an HP/HT project, further synergies can be found wherein MPD provides a solution to other traditional HP/HT challenges.
Well Design. MPD enables narrower margins to be drilled safely. It can also enable drilling deeper into pressure ramps, so that casings can be set deeper. But there are other, less-obvious benefits from MPD that can improve well design. To realize these, the decision to use MPD needs to be made early in the design phase. MPD allows the design envelope to be extended in the following ways:
- MPD allows drilling in a narrower margin between pore and fracture pressure.
- If managed-pressure-assisted cementing is used, tight-clearance equivalent circulating densities (ECDs) can be managed and it may be possible to eliminate underreaming from the well design.
- MPD can lead to a step change in well design by allowing a string to be removed.
Mud-Weight Selection. MPD removes reliance on mud weight as the sole element of the primary barrier, instead replacing it with a combination of mud weight, annular friction pressure, and SBP. Because SBP can be controlled easily, the criticality of selecting and maintaining the correct mud weight is reduced significantly.
When planning an MPD operation, there are three possible mud-weight strategies:
- Mode 1: Mud weight is greater than pore pressure. Use an overbalanced mud weight. If SBP is lost, the well will not flow. No SBP is held while drilling. SBP is applied on connections to replace ECD only.
- Mode 2: Mud-weight ECD is greater than pore pressure. Use a statically underbalanced mud that, in combination with ECD while drilling, overbalances pore pressure. If SBP is lost while drilling, ECD will prevent the well from flowing. If SBP and pumps are both lost together, there is potential for the well to flow. Little to no SBP is held while drilling. SBP is applied on connections to replace ECD.
- Mode 3: Mud weight+ECD+SBP is greater than pore pressure. Use a statically underbalanced mud that maximizes the operating window of the MPD system. Select a mud weight so that SBP is 60 to 80% of the rotating-control-device pressure rating on connections. SBP is also applied while drilling. If SBP is lost, there is potential for the well to flow.
Of these, Mode 3 provides the greatest operational flexibility and will deliver the most value. For narrow-margin wells drilled with an overbalanced mud weight (Mode 1), significant planning effort is needed to minimize ECD through mud rheology, casing design, drillstring design, and reduction of drilling parameters.
If a statically underbalanced mud is selected in combination with MPD (Mode 3), ECD is no longer an obstacle and planning efforts can be focused elsewhere. Planning must instead focus on the reliability of SBP as part of the primary barrier. Contingency procedures must be made for handling foreseeable issues such as reduced circulating times, mapping of subsurface boundaries, and optimization of mud weight for tripping.
HP/HT Well Control. MPD can be used to improve the robustness of the primary well-control barrier, and, if integrated into well-control procedures, can improve the way influxes are detected, controlled, and subsequently circulated out. MPD well control is a separate topic in its own right, but the following points provide a summary of some of the ways MPD can improve HP/HT well control.
- MPD can detect kicks earlier.
- With MPD, an influx can be controlled by increasing SBP in stages until flow out is equal to flow in. This can be done while continuing to circulate down the drillstring.
- After control of the influx has been established and its volume has been assessed, the influx may be circulated out using the MPD system without needing the rig’s choke manifold. By using the MPD choke, better control of bottomhole pressure is maintained during the driller’s-method circulation, and, if the rotating-control-device pressure limit allows, the string can still be rotated.
- Drilling with applied SBP in combination with accurate flow monitoring allows losses to be identified early and bottomhole pressure to be reduced quickly.
Keys to MPD Implementation
Drilling-Contractor Buy-In. The drilling contractor should be involved as early as possible. The drilling contractor will need to understand how MPD works, the system’s advantages and limitations, and why it is being used. The contractor may need to deviate from their own well-control standards or procedures, will need to agree to rig modifications, and will need to sanction tie-in to, and use of, their equipment.
Rig Selection. MPD can be implemented on almost any rig, but some rigs lend themselves to more-efficient MPD operations than others. Selecting the right rig can have a significant effect on performance. If an MPD-ready rig with previous MPD experience can be contracted, this will reduce rig-modification and crew-training requirements. Pit space, deck space, substructure space and blowout-preventer top flange, rig alignment over the well, and MPD friendliness of the drillpipe are all factors to consider as well.
MPD-System Functionality. It is important to design an MPD system with the right functionality to suit the application. It is often tempting, during the rig-modification phase, to reduce functionality to reduce cost or perceived complexity. However, a higher level of functionality often proves valuable, particularly when the subsurface conditions differ from prognosis.
Integrated HP/HT and MPD Drilling Procedures. To take advantage of improved techniques offered by MPD, and to ensure safe and efficient operations, MPD needs to be fully integrated into a set of combined HP/HT and MPD procedures. This can be written as a project-specific document, requiring input from the operations team, design team, drilling contractor, and MPD contractor.
Integrated HP/HT and MPD Training. The level of training required for an HP/HT MPD operation depends on the level of experience the rig and operator have and the operational risks. Highquality training is rarely a poor investment. A carefully designed, project-specific training course will reduce the risk of major incidents and will improve operational performance. Experience from previous MPD projects has shown that high-quality training results in crew buy-in and a common understanding of the MPD-system capabilities.
Integrating Managed-Pressure Drilling Into HP/HT-Well Planning
01 April 2018
Some technologies important for HP/HT have steadily developed and have seen increased uptake even throughout the downturn. These technologies have the potential to revolutionize the performance and safety of HP/HT drilling operations.
Successful Recovery and Stimulation in a Long HP/HT Horizontal Well in One Intervention
The case history presented in the complete paper describes the performance of an acid-fracturing intervention in an HP/HT well in which this intervention was the last procedure considered to evaluate the productivity of a Marrat Formation well.
Ultradeep-Field Study: Extreme-Underbalanced HP/HT Coiled-Tubing-Conveyed Perforating
An early commitment to integrate MPD into an HP/HT drilling operation can make MPD more than just an enabling tool and turn it into a performance tool that offers significant operational benefits.
Don't miss out on the latest technology delivered to your email weekly. Sign up for the JPT newsletter. If you are not logged in, you will receive a confirmation email that you will need to click on to confirm you want to receive the newsletter.
16 April 2018