As stated previously the main
components of the MPD system are RCD, choke and choke manifold, flow meter, and
surface gas separator and the MPD pump that provides the surface back pressure.
Each of those parts has its flaws and limitations that are going to be defined
in the following chapters.

3.2.2.1. RCD limitations and problems

RCD is a device that is used to
provide the active sealing around drill pipes allowing simultaneous rotation
and linear movement of the drill string and diverts the fluid and cuttings flow
to the choke manifold and gas separator. RCD is usually installed above the
annular preventer of the conventional BOP stack. It is obvious that due to this
alignment with the BOP stack the available spacing with the drill floor is much
smaller. Due to the height on which the RCD has to be installed and the weight
of RCD provide very large risk to the rig personnel. Also for installation of
the RCD element, we need to have proper lifting devices to assure secure
operation. The main components of RCD are the sealing element and the bearing
element. Sealing element is the working part of the whole device because it is
in contact with the rotating and moving drill string and the pressures that come
to the wellhead that has to be circulated through the choke and the choke
manifold. This means that failure of the sealing element can result in leakage
of the hydrocarbons to the rig floor. Those are the dominant reasons for the
failures of the sealing element. Stripping of a drill string is a procedure in
which the drill string is run to the well by its own weight through the closed
preventer, in this case, the RCD. While stripping the weight of the drill
string should be larger than the sum of the friction through the preventer and
the force that influx impose to the drill string inside the well. Friction
between the drill string causes wear of the sealing element and because of
that, it has to be changed. Another parameter that dictates the durability of
the sealing element is the type of elastomer in use. If the elastomer is paired
with the nonmatching fluid type the failure of the component occurs very fast.
Every sealing element is also predicted for work in the specified temperature
ranges, and the temperature higher than specified causes a failure of the
working element. Also, the type of the drill pipes that are used and the type
of the rotation that is applied on the rig, Rotary Table or Top Drive, defines
the durability of the sealing element. Using the square Kelly for the rotation
from the Rotary Table to the drill string is not recommended due to the serious
wear to the sealing element caused by the edges of the Kelly. Bent drill pipe
can cause wear to the sealing element due to the uneven loading to the sealing
element. Last but not least, if the sealing element is exposed to the working
pressures that exceed its rating, in that case, the sealing element loosens up.
Because of that, the leakage can occur at the pressures lower than the rating
pressures of the sealing element.

3.2.2.2. Choke limitations and problems

Choke manifold and chokes are used
to open, fully block or throttle the fluid flow through the choke line. They
are the main components that allow exerting a certain amount of surface
backpressure needed to control downhole conditions. Types of chokes usually
used are a manual choke, semi-automated choke, and PLC choke. Using the
statically underbalanced fluid main advantage is substantially higher ROP
comparing to the conventional drilling. Maximum allowable
overbalance/underbalanced pressure drop is defined by the pressure ratings of
the mud separator connected to the MPD choke manifold and the pressure of the
formation that is being drilled.  But the
consequence of the underbalanced pressures especially while drilling is the
possible influx and much higher concentration of cuttings inside the drilling
fluid. With the higher concentration of the cuttings in the drilling fluid,
there is a large possibility for the increase of density of the drilling fluid
that is used. If that is the case, then we are not avoiding one problem that we
wanted to avoid in the first place and that is the high equivalent circulating
density. Secondly, if we are drilling with high ROP through the gaseous zone we
can cause the kick due to the liberation of the gas from the cuttings being
drilled. This will cause even larger mud density gradient drop and invite more
influx into the wellbore. Last but not least increased concentration of the
cuttings causes wear of drill pipes and operating choke. If the choke is worn down,
then the operators are not able to achieve the wanted amount of backpressure
while drilling. This results in larger amounts of influx that can be hard to
operate and instead of continuing the drilling procedure, we need to start the
well control operation. Also, with the highest concentration of the fluid,
there is a high possibility of cuttings beds formations and plugging of the
choke lines. The reason for that is the enlargement of the flowing area after
the choke. Inside the larger area fluid flows slower and because of that, there
is a possibility of deposition of the cuttings in the choke lines. This also
triggers additional pressure drop in the choke like while circulating the
fluid.

 

 

4. Solutions Regarding Conventional Drilling Problems and MPD problems

In this chapter the solutions for
conventional drilling problems as well as for the MPD drilling problems. First,
the solutions for the conventional drilling problems and limitations will be
given and followed by solutions for MPD problems and limitations.

4.1. The reduction of the Invisible Lost Time

As mentioned before the main cause
of Invisible Lost time is the high mud density which is used in the drilling
program to secure the well and avoid any influx. The main advantage of the MPD
technology is the ability to use a certain amount of surface backpressure to
manage the values of bottom hole pressure. Surface backpressure can only be
applied if the circulating system is not open to atmosphere, as it is the case
with the MPD. Two main components that facilitate the application of the
surface backpressure are the RCD and the choke within the choke manifold. The
RCD provides the dynamic closure of the well while rotating and circulating and
simultaneously redirects the flow through the choke. The size of the choke
opening dictates the amount of surface backpressure that is applied at the
moment. The advantage of this method is obvious. The system allows active
management of the Bottom Hole Pressure by adjusting the choke opening. In that
way, we are able not only to react to the kick situations. It is possible to
react to the loss of fluid by opening the choke even more.  There is no need to stop the drilling process
to weight up the mud or to lower the density of the mud. In this way, by active
management of annular pressure profile, we are able to extend the sections of
the well that are being drilled by the same bit and use a smaller number of
casing strings to reach our target. Because there is no weighing up of the mud
by using the inert solids we are reducing the torque and drag in the system
decreasing the erosion of the tools inside the hole. Last but not least, by
surface backpressure adjustments the ROP of the system can be increased and in
that way the Invisible Lost Time is decreased.

4.2. Solutions of the well control issues

 In conventional drilling, the only way to
counteract the subsurface pressure is through the mud weight of the drilling
fluid or increase of the pump rate. The largest problem of the well control
practices is if the High-Pressure Low Volume gas zones are encountered. They
are usually encountered in the shallow zones and the problem is if the mud
weight of the fluid is increased significantly then there is a large
possibility to cause the fracture of the rocks. If the rock is fractured then
the lost circulation occurs, our fluid level in the well drops, bottom hole
pressure decreases and as a result, even larger kick occurs. This phenomenon is
called the Ballooning effect. The MPD method provides a solution to the problem
of the High-Pressure Low Volume gas zones. The main focus of this technique is
to de-energize the gas bubble by allowing the gas to expand in the well. The
expansion of the gas is allowed and controlled by the management of the surface
backpressure and continuously monitored by the flowmeter. This procedure causes
the depletion of the high pressured zone, and when the zone is depleted it is
much easier to control it. The second advantage is that the kick, due to the
closed circulating system, can be recognized much faster. If the kick is recognized
as fast as possible that means that is will be much smaller and it will be much
easier to circulate out the kick. The kick is usually recognized by the precise
flow meter, usually Coriolis flow meter.

4.3. Cost reduction by implementation of the MPD

The main source of cost reduction
regarded to the usage of MPD technology in comparison to the conventional
drilling technology is the reduction of total time needed to drill a well and
reduced number of additives used for mud conditioning. Increasing the ROP the
total time for drilling a well is decreased. Consequently, we need a smaller
number of days to drill a well so the cumulative costs of daily rates for the
rig are smaller. The number of bits used is also reduced, and the number of
tripping operations is reduced significantly. Regarding the drilling mud, the
sources for cost reduction are numerous. Because we are applying a desired
amount of backpressure there is no need to weight up the mud. Certain additives
like polymers, barite, and others are not added to the system and because of
that, the costs from additives addition standpoint are reduced. Lost fluid
problems can also be avoided by the certain reduce of the surface backpressure,
a there is no need for an injection of the LCM materials or installation of
cement plugs to overcome the fluid loss zones. Since the smaller number of
casing strings are used, less time is spent on making the connections, running
the casing strings and waiting for cement, and those are also a source of cost
reduction in comparison to the conventional drilling.

4.4. Planning of the drilling program to avoid the wear of the RCD’s
sealing element

The RCD is a working element that
closes the annular when we are using the Managed Pressure Drilling Technique.
It is installed upon the annular preventer of the conventional BOP. Due to the
weight of the RCD and the height on which it is installed, the process of
installation of the RCD element can be very dangerous for the rig crew. Due to
the possible influx that has been circulated it is possible that certain amount
of gas was trapped in the BOP stack, so additional attention has to be
considered while performing this kind of actions. It is recommended that only
trained personnel should perform these operations. Change of the sealing
element can also cause fatal consequences so the best case is when the change
of the sealing element is performed without any personnel near the RCD. It is
possible to change the sealing element without the presence of the rig
personnel by utilization of the special hydraulically operated clamping
systems. This system opens the clamps. The system is operated from the control
panel. It remotely relieves the pressure from the RCD. After that, the sealing
element is also remotely released and raised through the rotary table on the
drill pipes. After it is fixed or changed the sealing element is put onto the
drill pipes, put back through the rotary table into the RCD bearing element and
remotely put back in the place by the commands from the control panel. If we
are drilling with the underbalanced fluid system, then we have to define a
special procedure that solves the problems when hydrostatic pressure of the
fluid in the well is not sufficient to control the influx into the well. Those
actions are:

1.     Flow check

2.     Hold of the pressure while changing
the RCD element

3.     Circulation of the background gas

4.     Tripping in and out of the hole

Because the system is closed to the
atmosphere, the direction of the fluid discharge goes through the choke and
choke manifold. The additional line of discharge from the flow meter to the
trip tank should be added to be able to perform the flow check. Special MPD
pump has to be installed so that the backpressure can be applied to the bottom
of the hole. This pump withdraws the fluid from the trip tank so the best
solution, in this case, is to perform the flow check with dual compartment trip
tank. If we want to hold the pressure while we are changing the RCD element or
just on part of it, the easiest way to trap the gas is by closing the annular
or pipe ram on the BOP stack under the RCD. This action provides security to
the all rig personnel. Tripping in and out of the hole when is specially
complexed when statically underbalanced fluid is being used. To perform
stripping operation pipe weight should be larger than the sum of the friction
through the wellbore if the well is deviated or horizontal, friction through
RCD sealing element and the pipe uplifting force during when the influx is
inside the wellbore and the acting surface backpressure. At one point when
running out of the hole, we will have the pipe light situation. To solve this
situation a drill string safety valve should be installed so that we can pump
the weighted fluid down the drill string into the hole to kill the well. Another
solution is to inject a heavy pill in the open hole of even into the cased
section. The last solution for the pipe light situation is the snubbing unit,
of course, if it is available.

4.5. Choke problems solutions

Including the sealing
element of the RCD, it is the vital part of the MPD equipment. Because it is
constantly used during the whole process, it is exposed to the high pressures
and erosion due to throttling of the fluid flow from the well. Equipment
thorough which the backpressure can be achieved is either the BOP stack chokes
or the special MPD choke and kill Lines with appropriate MPD pump. This system
allows the application of the surface backpressure without wearing the BOP
stack system. Part of the system that can get plugged is the flow meter, choke
or choke line. The result of plugs is higher pressure drop in the system that
can cause the fracture of the formation. To solve this problem redundant pipes
and chokes and relief valves have to be installed. The following picture show
classic full redundant MPD system.