fluctuations and/or pack‑off. Slowly move the BHA away from
the problem zone and use the technique described previously
to verify the presence of cuttings. Back‑reaming can continue
at a reduced speed which allows an acceptable equilibrium
bed height to be maintained.
Ì
It is generally recommended that back‑reaming be continued
to 30 ‑ 40˚ inclination, where a further hole cleaning cycle is
performed before pulling the remaining stands on elevators.
Cuttings beds do not recognise cased hole, only wellbore
inclination. Therefore the shoe does not represent any
particular ‘safe zone’ above which the trip out can proceed
unhindered. The same precautions must be maintained
above the shoe.
If it is preferred not to back‑ream once inside casing, then
a full clean‑up cycle should be performed with the BHA
below the shoe and the remainder of the trip completed on
elevators, using the drag trending as described previously.
Over‑gaugehole
Over‑gauge intervals of hole, whether intentional or resulting from
wellbore instability, present a particular challenge to successfully
tripping at high inclination.
Ì
As equilibrium bed height is dictated partly by the annular
velocity of the fluid above the cuttings bed, it follows that in
sections of over‑gauge hole the cuttings bed will grow until the
annular volume above the cuttings bed is approximately equal to
that of the gauge hole (i.e. the annular fluid velocity will seek to
return to an equilibrium condition where it is sufficiently high to
mobilise the surface layer of the cuttings bed). (Figure 3.)
Ì
Large volumes of cuttings can accumulate in over‑gauge
intervals. As the BHA approaches, the cuttings are mobilised and
surge up‑hole. When they encounter gauge wellbore, they will
struggle to be accommodated within the reduced annular volume
and erratic stand‑pipe pressures will be the first indication of an
increasing pack‑off risk (Figure 4).
Ì
Every effort should be made to avoid significant hole
enlargement as a result of formation instability. This requires
an appropriate lower mud weight limit sufficient to prevent
breakout, hence a wellbore stability study is recommended.
Ì
If hole enlargement is suspected (e.g. cavings while drilling,
pack‑offs on previous trips) then the driller must exercise
particular care when approaching these intervals. The same
caution is required when approaching casing shoe rat‑holes,
or transitions from under‑reamed back to gauge hole. Correct
remedial responses are essential.
Conclusion
As well complexity and step‑out increases, so do the consequences
of stuck‑pipe or irreversible wellbore damage. The time and cost
to recover from such events can be significant, and stakeholders
may incorrectly assume that such wells are inherently ‘high risk’,
reducing their appetite for such wells in future. When assessed using
conventional wells and infrastructure, many marginal prospects
become uneconomic and may not reach sanction, leaving reserves
undeveloped.
It is concerning that many such events are subsequently found
to have been avoidable. While many operators have in place
comprehensive well planning processes to identify and address key
risk factors, some are more reliant on the experience and knowledge
of the individuals within their teams. The key to success in complex
wells lies in identifying and addressing each risk at the planning
stage, and providing a trained crew who appreciate the risks and
understand the why particular drilling practices are necessary.
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