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Oilfield Technology
September
2016
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Base fluid type can have a significant impact on the viscosity of
the invert emulsions (NAF). Some drilling fluids companies use
base fluids with low kinematic viscosities which contribute less
to the viscosity of the overall fluid system. Thus, with this effect,
lower overall viscosity fluid will have lower ECD. Examples of
such fluids are different mineral oils (e.g. Escaid
TM
110) and linear
paraffins (used in externals phases of invert emulsions).
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WARP
TM
– is a micronised weighting agent technology (10 times
smaller than conventional barite) which ensures low viscosity in
high mud weight systems to improve ECD management and yet,
high enough viscosity to avoid barite sag. Also, with decreased
viscosities and ECD, higher flowrates can be used to improve
hole cleaning. MI-Swaco successfully utilised WARP technology
in their drilling fluids in ERD wells offshore Canada, Holland,
Norway, Gulf of Mexico, etc.
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Ultra-Fine Grind (UFG) barite – this type of barite is used as a
weighting agent, it has a smaller size than regular API barite.
Similarly to the above-mentioned WARP, finer particle sizes
have lower settling velocities, thus reduce the risk of barite sag.
This feature enables to lower rheologies of the drilling fluid to
reduce ECD.
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Drill string design can drive ECD issues, particularly in smaller
holes (i.e. 8 1/2 in. and smaller) as bigger holes tend to be
less sensitive to tool joints and drill pipe OD. One of the
most common mitigation measures is using a tapered string
(multiple sizes of drill pipe can be used) to lower ECD. This
approach helps to balance hydraulics and torque requirements
to drill an ERD well. Another approach is using drill pipe with
smaller OD tool joints designed for ERD wells.
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Bits and BHA components should have maximum
junk-slot and flow-by areas respectively. This is a greatly
underestimated issue and frequently results in strings
being stuck while tripping or cyclic loading of the wellbore
due to pressure surges/ECD spikes created by large OD
string components such as sleeve stabilisers and small
junk-slot/flow-by areas. Instead, integral blade stabilisers
must be used in BHA components. It is considered
low-hanging fruit to address these issues at the planning
stage which will yield a high rate of return.
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Using a pressure while drilling (PWD) tool is money well spent
in wells with ECD issues. Real time ECD measurements provide
a good idea of the downhole environment while drilling
and appropriate actions can be taken to mitigate problems
proactively. However, it is important to keep in mind that PWD
tools actually measure pressure at the PWD sensor, which is
often located far above the bit. As the highest ECD loads occur
at the bit, the PWD tool often underestimates the actual ECD
and the difference can be significant in narrow mud weight
window scenarios.
Operational stagemitigators
RPM effect on ECD can be quite large. Oney et al. in their paper
on the effects of drill string deflection and RPM on frictional
losses, performed a thorough literature review of this subject
and conducted lab and field tests. It was concluded that RPM
can significantly increase or decrease ECD due to shear thinning
ability of yield power law (YPL) fluids, i.e. drilling fluids. Multiple
variables affect ECD in a drilling process. The combination of
particular RPM and flowrate used will define the final effect on
ECD. Particularly viscous coupling effect along with stirred up
cuttings to the conveyor belt, exasperates ECD fluctuations.
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Flow rate has a major impact on ECD. However hole cleaning
must Be kept in mind when lowering the flowrate to reduce ECD.
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ROP reduction, i.e. the generation of fewer cuttings
suspended in the fluid directly affects ECD. A cost/benefit
analysis must be performed as always before deciding on
control drilling.
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Reaming and slide drilling practices can have an impact on
ECD as they will create cuttings accumulations that will be
disturbed suddenly. Very gentle practices must be exercised to
minimise ECD spikes. Breaking circulation must be performed
at lowest possible flowrates.
Newtechnology
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IReelWell drilling method (RDM) was developed by a group
of companies in a joint industry project to enable drilling
ERD wells beyond 16 000 m mitigating ECD, torque and drag
and equipment limits. The main enabler is the utilisation of
dual aluminium drill string, which has dual walls that allow
pumping down the drilling fluid in one channel and returns
with cuttings up to surface through the other. This eliminates
ECD issues. Additionally, heavy-over-light (HOL) technique,
which involves using a heavier fluid in the annulus and a
lighter active fluid inside DP, increases buoyancy, therefore,
decreasing T&D (Figure 4).
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MPD – managed pressure (dual-gradient, mud cap, constant
bottom hole pressure etc.) and underbalanced drilling
methods are the ultimate ‘sledgehammer’ in mitigating
severe ECD issues. MPD is no longer considered to be a
new technology and equipment reliability has increased
dramatically, however there is no doubt that it introduces
additional unwelcome complexity. Thus, the added
operational complexities make their implementation
unattractive on already complex wells.
Conclusion
Managing the MW window is the key challenge in balancing
wellbore stability with losses. This article outlines the importance
of solving the ECD issues at the planning stage (mitigation) where
more options are available versus the operational (cure) stage. As
outlined above, there are multiple options to address the issue,
however, each one has a different impact on operations, e.g. using
MPD has a great impact on ECD reduction, however, it has major
operational complexities that have to be dealt with. To avoid such
‘side effects’ of some unconventional methods, operators often
choose multiple minor changes such as altering mud rheology
or drilling parameters in combination. Regardless of the ECD
reduction technique selected, detailed planning and modelling at
the well design stage will go a long way in reducing the risk of not
achieving the well objectives.
References
1.
Armstrong N.R, Evans A.M., ‘Extended Reach Drilling - Offshore California.
Extending Capabilities and Improving Performance’, SPE/IADC Drilling
Conference and Exhibition, Amsterdam, (2011).
2.
Murray D.,Sanders M.W., Houston K., Hogg H. and Wylie G., ‘Case Study -
ECD Management Strategy Solves Lost Circulation Issues On Complex Salt
Diapirs/Paleocene Reservoir’, SPE ATCE, New Orleans, 2013.
3.
Vestavik O., Egorenkov M., Schmalhorst B., Kverneland J., Falcao J.,
‘Extended Reach Drilling – new solution with a unique potential’, SPE/IADC
Drilling Conference and Exhibition, Amsterdam, (2013).
4.
Oney E., Ozbayoglu M., Miska S., Yu M., Takach N., Saasen A., May R., ‘Effect
of Drillstring Deflection and Rotary Speed on Annular Frictional Pressure
Losses’, Journal of Energy Resources Technology, ASME, (2014).
5.
Training slides and ERD manual, Merlin ERD, (2016).
6.
WARP Advanced Fluids Technology MI-SWACO brochure, (2009).
7.
Prebensen O.L., M-I SWACO’s European Technical Center, ‘New water-based
drilling fluid with treated micronised barite slurries may help reduce ECD’,
Drilling Contractor magazine of IADC, (2009).
