Maris International Ltd.
Continuous Circulation Drilling
Drilling problems
minimised or eliminated
Continuous Circulation
Drilling
Improved
safety & production
1. “Continuous Circulation” Drilling
compared with “Stop/Start” circulation + MPD
The circulating mud pressure (or Equivalent Circulating Density – ECD) in the
well bore between the bit and the last shoe is intentionally kept below the
Fracture Pressure and above the Pore Pressure of the exposed formation.
(This ‘Frac Pressure/Pore Pressure window’ is discussed under section 2.6).
For offshore wells, particularly in deep water, the circulating mud pressure in
the well bore at, or near, the last casing shoe can approach the formation
Fracture Pressure as the circulating mud pressure at, or near, the bit
approaches the Pore Pressure, requiring casing to be run before drilling
further.
Pressure
Connections
etc
Temporary
surge
Temporary
swab
Depth
Circulating
pressure
Static
pressure
“Stop/start circulation” drilling
When drilling conventionally, before each
connection is made, the mud circulation
is stopped and the mud pressure in the
uncased
borehole,
drops
rapidly,
generally overshooting balance by
several hundred psi before rising to the
static level. This negative pressure
surge, or swab, may induce flow from the
formation if it falls below the formation
pore pressure.
After a connection is made, circulation is
re-started and the mud pressure in the
borehole,
rises
rapidly,
generally
overshooting by several hundred psi,
before decreasing to the circulating level.
This surge pressure may exceed the
fracture pressure anywhere in the
exposed wellbore
In conventional drilling, this “stop/start” cycling of pressure across the full
length of the exposed formation, occurs every 30, 60 or 90 ft of hole being
drilled. With each connection the exposed formation is repeatedly
depressured before being “pumped up” or supercharged before normal
circulation pressure is re-established.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
Clearly, the solution is to keep the circulation of drilling fluids steady and
continuous, throughout the drilling of each section, or to try to compensate
for these wild pressure variations each time circulation is stopped.
Until recently, continuous circulation was not commercially available. As a
result several competing Managed Pressure Drilling (MPD) technologies
came on the market to try to maintain a steady/constant downhole pressure,
while circulation was stopped to make connections.
However, none of these commercially developed MPD techniques,
completely compensate for the pressure variations when circulation is
stopped, as will be explained later.
The solution
Pressure
Connections
etc
.
Depth
Circulating
pressure
“Continuous Circulation” Drilling
Ideally when drilling, circulation of the
drilling fluid should be continuous.
This is now commercially possible
with the Continuous Circulation
System (CCS). Located on the rig
floor, the CCS safely maintains
uninterrupted circulation to the drill
string, at up to 5,000psi, while the Top
Drive is disconnected. Connections
are made or broken within a pressure
chamber
constructed
from
conventional blow out preventer
components, using “Iron Roughneck”
technology
With Continuous Circulation, the
circulating pressure in the borehole
(the ECD) can be carefully controlled
to stay within narrow “Frac Pressure /
Pore Pressure” windows.
Additionally, with uninterrupted flow, the fluid dynamic regime established
while drilling, is kept “Steady State”. The annular flow, mud temperature and
density, hole cleaning, mud processing and cuttings mobility are all constant
and stable.
Consequently any small change in the returning flowrate, caused by
fracturing, or the beginning of lost circulation, or a kick, is far easier to detect
and respond to.
For the driller, this improved well control and safety is significant.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
Top Drive
Pressure
Mud
in
Mud
out
Coupler
a
b
Mud
Subsea BOP
Pressure in
drill string
Cased
Hole
Fracture
Pressure
Last
g
e
Hydrostatic
Pressure
Pressure in
annulus
Pore
Pressure
Exposed
Formation
Pressure in drill string and annulus
due to static pressure only
Bi
f
d
c
Depth
Stable Pressure Gradient across the Exposed Formation
when Drilling with Continuous Circulation
‘Steady State’ pressure, flow, turbulence, ECD, hole cleaning, cuttings mobility,
temperature and density, plus increased safety and subsequent production.
Managed Pressure Drilling (MPD) techniques may be used to partially
compensate for the loss of pressure and ECD, when circulation is
stopped.
To maintain the down hole pressure, drilling fluid has to be pumped into the
annulus against a closed drill string, and the annulus has to be ‘modelled’ to
calculate ‘how much’ to pump. The accuracy and reliability of the computer
model, which has to be continuously ‘built in real time’, is a significant
challenge. Circulating continuously avoids most of these complications and
has several other important benefits.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
Fracture
Pressure
Downhole Pressure Maintenance
If the section has been drilled to its limit,
the circulating annular pressure, (d to e),
is, typically, already close to the frac
pressure near the last casing shoe (e) and
close to the pore pressure near the bit (d).
e
Pressure in
annulus, when
circulating
Pore
Pressure
Static Annulus
When circulation stops, the static mud
pressure gradient across the exposed
formation is lower than the dynamic
pressure gradient.
In this situation, most MPD providers aim
to keep the bottom hole pressure steady
(at d), by imposing pressure on the
annulus at the surface, resulting in the
pressure at, or near, the last casing shoe
(at Y) being greater than when circulating
and may enter the frac pressure zone.
d
Fracture
Pressure
Y
Pressure in
annulus, when
static & back
pressured
e
Pore
Pressure
Alternatively, if the pressure at the last
casing shoe is kept steady, the pressure at,
or near, the bit (at Z) will be less than when
circulating and may enter the pore
pressure zone.
Although the dynamic pressure loss up the
annulus, across the exposed formation,
when circulating, is small, it is nevertheless
quite significant and, generally, beneficial.
MPD methods do partially compensate for
pressure loss, if and when circulation is
stopped; and so assist in minimising the
occurrence of several drilling problems
But MPD methods do not compensate for
many of the disadvantages of “stop/start
circulation”, including reduced safety.
d
Fracture
Pressure
Pressure in
annulus, when
static & back
pressured
e
Pore
Pressure
d
Z
Stopping and re-starting circulation, when making connections, has been
a major cause of most typical drilling problems, ever since rotary drilling with
jointed tubing, replaced cable drilling.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
Continuous Circulation Drilling
Continuous Circulation
Drilling
Drilling problems
minimised or eliminated
Improved
safety & production
16 Drilling Problems :Minimised or Eliminated by Continuous Circulation
Minimised Problems:
Eliminated Problems:
Formation Fracturing
Surging on stop / start circulation
Lost circulation
Formation pumping & Balooning
Differential Sticking
ECD variations
Stuck Pipe
Well de-stabilisation in UBD
Slugging of Cuttings Returns
Static Cuttings Settlement
Narrow Pore/Frac pressure windows
Connection Kicks
Lengths of sections in ERD wells
Disconnecting mud from drill string
Bit wear / ROP / Surge & swab
P & T variations in HPHT wells
“Why on earth, for a hundred years, we stopped the circulation of drilling
fluids, every time we wanted to make a connection; and disconnected the
mud from the drill string just when connection kicks could occur, is an
enigma.
The answer is that we had no means of maintaining circulation; until now,
when the technologies of BOPs, Iron Roughnecks, Snubbing and
Programming have come together to give us a commercially viable, reliable
and safe mechanised system to maintain continuous circulation throughout
the drilling of each section.”
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.0 Introduction to Drilling Problems
Caused by ‘Stop/start’ Circulation
NPT (Non Productive Time)
Much drilling NPT is attributable to Lost Circulation, Kicks, Sloughing, Stuck
Pipe and Differential Sticking, all of which are largely caused by
stopping/starting the circulation of drilling fluids.
Continuous Circulation Drilling or Stop/start circulation + MPD
MPD (equipment, techniques and real time computer modelling) compensate
partially for the loss in downhole pressure, when circulation is stopped.
However, MPD does not compensate for the other disadvantages of
“stop/start circulation” such as:- loss of steady state dynamics, adverse
decrease in annular pressure, cuttings settlement, rising temperature and
reducing density of the static drilling fluid and time taken to circulate out
cuttings from the uncased hole, before stopping circulation. Also, after
making a connection, circulating out the potentially unstable 3 phase annular
accumulations of gas, produced fluids or slugs of cuttings and debris, before
re-commencing drilling.
Adding MPD (Rotating BoP/annulus chokes) to Continuous Circulation
Drilling
Produces the additional benefit of being able to throttle the returning mud in
the annulus to maintain downhole pressure, whenever tripping out of the
hole. This avoids having to increase the circulation rate as the drill string is
withdrawn, or displace to heavier mud, to achieve the same result.
“The most trouble free drilling” is achieved, when the downhole ‘steady
state fluid dynamic regime’, which has been well established while drilling, is
left undisturbed throughout the drilling of each section. The hole is cleaned
continuously and the only variations detected in flow, pressure, temperature
and density, will be due to the formation being drilled.
The feedback, to the drillers, is therefore far more sensitive to downhole
changes and is safer.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.1 Formation Fracturing
Minimised by Continuous Circulation
The problem
Formation fracture occurs when the pressure in the well bore exceeds the
Fracture Pressure of the formation. The Fracture Pressure can be defined as
the hydraulic pressure imposed by the fluid in the wellbore at which the
formation breaks down and the fluid penetrates the surrounding formation.
Once fractured, the formation can absorb mud at a lower pressure than the
initial Fracture Pressure.
Controlling the circulating pressure in the annulus between the bit and last
casing shoe to navigate down a narrow frac pressure / pore pressure
corridor, is challenging; particularly, when stop/start circulation produces
such static and transient pressure variations as shown below.
Conventional Drilling
The solution
Pressure
Pressure
Connections
Connections
etc
etc
Temporary
surge
Temporary
swab
Depth
Circulating
pressure
Depth
Static
pressure
Circulating
pressure
“Continuous Circulation” Drilling
“Stop/start Circulation” drilling
Continuous Circulation maintains a steady ECD and pressure gradient
across the exposed formation surface. With the best MPD technique the
pressure gradient is supplemented by the applied annulus pressure at the
surface which can cause wellbore instability.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.2 Lost Circulation
Minimised by Continuous Circulation
The problem
If circulation fluid flows into the formation, this is experienced as a drop in the
returning mud flow termed ‘Lost Circulation’. Lost circulation, as the result of
formation fracture or drilling into a porous formation, is largely proportional to
the positive differential pressure of the circulating fluid over the formation
pressure.
‘Near wellbore’ damage to the recently drilled hole is often caused by
‘pumping up’ the formation with significant changes in annular pressure. This
can turn a reasonably ‘circular’ small diameter hole into a larger ‘irregular’
cavity, which is more vulnerable to further collapse.
Lost Circulation is expensive in mud usage and because of consequent
circulation pressure reduction, can induce a kick elsewhere in the borehole.
The solution
The downhole steady state fluid dynamics established while drilling, is best
left undisturbed. Stopping and re-starting circulation take the dynamics
through many changes in turbulence (quantity and location) and, in unstable
formations, inevitably erode the exposed formation wall, more than if
circulation is continuous and steady.
Continuous circulation therefore, provides the best conditions for establishing
a minimum filter cake and minimising erosion of the exposed formation.
The steady pressure, throughout the drilling of the section, is most likely to
minimise the loss of mud to the formation. The continuous return flow means
that any variation can be taken as an early sign of lost circulation or a kick
and the response can be quicker and therefore more effective.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.3 Differential Sticking
Minimised by Continuous Circulation
The problem
P2
P1
P2
P3
P3
Initial sticking
P2
P3
P1
P2
P3
Increased sticking a period of time
Where P1 is the Annular pressure and P3 is close
to the pore pressure and P1 > P2 > P3.
Differential Pressure Sticking – A
condition wherein the drill string
becomes stuck against the wall of the
hole and will not rotate. Conditions
normally
include
a
permeable
formation, a nearly impermeable (thick)
filter cake and a relatively high
pressure differential across the filter
cake between the well bore and the
formation. Once the pipe is embedded
in the filter cake, it is forcibly held
there.
‘Differentially stuck pipe’ is a major
cost, involving significant NPT (Non
Productive Time) on the rig. Attempts
to free a stuck pipe can result in
parting the drill string, followed by
fishing or side tracking, adding many
days to the well construction operation.
During such attempts, any mud
pressure reduction may induce a kick,
which may be difficult to contain if
circulation
is
interrupted
during
connections.
The lower section of the drill string –
usually the drill collars, is under
compression and in vertical wells is
picked up off bottom to place the
whole drill string in tension before
ceasing rotation. This minimises the
possibility of contact with the wall of
the hole.
The prime cause, though, is the effective differential (balance) pressure,
which includes both the mud hydrostatic pressure and the dynamic
circulation pressure head. Conventionally, this is set sufficient high to allow
the static pressure alone to control the well when circulation is stopped.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
The solution
Continuous Circulation allows the differential between the mud pressure and
the pore pressure to be kept continuously at the optimum pressure for drilling
the particular formation.
The lower ‘continuous circulation’ differential pressure, reduces the lateral
force on the drill string; and, provided the drill string is in tension before
rotation is stopped, the possibility of differential sticking is greatly reduced.
If, while drilling with continuous circulation the drill string still becomes stuck,
the differential pressure can be temporarily adjusted to underbalance the
formation, reverse the sticking force and free the pipe, allowing drilling to be
resumed without undue delay.
Opinions vary on the effect of circulation and rotation on differential sticking
and, certainly, the longer a stuck pipe is left without taking remedial steps the
more stuck the pipe becomes.
The general agreement is that the following actions minimise the possibility
of differential sticking - in order of importance:
a) Maintain a low differential (balance) pressure, while drilling.
(Possible with Continuous Circulation or Stop/Start Circulation + MPD)
b) Avoid ‘pumping up’ the formation with stop/start circulation.
(There are no pressure changes at all, with Continuous Circulation Drilling)
c) Maximise lubricity and minimise gel in the drilling fluid.
(This is achievable with Continuous Circulation Drilling)
d) Raise bit off bottom to ensure drill string is in tension, before stopping
rotation. (Continuous Circulation turbulence helps to keep the drill string
‘loose’ while stationary); and re-start rotation before returning to bottom.
e) Rotate drill string, slowly, at rotary table level, while it is disconnected for
lengthy periods of time. (Likely development for the CCS in future).
If differential sticking occurs while drilling, be prepared to go temporarily
underbalanced, to reverse the sticking force, and pull out of hole, without any
rotational torque. (Much easier to perform safely with mud continuously
circulating, in Continuous Circulation Drilling.)
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.4 Stuck Pipe
Minimised by Continuous Circulation
The problem
‘Stuck Pipe’ is another major cost in certain drilling formations, involving
significant NPT (Non Productive Time). ‘Stuck Pipe’ can refer to the bit,
collars, BHA (bottom Hole Assembly), or the drill pipe itself, all of which
become stuck for several different reasons:
Attempts to free stuck pipe can result in parting the drill string or backing off
intentionally and then having to fish or side track, adding many days to the
well construction operation.
‘Stuck Pipe’ may be caused by:
The borehole wall collapsing in friable or unconsolidated formations or
plastically deforming inwards and reducing the hole size, both of which are
encouraged by stopping/starting mud circulation, causing step changes in
the downhole pressure with surges in both directions.
Cuttings and debris settle in near vertical holes, when circulation stops
unless high gel strength drilling fluid is employed. However this can impose
even higher initial surge pressure on the well bore when circulation is
resumed.
In long laterals, the cuttings have only to settle a few inches to the lower
side of the hole when circulation and rotation cease. Re-starting circulation
and rotation only moves some of the accumulated cuttings up the hole
before stopping for the next connection. This results in the formation of piles
of debris or “weirs” at intervals, reducing the effective diameter and
increasing the potential for stuck drill collars, BHA or Bit.
The prime cause is insufficient and inefficient hole cleaning, aggravated by
stopping the circulation during connections.
The solution
By using a CCS, circulation is uninterrupted and the transport of cuttings and
debris continuous with no opportunity to settle and build up. The continuous
mud flow keeps cuttings moving and the hole clean while connections are
being made.
Continuous circulation does not ‘pump up’ the formation, or change the down
hole ‘pressure dynamics’, both of which can damage the borehole wall and
make it increasingly unstable.
The ‘steady state’ downhole pressure and flow conditions, maintained by
continuous circulation, are the best way to construct boreholes in sensitive
formations and will minimise the chances of stuck pipe.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.5 Slugging of Cuttings Return
Minimised by Continuous Circulation
The problem
Stopping/starting the mud circulation, inevitably causes significant variations
in mud density and content across the shale shakers, after restarting
circulation This is mainly due to cuttings and debris settling and the mud
heating up down hole when circulation stops.
To minimise this, cuttings are normally circulated back in the annulus above
the drill collars after drilling has stopped and before circulation is interrupted.
On restarting circulation the accumulated cuttings and debris have to be
mobilised before restarting drilling.
The solution
When circulating continuously, the transport of cuttings and debris is
continuous back to the shale shakers, with a steady temperature, minimal
density changes and no slugging.
Mud flow continues at a steady flow and temperature during tool joint
connections, cleaning the hole and increasing the efficiency of cuttings
separation across the shale shakers and the mud treatment process.
Since the mud flow is continuous, the gel strength can be reduced and the
chemical composition optimised to provide a thin, tough filter cake.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.6 Narrow Frac/Pore pressure windows
Minimised by Continuous Circulation
The problem
Pore Pressure and Fracture Pressure convergence can leave a very small
operating window, which is a major issue for MPD (Managed Pressure
Drilling). However all MPD techniques, computer programs and equipment
aim to keep the down hole pressure steady as circulation is stopped.
Unfortunately, the very nature of circulation is that there is a dynamic
pressure drop in the annulus between the bit and the last casing shoe.
There is therefore no MPD method that can maintain a constant pressure
across the exposed formation between the bit and the last casing shoe,
except continuous circulation.
The challenge is not just to keep the dynamic circulating pressure within the
Frac pressure/Pore pressure ‘window’. The gradient within the ‘window’ is
typically different from the circulating pressure gradient, particularly offshore
and in deeper waters.
As the circulating pressure gradient moves from close to the Frac Pressure
at the last casing shoe, to close to the Pore Pressure at the bit, (rarely vice
versa) the stage is reached where the section has to be cased. The casing
programme, particularly in deep water can involve an inconveniently large
number of cased sections, starting with overly large conductors and ending
with uneconomically small production strings.
The solution
By maintaining continuous circulation, during tool joint connections, the
circulating pressure in the borehole can be carefully set and controlled to
remain steady across the whole exposed formation from the bit to the last
shoe.
Additionally, because there is no change in the downhole pressure regime,
(static plus dynamic head and an established turbulent flow) the borehole
fluid dynamic situation can therefore be called truly ‘steady state’. This
enables an extended length of the each hole section to be drilled through
narrow frac pressure / pore pressure windows.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.7 Lengths of sections in ERD wells problem
Minimised by Continuous Circulation
The problem
In lateral wells and Extended Reach Drilling (ERD) wells in particular, the
increase in circulating pressure at the bit, can be solely due to the increasing
dynamic (or friction) pressure component as the section extends horizontally.
The Pore and Fracture pressure may not vary much from the last shoe to the
bit but the circulating pressure at the bit increases with the length of the
section.
When circulation ceases, the mud pressure at the last casing shoe (and at
the bit) may be close to the pore pressure. When circulating, the mud
pressure at the bit will be approaching the frac pressure. The section length
will be limited by the difference between the Pore and Frac pressure. This, in
stop/start circulation, has to exceed the entire dynamic pressure drop from
bit to surface.
On long laterals, the settlement of cutting on the low side of the hole when
circulation ceases, can, in some formations, cause serious problems. In such
conditions, cuttings have to be circulated out of the uncased section, after
stopping drilling to make a connection.
The solution
By maintaining continuous circulation, during tool joint connections, the mud
pressure in the borehole can be carefully set and controlled to remain
steady across the whole exposed formation from the bit to the last casing
shoe.
In long laterals and ERD wells, continuous circulation allows each nearhorizontal section to be extended. As above, each section length will be
limited by the difference between the Pore and Frac pressure. Utilising
continuous circulation drilling, this only has to exceed the dynamic pressure
drop from bit to the last casing shoe. Consequently the section can be much
longer before casing is required, which is most valuable.
Continuous circulation allows the continuous transportation of cuttings
throughout the making of connections, eliminating any time for stagnation
and minimising cuttings settlement.
In lateral boreholes, the continuous ‘steady state’ mud pressure, flow,
temperature and density, provide the best means of constructing and
maintaining a long and stable open hole, before committing casing.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.8 Bit wear / ROP / Surge & swab problems
Minimised by Continuous Circulation
The problem
An extensive range of bits has been developed to drill a wide variety of
formations from clay to granite. However, a particular bit does not perform
well in circumstances for which it was not designed. Excessive bit wear can
occur when it is required to drill through debris accumulated during lengthy
periods without circulation, or re-enter a hole that has been ‘pumped up’ by
stop/start circulation, during the tripping out, and in.
Rate of Penetration (ROP) is generally reduced by a high overbalance
between the circulating and formation pressures. In practise the mud
weight/density is calculated to provide a small overbalance on formation
pressure when circulation ceases, to control the possibility of influx from the
formation, or kicks.
With circulation stopped during tripping, axial motion of the pipe out of and
into the well, produces negative and positive pressure fluctuations, swabbing
and surging, which can cause hole damage, including the possibility of an
influx of formation fluid or gas. Either way, some ‘pumping up’ of the
formation is inevitable.
The Solution
Continuous circulation provides a steady state down hole pressure and flow
regime, which greatly reduces the possibility of damage to the wellbore.
Continuous hole cleaning avoids settlement or accumulation of cuttings and
debris, allowing the bit to be returned directly to bottom to restart drilling
following connections.
The steady state, down hole, allows the circulating pressure to be controlled
by flow rate alone, at the optimum low differential pressure for the formation
being drilled, which generally allows the bit to penetrate faster.
By keeping the circulation continuous throughout the tripping operation, the
drill string can be tripped faster without the damaging swab and surge effects
on the wellbore. Additionally, the well remains under continuous control with
circulation connected and flowing at all times.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.9 Surging on stop / start circulation
Eliminated by Continuous Circulation
The problem
Each time circulation is started, the normal pump pressure is exceeded by a
‘dynamic pressure increment’ required to shear the gel of the static mud
column and start it moving. This relatively sudden increase in pressure
produces a ‘surge’ which can fracture and/or weaken the exposed formation.
Each time circulation is stopped, the down hole pressure momentarily
decreases below the normal static mud pressure. This relatively sudden
decrease in pressure produces a ‘negative surge’ (or swab) which can
initiate formation collapse or an influx of fluid (or kick) from the exposed
formation.
Either way, stopping/starting circulation effectively pressurises or
depressurises the exposed formation. This has been happening ever since
rotary drilling commenced some one hundred years ago. This ‘pumping’ of
the formation and stopping/starting circulation is the main contributor to most
drilling problems caused by weakening of the exposed formation and is
evidenced by the considerable current interest in MPD (Managed Pressure
Drilling), which aims to compensate for the interruption of circulation to the
wellbore.
The solution
Maintain continuous circulation.
The most simple, effective and safe MPD solution is to circulate
continuously.
As new wells are drilled in increasingly difficult pressure regimes and old or
depleted wells are re-entered and deepened, the pressure variations due to
stopping/starting circulation are increasingly unacceptable.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.10 Formation pumping & Balooning
Eliminated by Continuous Circulation
The problem
Formation ‘pumping’ is the undesirable effect of varying the downhole mud
pressure up and down repeatedly. This has been considered as inevitable
for many years and only recently has MPD (Managed Pressure Drilling)
become an acceptable drilling technique
At the upper extreme, the surge pressure on starting circulation can ‘charge
up’ a permeable formation and inject drilling fluid into the formation or even
fracture it.
Balooning is the ability of the formation to return this fluid to the wellbore
when the circulating pressure drops and stabilises, giving the impression of a
kick initiating. Over an open hole of several thousand meters, this can be a
considerable and misleading volume.
The solution
Establish continuous circulation and avoid any significant pressure variations
to be compensated for with alternative MPD methods. The rightful use of
MPD can then be focussed on controlling the ‘Continuous Circulation’
pressure at the optimum for the formation being drilled.
The steady state, maintained by a CCS (Continuous Circulation System),
provides the calm setting for reading downhole pressure variations more
meaningfully, since any variations in pressure, or flow, will be due to the
formation being drilled, which should also improve well control and safety.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.11 ECD variations
Eliminated by Continuous Circulation
The problem
Many drilling engineers use ECD (Equivalent Circulating Density), instead of
circulating pressure when planning how to drill through a particular
formation. “Controlling the ECD” is a main objective for the drilling team.
The ECD is the sum of the density of the mud in the hole plus the density
equivalent of the dynamic pressure drop generated by circulating.
To be able to maintain a steady, unchanging ECD, would greatly assist the
drilling of formations with narrow pore/frac pressure windows and minimise
differential pressure to increase ROP. Controlling the ECD has the potential
to minimise or eliminate a range of drilling problems, allowing a longer hole
section to be drilled before running casing.
The solution
Maintain continuous circulation with a CCS and achieve all the benefits of a
constant ECD. Of all of the MPD methods, continuous circulation, is the
simplest and most effective and is the only method that maintains the steady
and unchanged down hole pressure regime, across the entire exposed
formation, while making connections.
.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.12 Well de-stabilisation in UBD
Eliminated by Continuous Circulation
The problem
Underbalanced Drilling (UBD) can be defined as a drilling activity employing
appropriate equipment and controls where the pressure exerted in the
wellbore is intentionally less than the pore pressure in any part of the
exposed formation with the intention of bringing formation fluids to the
surface.
The wellbore pressure must be maintained as near constant as possible,
which is difficult when making connections; not just because of the loss of
ECD when circulation is stopped, but because formation fluids, gas in
particular, accumulate in the annulus, upsetting the pressure balance and
have to be circulated out before resuming drilling
Circulating out the formation fluids and re-establishing a steady state down
hole can take up several hours of Non Productive Time (NPT) per
connection.
The solution
Maintaining continuous circulation, allows the ECD to be kept constant and
the ‘underbalance’, ‘balance’ or ‘near balance’ pressure, to be more easily
controlled.
By circulating continuously during connections, there are no accumulated
fluids to circulate out and the steady state down hole pressure regime is
maintained throughout, allowing drilling to recommence without delay.
Furthermore, with reservoir fluids flowing into the wellbore, it is intrinsically
safer to have uninterrupted circulation, particularly, while making
connections.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.13 Static Cuttings Settlement
Eliminated by Continuous Circulation
The problem
When circulation is interrupted to make a connection, cuttings in the
wellbore annulus start to fall even in high gel strength mud. In near vertical
holes, cuttings and debris could accumulate sufficiently to trap the collars,
BHA or bit. In lateral holes, the solids need only sink a few inches to settle
on the low side and potentially trap the drill string when drilling
recommences.
This is minimised by taking extra time to circulate solids out of the uncased
hole before stopping circulation.
The solution
Using the CCS to circulate continuously, the mud is never static and settling
is avoided as the circulating fluid continues to clean the hole during
connections. The uninterrupted flow of drilling fluid in lateral wells minimises
settlement and reduces the build up of solids at intervals or “weirs” in the
annulus.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.14 Connection Kicks
Eliminated by Continuous Circulation
The problem
Connection kicks are those influxes of formation fluids into the wellbore,
which occur during conventional connection operations, caused by the drop
in downhole pressure when circulation of drilling fluid is stopped.
The downhole mud pressure, at the exposed formation in the annulus, is the
sum of the hydrostatic mud pressure and the dynamic (or friction) pressure
generated to move the mud back up the annulus to the surface; plus any
back pressure imposed at the surface, on the returning mud. Until now, the
Top Drive (or Kelly) could not be disconnected without first stopping the mud
circulation. With stop/start circulation, the dynamic pressure is lost when
circulation stops every time a connection is made. Additionally, on stopping
circulation, the pressure drops temporarily below the static mud pressure
which can initiate a kick when the “safety margin” on the static mud column
is small.
Furthermore, a ‘connection kick’ may not be detected immediately and a tool
joint in the drill string at rig floor level may have been disconnected, when the
influx of formation fluids is detected. With the mud disconnected, circulation
cannot be immediately started to limit the kick. The drill pipe BoP must be
made up and a connection made before the mud pumps are re-started. This
has been the situation on all rotary drilling rigs for over a hundred years
The solution
With continuous circulation connection kicks are eliminated.
Continuous circulation maintains the established ‘steady dynamic state’,
across the exposed formation from bit to the last casing shoe, in terms of
both pressure and flow regime, such that there should be no induced flow
from the formation.
If during or after a connection, any change in pressure or flow occurs, it can
be interpreted as an exchange between the annulus and the formation,
providing a more sensitive and immediate signal of what is happening
downhole.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.15 Disconnecting mud from the drill string
Eliminated by Continuous Circulation
The problem
Stopping circulation and disconnecting the mud supply from the drill string
while drilling is ‘intrinsically unsafe’. In rotary drilling with jointed pipe, it was
inevitable when making a connection to add or remove drill pipe to/from the
drill string.
‘Intrinsically unsafe’, as evidenced historically by the number of occasions
when a kick has been initiated during a connection and developed into a
blow out situation before drill pipe could be re-connected and circulation reestablished.
Even with MPD techniques that try to compensate for the drop in pressure
when circulation is stopped, making a connection during a drilling operation,
is still not without a measure of risk, requiring both a sensitivity to detect
down hole changes and an ability to respond quickly.
The solution
Using a CCS to provide continuous circulation while making connections,
facilitates continuous and improved well control.
The valves which control the mud flow between Top Drive and CCS are
standard, accessible and safe proven equipment. No additional valves or
components are required in the drill string itself.
Continuous mud connection to the drill string allows steady running of the
mud pumps, which is preferable and the continuous flow over the shale
shakers, allows a higher separation efficiency, and circulation while making
connections improved hole cleaning.
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L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
2.16 P & T variations in HPHT wells
Eliminated by Continuous Circulation
The problem
Increasingly, wells have to be drilled to access hydrocarbons in HPHT fields.
In these conditions when circulation is stopped to make a connection, the
static mud column heats up and can change its properties. This change in
properties can significantly affect the accuracy of hydraulic calculations;
circulation rate, pressure, etc.
In HPHT wells, pressure surges on stopping/starting circulation can be
significant, potentially causing well-bore ballooning on start up, and fluid
influx and formation collapse when the pumps are stopped. Stabilising the
mud column before and after making a connection can be time consuming
and add to NPT.
The solution
Using continuous circulation, an uninterrupted ‘steady state’ flow of mud is
established with uniform parameters throughout the system making the
hydraulics consistent and easier to calculate.
With continuous circulation, high dynamic pressure drops in the annulus can
be accommodated since these are not lost during connections.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
Continuous Circulation Drilling
Drilling problems
minimised or eliminated
Continuous Circulation
Drilling
Improved
safety & production
3. Improved Safety and Production
Facilitated by Continuous Circulation Drilling
3.1 Improved Safety
Conventional practice
While drilling ahead, the downhole pressure, temperature and dynamic flow
regime, settle down to a near steady state, affected only by the formation
being drilled.
When stopping drilling, to add another stand of drill pipe, the mud pumps are
stopped, usually after first circulating mud and cuttings away from the collars
and BHA or back to the last shoe in lateral wells.
As the circulation reduces to zero, the steady state is lost and the dynamic
flow regime passes through several turbulent and laminar flow patterns,
which encourage additional scouring and debris production.
As the circulation shuts down, a negative surge temporarily reduces the
downhole pressure to below the static mud pressure. This occurs across the
whole of the exposed formation from the bit to the last shoe and may
encourage an influx of fluid from the formation, “a kick”, or it may allow fluid
to be relieved from the formation from being ‘charged’ by the higher drilling
pressure or ballooning. Both the changes in flow and pressure tend to
damage the uncased hole.
Having upset the downhole steady state regime and significantly lowered the
downhole pressure often causing a fluid influx, the mud supply, which is the
primary means of well control, is not only shut off by breaking the tool joint
between the top drive and the drill string but disconnected completely.
While making connections, the downhole mud heats up, cuttings sink and
settle (depending on the gel strength), formation fluids accumulate (gas
being a major concern), the shale shakers are out of use and the mud
remains disconnected from the drill string. The ability of the driller to
interpret the signs of what is happening down hole is significantly
compromised.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
On reconnecting the drill string and restarting circulation, the downhole
pressure surges temporarily above the steady circulating pressure and
changing flow patterns further damage the formation wall. Accumulated
cuttings, debris, and formation fluids then have to be circulated out of the
well, before a steady state for drilling can be re-established.
Continuous Circulation Drilling
By continuously circulating, the downhole pressure, temperature and
dynamic flow regime, remain in a steady state, affected only by the formation
just drilled.
There are no pressure surges, no accumulated formation fluids, no static
settlement of cuttings, no pumping of the formation and, above all, the mud
is connected and flowing at all times.
The connection operation is simplified and the exposed formation is not only
treated with more respect, but the well remains under better control. The
driller is able to respond to any well situation without having to first
reconnect the mud supply and start up the mud pumps.
With continuous circulation maintaining a steady state throughout the
connection operation, any variation in downhole mud pressure or flow will be
due to the formation and can be more easily and quickly interpreted as such.
The CCS is constructed from recognised, standard BOP components and
the connection operation is safely contained entirely within this assembly.
The drill string is conventional without the necessity to introduce any valves
or other mechanical items. In the event of any difficulty with the connection,
the CCS allows remedial action to take place safely under pressure, while
keeping the well under control at all times.
L.Ayling – ITF JIP Director – July 2009.
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Maris International Ltd.
3.2 Improved Production
Conventional practice
The drilling team construct wells to produce oil and gas from pay zones that
are increasingly difficult to reach but, having done so, the ability of the pay
zone to produce fluids is largely dependant on the extent to which drilling
fluid has been squeezed into or lost to the formation.
Fracturing and/or lost circulation within the pay zone may decrease the
subsequent flow rate of production fluids Even if this is avoided, a major
factor in subsequent productivity will be the extent of ‘over balance’ pressure
that existed during the drilling of the pay zone.
Generally, this incremental over balance is set to ensure that the static mud
pressure is in excess of the formation pore pressure to prevent a kick
occurring.
Increasingly, MPD techniques are used to allow NBD (near
balance drilling) or UBD (under balance drilling).
The circulating, or drilling, pressure exceeds this static pressure by the
dynamic, or friction, pressure required to push the mud back to surface.
Hence the balance when drilling is usually several times the balance when
circulation stops. It is this drilling circulation pressure which forces drilling
fluids into the formation.
The drilling fluid includes barites to increase
density, gelling additives to support cuttings when static, bit lubricant and
additives to produce mud filter cake to reduce fluid loss to the formation.
The subsequent production rate of hydrocarbons achievable from the well is
fundamental to the well economics.
Continuous Circulation Drilling
By maintaining a continuous circulation flow, the circulating pressure and
balance, ECD, can be set at the optimum to control the well and maximise bit
RoP, the static mud density can be reduced as can the fluid loss into the
producing formation.
With steady state pressure, temperature and mud flow, throughout the drilling
of the pay zone, the chances of exposing a cleaner, undamaged production
formation are greatly enhanced. Generally, the higher production rate will be
very worth while.
L.Ayling – ITF JIP Director – July 2009.
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