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Welcome to this presentation about Hole Cleaning in Directional
Wells. It has been prepared by Sedco Forex, to give you a better
understanding of the cuttings-transport phenomenon and how it
affects drilling operations, specifically hole cleaning. All the
recommendations presented here should be applied within operational
needs.
Why do we want to disseminate this knowledge? Because great
amounts of money and time are spent, worldwide, every year
combating stuck-pipe problems. In the majority of the cases, the
factors that caused these problems including hole cleaning could
have been avoided or controlled.
It is very important to remember that poor hole cleaning, if not
addressed properly, could eventually cause stuck pipe and other
major problems, making the entire drilling operation
uneconomical.
By the end of this presentation you should have a good idea
about how to minimize hole-cleaning problems in directional
wells.
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SFM-HC-951
HOLE CLEANING IN DIRECTIONAL WELLS
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Technology TransferHole Cleaning in Directional Wells
Page 2
One of the main functions performed by any drilling fluid is the
removal of cuttings from the bottom of the hole and their effective
transport to the surface.
If this function is not performed efficiently, cuttings
generated through the drilling process will begin to accumulate in
the annulus. In directional wells, this problem is accentuated by
gravitational forces.
When not circulating, it is equally important that the drilling
fluid be able to suspend the cuttings. It is here where Gel
Strength and Yield Point become critical. For more information
about this topic please see Appendix 1.
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SFM-HC-952
Introduction
l Drilling Fluidsl Cuttings Accumulation
l Suspension Properties
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Technology TransferHole Cleaning in Directional Wells
Page 3
During drilling, the velocity of the drilling fluid must exert a
force high enough to counteract the effects of gravity, which will
tend to make the cutting drop to the bottom of the well. Usually,
enough velocity is achieved by the drilling fluid to perform this
task efficiently in vertical wells.
On the other hand, directional wells pose a more difficult
problem. Influenced by gravity, the cutting will still try to drop,
but due to the inclination of the well it does not have to travel
too far before it reaches the lower side of the wellbore. In this
situation, the velocity of the drilling fluid has to be higher in
order to keep the cutting moving up towards the surface.
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SFM-HC-953
Particle Movement
Mud Velocity
Mud Velocity
Particle Velocity
Particle Velocity
Vertical Well Directional Well
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Technology TransferHole Cleaning in Directional Wells
Page 4
If the cutting-displacement function is not performed
efficiently by the drilling fluid, cuttings generated through the
drilling process will begin to accumulate in the well. For a
particular well section, the ratio between the volume of cuttings
and the annular volume is known as annular cuttings concentration
(ACC).
When the annular cuttings concentration in directional wells
reaches a certain level, a phenomenon known as a cuttings bed
occurs. It is a massive amount of drilled cuttings that, due to
inadequate fluid velocity, accumulates in the wellbore and is
heavily influenced by hole inclination.
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SFM-HC-954
Cuttings Concentration
ACC VolumeofCuttingsAnnularVolume
Not to scale
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Technology TransferHole Cleaning in Directional Wells
Page 5
Experience shows that high annular cuttings concentration in the
form of cuttings beds is the main factor that causes severe hole
problems like:
High torque & drag (manifested as lack of tool-face
control/steering, and poor weight transfer), stuck pipe (costly and
time consuming), and difficulty when running/cementing casing.
Therefore, it should be carefully considered regarding hole
cleaning in directional wells.
Numerous studies have shown that to limit the formation of a
cuttings bed, the annular mud velocity in directional wells has to
be significantly higher than in vertical wells.
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SFM-HC-955
Cuttings Beds
l Poor hole cleaning may cause: High Torque and Drag
Stuck Pipe Casing Problems
l Vmud (directional) > Vmud (vertical)
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Technology TransferHole Cleaning in Directional Wells
Page 6
Lets take a look at the more important factors that affect hole
cleaning. They are:
Flow Rate
Flow Type (laminar/turbulent)
Wellbore Inclination
Fluid Rheology & Density
Rate of Penetration
In addition, other parameters like Particle Settling Velocity,
Particle Geometry and Distribution, Drillpipe Eccentricity, and
Drillpipe Rotation also influence hole cleaning.
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SFM-HC-956
Cuttings Transport
l Flow Rate (annular velocity)l Flow Type
(laminar/turbulent)
l Wellbore Inclinationl Fluid Rheologyl Rate of Penetration
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Technology TransferHole Cleaning in Directional Wells
Page 7
For the purpose of hole cleaning efficiency, all well sections
can be categorized into three main types (according to their degree
of inclination) :
Type I, includes the vertical (0 - 10), and low-inclination well
sections (10 - 40).
Type II, includes well sections with an inclination between 40
and 60 degrees.
Type III, includes well sections with inclinations over 60
degrees.
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SFM-HC-957
Types of Wellbores
l Type I > (0 to 40)l Type II > (40 to 60)
l Type III > (over 60)
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Technology TransferHole Cleaning in Directional Wells
Page 8
Full-scale experiments have shown that in wellbores with a 10
inclination, movement and concentration of cuttings is only
slightly worse than in vertical wells. Therefore, inadvertent hole
deviations up to ten degrees, in vertical-hole drilling, should not
pose any special hole-cleaning problems.
It has also been found that, in vertical sections,
high-viscosity muds provide better cuttings transport than
low-viscosity muds.
In addition, pipe-rotation effects are minor (specially in
turbulent flow).
Finally, cuttings transport in vertical sections is nearly the
same for all drillpipe eccentricities.
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SFM-HC-958
Type I Vertical Section (0- 10)
l Hole Cleaningl Fluid Viscosity
l Pipe Rotationl Drillpipe Eccentricity
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Technology TransferHole Cleaning in Directional Wells
Page 9
Compared to vertical sections, low-angle well sections require
higher annular velocities in order to achieve effective hole
cleaning.
Because of increasing inclination, more and more cuttings are
forced toward the low side of the annulus, where they slide
downward and are eventually lifted and re-enter the high-velocity
region at the middle of the annulus. After this, they are swept
upward, and the process is repeated. This phenomenon is described
as particle recycling, and has a detrimental effect on hole
cleaning.
Also, it can be said that for this type of well section, the
effect of laminar flow dominates cuttings transport. Therefore,
viscosity, yield point, and initial gel strengths have a
significant effect on annular cuttings concentration and hole
cleaning efficiency.
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SFM-HC-959
Type I Low-Angle Sections (10- 40)
l Annular Velocityl Particle Recycling
l Laminar Flow
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Technology TransferHole Cleaning in Directional Wells
Page 10
This slide illustrates the phenomenon known as particle
recycling, which occurs as a result of wellbore inclination and
fluid-velocity differential, and has a detrimental effect on hole
cleaning. For illustration purposes, we will follow this process
for only one cutting.
Because of increasing inclination, the cutting is forced toward
the low side of the annulus, where it travels downward due to a
lack of lifting force in the flow (low velocity near the wall).
At some point, due to a higher shear stress, the cutting is
lifted and re-enters the high-velocity region at the middle of the
annulus.
Then, it is swept upward and continues to travel until
its tendency to drop overcomes the lifting force in the flow and
it is forced toward the low side of the annulus again.
This process can be repeated many times resulting in the
cuttings shape being altered through grinding. Measures used to
minimize this problem include viscous sweeps. For more information
see Appendix 1.
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SFM-HC-9510
Particle Recycling
(10 - 40)
1
3
2
4
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Technology TransferHole Cleaning in Directional Wells
Page 11
In well sections with inclinations greater than 40 degrees,
cuttings do not recycle as readily as in lower-angle sections. This
is because gravity tends to hold them down on the low side of the
hole.
Well sections with inclinations between 40 and 60 are considered
critical, not only because a cuttings bed develops, but because it
is unstable and prone to sliding downward (avalanching). The
consequence of avalanching is an instantaneous build-up of cuttings
around the drillpipe and/or BHA which, if not treated properly, can
result in stuck pipe.
Turbulent flow exhibits a desirable, eroding effect on cuttings
beds.
Pipe movement (rotation/reciprocation) mechanically disturbs
cuttings beds.
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SFM-HC-9511
Type II Crit.-Angle Sections (40- 60)
l Cuttings Behaviorl Cuttings Beds
l Annular Velocityl Pipe Movement
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Technology TransferHole Cleaning in Directional Wells
Page 12
At high angles of inclination the formation of a cuttings bed is
almost instantaneous, and its thickness is governed primarily by
annular velocity.
The cuttings bed that forms at angles greater than 60 degrees is
stable, this means it will not avalanche.
The waving, vortex-like, character of turbulent flow has a
destructive influence on the bed being formed. There is a tendency
for cuttings to be withdrawn (lifted) from the bed and displaced
upwards in the annulus, where such a process may occur again. This
kind of interaction, together with the flat velocity-profile
typical of turbulent flow, leads to better hole cleaning.
Pipe movement (rotation/reciprocation) mechanically disturbs
cuttings beds.
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SFM-HC-9512
Type III High-Angle Sections (60+)
l Instantaneous Bed Formationl Stationary Cuttings Bed
l Turbulent Flowl Pipe Movement
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Technology TransferHole Cleaning in Directional Wells
Page 13
The following are some field guidelines that Sedco Forex
recommends for effective hole cleaning in low-angle well
sections:
General use of laminar flow.
Maintaining a high Yield Point and Gel Strength to reduce the
settling of cuttings when pumps are off. Always ensuring that the
hole is clean before turning the pumps off.
Maximizing YP/PV ratio.
Using viscous sweeps to reduce the effects of particle
recycling.
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SFM-HC-9513
Field Guidelines
Type I Well Sections (0 - 40)l Laminar Flow
l High Yield Point/ Gel Strengthl Maximize YP/PVl Use Viscous
Sweeps
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Technology TransferHole Cleaning in Directional Wells
Page 14
In the well sections over 40 degrees, attention must focus on
minimizing cuttings beds.
For the range of intermediate inclinations (40 - 60), turbulent
flow is recommended. Since cuttings transport in turbulent flow is
not affected by rheological properties, lower mud parameters (i.e.
YP, PV) may be used. However, static mud parameters such as gel
strength are usually desirable even if turbulent flow is
preferable. If turbulent flow cannot be used because of other
adverse factors, like wellbore instability, annular velocity should
be kept as high as possible.
Rotating and/or reciprocating drillpipe has a mechanical,
destructive influence on the cuttings bed (this influence is the
main factor that provides a higher cleaning rate for higher RPM).
As pipe rotation is typically governed by directional-drilling
needs, periodic wiper trips should be considered.
Combination sweeps (low viscosity/high density) are effective at
eroding the cuttings bed, and carrying the cuttings to surface.
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SFM-HC-9514
Field Guidelines (cont.)
Type II Well Sections (40 - 60)l Turbulent Flow
l Mechanical Agitationl Combination Sweeps
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Technology TransferHole Cleaning in Directional Wells
Page 15
Turbulent flow is preferable in high-angle wells. Generally, the
same recommendations as those described for intermediate-angle
wells are applicable in this region for turbulent flow; however,
the requirements to ensure a mud gel strength are less important.
If turbulent flow cannot be achieved, the YP/PV ratio should be
maintained as high as possible.
Again, rotating and/or reciprocating drillpipe has a mechanical,
destructive influence on the cuttings bed. It is this influence
that provides a higher cleaning rate for higher RPM, particularly
at high inclinations where a considerable cuttings bed is formed.
Periodic wiper trips should be used if drillpipe rotation is
restricted.
Combination sweeps (low viscosity/high density) are effective at
eroding the cuttings bed, and carrying the cuttings to surface. The
effectiveness of a cuttings-bed-maintenance program can be
determined through several indicators.
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SFM-HC-9515
Field Guidelines (cont.)
Type III Well Sections (60 +)l Turbulent Flow
l Mechanical Agitationl Combination Sweeps
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Technology TransferHole Cleaning in Directional Wells
Page 16
The following are some effective indicators of poor hole
cleaning:
Abnormally low or intermittent cuttings volume discharge.
High pick-up weight.
Poor weight transfer. A higher than normal surface weight
(slack-off) is required to get a pressure-drop response from the
mud motor.
Difficulty orienting the motor, due to excessive friction
between the cuttings and the drillstring.
Excessively ground cuttings, due to extended particle recycling
and drillpipe interaction with the cuttings bed.
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SFM-HC-9516
Field Guidelines (cont.)
Poor-Hole-Cleaning Indicatorsl Reduced Cuttings Return
l High Pick-Up Weightl Poor Weight Transferl Difficulty
Orienting Motor
l Excessive Grinding of Cuttings
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Technology TransferHole Cleaning in Directional Wells
Page 17
Lets take a look at a recent drilling program implemented by a
major operator in the UK.
This was an extended-reach drilling program comprised of several
wells with an average total depth of 23000 ft (7012 m), and a
horizontal departure of 19500 ft (5945 m). Departure-to-depth ratio
of 4:1.
Hole cleaning was identified as a critical factor in the success
of this project.
The entire hydraulics program was designed to achieve maximum
annular velocity. Flow rates up to 1150 gpm were used, in
conjunction with 65/8 drillpipe, generating annular velocities up
to 264 ft/min, and standpipe pressures up to 4500 psi. No
hole-cleaning problems were experienced in either the 121/4 or 81/2
sections.
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SFM-HC-9517
An Operators Experience (1995)
l Extended-Reach Wells Total Depth of 23000 ft
Horiz. Departure of 19500 ft
l Hole Cleaning Challenge
l High Annular Velocity Flow Rate @ 1150 gpm (264 ft/min) 65/8
Drillpipe (SPP 4500 psi)
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Technology TransferHole Cleaning in Directional Wells
Page 18
In order to achieve the mentioned operational parameters,
significant equipment modifications were required. To meet
flow-rate demands three 1600 HP pumps were installed (together with
adequate power systems), and larger 65/8 drillpipe was used. This
helped lower frictional pressure losses (inside the pipe) and
increase annular velocity. As part of this process, surface lines
where also refitted to meet the high-pressure demands.
All these modifications had a significant cost; however, the
performance incentives offered by the operator to the drilling
contractor made this a profitable venture. We must remember that by
ensuring adequate hole cleaning the risk of stuck pipe and other
costly events is diminished this enabled the drilling contractor to
finish most of these challenging wells ahead of schedule.
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SFM-HC-9518
An Operators Experience (cont.)
l Equipment Requirements Three 1600 HP pumps
High HP power systems Larger 65/8 drillpipe
Larger surface lines
l Financial Considerations Cost
Profit
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Technology TransferHole Cleaning in Directional Wells
Page 19
Lets summarize the key points from this presentation:
High annular velocity equals improved hole cleaning. This
applies to any wellbore inclination.
Whether or not cuttings beds are stable is determined by the
well-section angle; however, anytime well inclination surpasses 40
expect a cuttings bed to form.
Turbulent flow erodes cuttings beds.
Mechanical agitation (pipe rotation/reciprocation) reduces bed
thickness.
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SFM-HC-9519
Summary
l High annular velocity equals improved hole cleaning
l Cuttings beds form at any angle over 40 degrees
l Turbulent flow erodes cuttings bedsl Drillpipe rotation and/or
reciprocation aids in
cuttings bed maintenance
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Technology TransferHole Cleaning in Directional Wells
Page 20
In laminar flow, carrying capacity is affected by rheology.
Therefore, increasing the ratio of yield point to plastic viscosity
(YP/PV) increases the carrying capacity of a drilling fluid. This
becomes important when turbulent flow cannot be achieved.
In turbulent flow, carrying capacity is not affected by
rheology.
Combination sweeps (low viscosity/high density) are effective in
removing cuttings.
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SFM-HC-9520
Summary (cont.)
l In laminar flow, cuttings transport is affected by
rheology
l In turbulent flow, cuttings transport is not affected by
rheology
l Combination sweeps are effective in hole cleaning
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Technology TransferHole Cleaning in Directional Wells
Page 21
To finish the summary, lets take a look at a graph which shows
the relationship between the types of well sections discussed
during this presentation, and the impact they have on operational
risk and cost based on hole cleaning.
Type I well sections exhibit a relatively low risk (low cost);
therefore, preventive measures to ensure adequate hole cleaning are
always necessary but not critical. An exception to this is the
riser, which because of its large diameter requires special
fluid-flow considerations.
Type II well sections have the highest risk level of the group.
This is due to the formation of cuttings beds and their tendency to
avalanche, primarily when circulation is stopped. Every effort
should be made to ensure adequate hole cleaning in these
sections.
Type III sections also have a high associated risk. Even though
avalanching is not a concern at this inclination range,
cuttings-bed maintenance is imperative in order to prevent costly
problems like high torque & drag, and stuck pipe.
THIS IS THE LAST SLIDE
SFM-HC-9521
Summary (cont.)
III
I
II
Section Type
Ris
k
Co
st
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Technology TransferHole Cleaning in Directional Wells
Page 22
This is an optional slide that can be used to explain the
phenomenon of flow diversion in eccentric annuli. Please refer to
Appendix 1, for more information.
Flow diversion from under the drillpipe, controlled by the
fluids Power-Law n factor, significantly affects cuttings transport
at higher angles of inclination. As the drillpipe is forced towards
the low side of the wellbore, actual fluid velocities under the
drillpipe can be drastically reduced compared to the calculated
average annular velocity levels, in which zero drillpipe
eccentricity is assumed. This accentuates the formation of cuttings
beds.
At low to intermediate angles, flow diversion has less an effect
(lower eccentricity).
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SFM-HC-9522
Flow Diversion
DP
150 ft/min
60 ft/min
100 ft/min100 ft/min
60 ft/min
10 ft/min
5 ft/min 5 ft/min
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This is an optional slide that can be used to explain the
Yield-Power Law model. Please refer to Appendix 1, for more
information.
Fluid rheological behavior is described using the more accurate
yield-power law (Herschel-Bulkley) model:
is the yield stress. A muds is important in evaluating its hole
cleaning and suspension abilities.
n is the flow index. The flow index governs the diversion of
flow from under the drillpipe to above the drillpipe, something
very important in highly-deviated/horizontal drilling where flow
under the drillpipe is critical to removing the cuttings beds or
preventing their formation.
K is the consistency index.
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SFM-HC-9523
Yield-Power Law
= o+K n
= o +Kn
to
to
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Technology TransferHole Cleaning in Directional Wells
Page 24
This is an optional slide that can be used to explain some
important comparisons between water-base and oil-base muds,
regarding hole-cleaning efficiency:
In highly deviated wells, and for lower values of yield point
and plastic viscosity, cleaning performance for both mud types is
roughly the same. However, at higher values of yield point and
plastic viscosity water-base muds provide better cleaning. The
general observation is that an increase in mud yield point and
plastic viscosity results in increased cuttings concentration for
both muds.
As a result from this higher cuttings concentration, torque
requirements for both muds increase with increasing yield point and
plastic viscosity, at higher hole inclinations.
Hole-cleaning performance of oil-base muds at critical angles
(40 to 60), is reduced by severe cuttings bed avalanching (due to
reduced friction).
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SFM-HC-9524
Water-Base vs. Oil-Base
l Performance (High-Angle) Similar at low YP and PV
Water-base is better at high YP and PV
l Torque vs. Hole Anglel Performance (Critical Angles)