ExxonMobil on the El-Capitan ERD well drilled from a platform
offshore California, USA (SPE paper #140265) used special drift
9 5/8 in. 40 ppf liner (8 3/4 in. drift) with a thinner wall which increased
annular clearance for the subsequent 8 1/2 in. hole section drill
string, thus reducing ECD.
Running the intermediate string as a liner rather than a full
string has a major effect on ECD in subsequent hole section drilling.
The liner can always be tied-back to surface if needed after the
subsequent hole section is drilled or left as a liner. However, there
are some downsides to this solution; increased annular clearance
in the section above liner top packer can lead to poor hole cleaning
due to limited flowrates of smart tools downhole (insufficient
annular velocity).
It is also possible to use flush casing connections if annular
clearances are tight. Also, a well-designed centraliser programme
that minimises the number used for practical cementing purposes
will reduce ECD while running and cementing casing.
Drilling fluidsprogramme
There are multiple options to alter drilling fluid properties and
types that will be influential to ECD issues. The relative merits of
each option should be reviewed thoroughly.
Running a lower mud weight is probably the easiest option
for reducing ECD; however it can create wellbore instability. This
introduces a delicate balancing act of controlling both losses and
wellbore instability. It often becomes impossible to solve both
issues and one of them will inevitably have to be prioritised over
another.
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Low mud rheologies, in other words, thinner mud will have
lower ECD. Rheology has a major impact on ECD, however
rheology must be maintained at an acceptable level for
hole cleaning purposes. Particularly, it is recommended to
ensure that the drilling fluid has a strong shear thinning
capability to ensure lower ECD fluctuations and adequate
hole cleaning.
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Gel strength control – flat gels are paramount. It is important to
ensure breaking circulation is started at a low flowrate to avoid
ECD spikes while breaking the gels. When practical, consider
starting pipe rotation first before starting up the pumps.
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High oil-water-ratio (OWR) – case study (SPE paper 166134)
by Murray et al., Shell UK limited, demonstrated the effect of
high OWR in non-aqueous drilling fluids (NAFs). The authors
successfully formulated and implemented a low cost, low-ECD
drilling fluid to address severe losses in the narrow margin
drilling of multiple wells in Pierce Field, UK North Sea. The
idea behind this formulation was to design a fluid system that
would address the challenges of barite sag, hole cleaning and
high ECD while being low cost and easy to replace if losses were
encountered. As mentioned previously, this is a tough balancing
act. The authors discovered that the key is high OWR (up to
85/15) combined with organophilic clay content. Regarding
viscosity, it was found that with increasing OWR, contributions
from the emulsion droplets reduce, hence dependence on
organophilic clays increases. Therefore, the concentration of
organophilic clay increases proportionally as OWR increases to
maintain certain rheological properties of the fluid. The high
content of such clay proved to be more resistant to barite sag.
Hence, higher OWR NAFs can more efficiently prevent barite
sag due to their increased organophilic clay content. With this
property in mind, it was concluded that viscosity of this type of
fluid can be safely reduced without exacerbating hole cleaning
and barite sag issues. This allowed for a significant reduction
in ECD.
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