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Oilfield Technology
May
2016
it interacts with the borehole wall or casing. Interactions
between the centraliser and the borehole, particularly in the
open hole where cuttings beds may exist and where contact
stress between the centraliser and formation may cause
indentation, can influence local friction during running.
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Open flow area around the centraliser must generally
be considered to ensure the pressure losses along the
annulus do not limit flow or create downhole pressures
that inadvertently fracture the formation during circulation
or cementing operations. Sufficient open area will also
minimise the potential for cuttings to pack off in the
annular space.
Product installationrobustness
In ERD wells, the distances travelled by centralisers under
severe side loading against competent formations can
lead to significant surface wear potential. Clear examples
of retrieved strings indicate there is merit in selecting
wear-tolerant centralisers. Cost savings offered by less
wear-resistant options should be weighed against the
potential for failure and the associated implications to
centraliser function and the overall cost of the casing run.
Robustness evaluations should consider:
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Wear anticipated for the overall travel distance to TD.
Wear of the centraliser may compromise standoff and
run-ability.
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Integrity of the centraliser as a whole during harsh
running conditions. Simply put, damaged centralisers
can increase local friction and cause challenges in
reaching TD.
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Application factors that increase wear potential. For
instance, centralisers used in casing drilling applications
can be expected to undergo much more severe surface
wear, and will benefit from surface treatments (such as
hard-facing) that diminish wear rates.
Local frictionmanagement
Friction coefficients between centraliser contact surfaces
and the borehole or surrounding pipe ID are often the focus
of centraliser product literature. Since contact surfaces on
centralisers are subjected to high loads, harsh conditions
that lead to wear, and will tend to interact with cuttings
beds and the formation in the open hole section, centraliser
shape and borehole quality should also be considered.
While there is merit in considering reductions in the drag
force on a given centraliser by minimising surface friction
coefficient, true benefits offered by ‘low friction’ options
should be established through field evidence.
A second important consideration is the management of
friction in situations where rotation is used to enable casing
and liner strings to reach TD. Affixing centralisers to the
outside of the string (through mechanisms such as crimping
or set-screws) allows rotational motion to occur at the
centraliser/borehole interface, creating a more favourable
axial friction condition that enables forward progress. In the
limit, a centraliser that rotates freely on the casing string
may simply act as a sleeve if the string is being rotated from
surface, providing no benefit to forward progress.
Global frictionmanagement
Many of the factors identified above are common to more
conventional horizontal well technology, and can be
considered in the same light for ERD wells. However, the
largest centraliser-related opportunity for technological
advancement in these types of wells clearly rests in the
management of casing string mechanics as the string is
pushed towards TD from surface. Compression in the casing
string, particularly in the heel section of the well as the string
approaches TD, creates higher potential for sinusoidal and
helical buckling than in shorter wells. Even with local friction
reduction measures, this situation should be anticipated, and
if possible, addressed with a buckling-tolerant design.
Traditional torque and drag (T&D) models provide a
mechanism for predicting the onset of buckling and its
associated implications on drag; this work is continuing in
the academic community, with a focus on more accurate
estimation of T&D behaviour. Volant has advanced its
understanding of the impact of centraliser spacing on
sinusoidal and helical buckling mechanics, specifically
in the context of side loads that are generated from the
buckling behaviour. Proper centraliser density, particularly
in high-compression zones of the string, can mitigate
buckling-related side loads and associated drag forces.
Centralisers will not prevent buckling, but they can be used to
limit the side load associated with a particular compressive
force, reducing the overall drag. This is the foundation for the
company’s centraliser placement strategy.
The company considers it mission-critical to
ground-truth the benefit of its placement strategy through
field application, and has been actively doing so over the
past three years. Volant performs T&D modelling before and
after ERD runs to both calibrate its placement basis and to
enable a better understanding of risk. As a result, operators
have had fewer surprises, learned from challenging wells,
experienced lower overall costs, and enjoyed the benefit of
higher reliability in reaching TD.
Summary
Successfully reaching farther with ERD well design requires
attention to all aspects of the well construction process,
and designing the well with an appropriate strategy for
centraliser selection and placement is no exception.
Industry stories of centralisers degrading downhole and
hampering casing installation are common and lead
to significant amounts of NPT for operators. While it is
easy to look at the cost of centralisers and elect to use
the most inexpensive option as a means to cut material
costs, one must be careful this is not done at the expense
of additional rig time spent to overcome inadequate
product performance. In most cases, very few hours of
rig NPT need to be saved to offset the cost of selecting an
application-appropriate centraliser solution.
Well-conditioned centralisation strategies can
significantly improve the chances of successfully reaching
TD in ERD casing and liner installations. In addition
to improving run-ability of existing string designs,
understanding the positive impact of coupling the right
centraliser product with an optimised placement strategy
can reduce completions costs by saving time, and may
enable operators to drill farther with the confidence that
they will be able to successfully complete those longer
wells.
