RCD provides dynamic closure of the
annular while drilling and rotating the drill string and simultaneously
redirects flow to the MPD choke manifold. The technology is based on applying
an advanced compound sealing rubber against the drill string or Kelly surface,
which provides an effective seal while allowing vertical movement of the pipe.
MPD choke manifold is a critical component of the MPD equipment. It creates the
variable flow restriction that controls the wellhead pressure, which in turn
maintains relatively constant BHP in both static and dynamic conditions. The
choke in MPD conditions is used to control well pressure. Unlike in
conventional drilling methods, MPD chokes are not used as a secondary
well-control equipment. MPD is intended to avoid continuous influx of formation
fluids to the surface. Any influx that can possibly occur can be countered by
the additional surface backpressure obtained at the choke. This means that a
choke in MPD operations is used for pressure control and less for flow control.
Choke manifold must have the same pressure rating as the preventer stack. The
choke system is part of the drilling equipment and should not be considered as
a part of the well-control equipment, and the RCD is a diverter and not a
blow-out preventer (BOP). Choke alone can be operated manually or
automatically. In automatic MPD choke systems, after an influx is detected, no
change in flow rate is required. The choke automatically closes to increase the
backpressure at the surface until the influx is controlled. After the influx is
controlled, the annular surface pressure is controlled to circulate the influx
out of the well.

 

Regarding the MPD process we have
two main categories:

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1.     Reactive MPD, and

2.     Proactive MPD.

Reactive MPD is described as when
attempting to drill with a conventional drilling method casing set points and
fluids program, the drilling program is tooled up at least with a RCD, choke
and perhaps drill string float valve. This tool layout enables more efficient
and safe way to deal with unexpected downhole pressure environmental limits,
e.g. lower or higher pore pressure or fracture pressure than expected. In
Proactive MPD the fluids and casing programs are designed from the beginning
with a casing, fluids and open hole program that takes full advantage of the
ability to more precise management of the pressure profile along the annular.

Except of these two main categories
we have numerous variations, and these are:

1.     Constant Bottom Hole Pressure- CBHP,

2.     HSE MPD,

3.     Pressurized Mud Cap Drilling MPD,

4.     Deepwater Dual Gradient MPD,

 

2.1. Constant Bottom Hole Pressure MPD

This variation is usually considered
when offset wells have often shown kick-loss scenarios, and when a lot of well
control procedures were implied on the offset wells due. Reason for that is the
narrow mud window. Typically, such tight mud windows manifest themselves first
by loss of returns immediately after the circulation started after pipe connection.
If the well wasn’t kicking in a static condition during connection this
indicates the ECD as a result of restarting the rigs mud pumps has exceeded the
formation pressure. This event describes the loss portion of kick-loss
scenario. Instinctively, the density of the mud is lowered to avoid the fluid
losses and formation breakdown. But it is important to emphasize that by
lowering the density of the mud there is a higher possibility that the kick
might occur during static conditions. This is the kick portion of kick-loss
scenario. CBHP method determines the BHP as following:

               BHP= Hydrostatic mud weight +
ECD + Backpressure

Understanding the relationship
between the rig pump rate and applied surface backpressure, via RCD and Choke
system, a constant bottom hole pressure can be achieved and maintained while
drilling, tripping or making connections.

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