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Managed Pressure Drilling

...MPD Pressure Control
...MPD Open Circulation System
...MPD Closed Circulation System
...MPD Operational Considerations

Closed Circulation System
In a closed circulation system, when the rig pumps are on, the fundamental pressure equation is

BHPdynamic = Pstatic + Pafp + Pbp

where Pstatic and Pafp are static and annular frictional pressure respectivel, and Pbp is the surface backpressure applied to the annulus by pressure control equipment connected to the wellhead.

This equation applies when the rig mud pumps are on and mud is circulating.

Under static conditions when the rig pumps are off Pafp = 0 and BHPdynamic becomes

BHPstatic = Pstatic + Pbp

In a closed system the backpressure term, Pbp, is always present during a connection and depending on the MPD application, while drilling, during tripping, even during unpanned stoppages for rig repairs.

It is through control of the backpressure that the BHP can be maintained at a constant value from dynamic to static conditions, i.e. from pumps-on to pumps-off without raising the static mud weight.

However, not all constant bottomhole pressure applications require dynamic and static BHP to be equal just as long as they are both within the specified safe operating limits.

A closed circulation system -

...is one in which the wellhead is sealed off and surface backpressure is applied to the fluid in the annulus by restricting its flow out of the well through a choke.

One of inherent risks of an open circulation system that a closed system mitigates with backpressure control is wellbore instability induced by repeated pressure charging and relaxation associated with pumps-on and pumps-off.

In a closed system, when the rig pumps stop, the choke manifold closes to increase the backpressure and compensate for the loss of the annular frictional pressures.

In this way the system maintains the static BHP at or very close to the level of the dynamic BHP. In other words, the system maintains a constant BHP within the limits of the setpoint.

Closed system backpressure control expands drilling’s ability to control the BHP by expanding its ability to manage static and annular frictional pressure and by giving it the additional ability to apply and manage backpressure.

All MPD systems that provide constant BHP rely on a rotating control device (RCD) as the primary wellhead pressure seal - mounted below the drill floor and above the annular blowout preventer (BOP). The piping and instrumentation drawing illustrated below shows the location of the RCD and the typical MPD piping connections in a closed system.

MPD systems designed to maintain constant BHP manage annulus backpressure with a fit-for-purpose choke manifold connected to the RCD. Choke systems for CBHP will differ from each other by, among other things the:

Control method - manual, semi-automated, automated
Integration of choke control and hydraulics model
Use of pressure while drilling (PWD) data to calibrate the hydraulics model
Real-time capability and speed of the hydraulics model
The ability to create backpressure with an independent backpressure pump

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