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Oilfield Technology
June
2015
The trip would progress as follows:
Ì
Complete hole cleaning cycle.
Ì
Commence trip out on elevators:
Record the pick‑up weight for each stand pulled (at a
consistent pick‑up speed for accurate readings).
Plot recorded values over the T&D road‑map. Review the
actual trend versus the theoretical trend in real time.
If the over‑pull (i.e. departure from trend) reaches the
pre‑determined limit, this automatically triggers remedial
action by the driller.
•
A nominal over‑pull limit of 30 000 lb is typically used by
Merlin ERD. Above this value this risk of becoming stuck
has been found to increase significantly.
•
Traditional concepts such as limiting the maximum
over‑pull to a percentage of BHA weight below the jars
have little relevance in high inclination wells, as the BHA
may have little or no weight (as distinct from drag). The
slack‑off weight required to move the BHA downhole
away from the obstruction must come from the drill
string above the tight spot.
Assume the over‑pull is due to cuttings. Stop pulling and move
downhole away from the tight spot until there is no excess drag.
As a minimum, the BHA should be run in hole by the length of
the drill collars plus one stand. This may have to be increased
according to position within the wellbore and perceived risk
(e.g. cuttings slumping within the avalanche zone).
With the BHA positioned safely below the tight spot, slowly
stage up circulation: observe for pressure spikes and
allow pressures to stabilise between stages. Stage up drill
string rotation, monitoring for pack‑off indicators. Increase
parameters to above hole cleaning thresholds.
Circulate for 20 ‑ 30 min. The intention here is not to clean‑up
the wellbore, but to determine if cuttings are responsible for
the tight spot.
Shut down rotary and circulation, and pull
back through the tight spot depth.
•
If the tight spot has moved up‑hole,
then a cuttings bed has been
confirmed. An intermediate hole
cleaning cycle should be performed
to reduce the residual bed height to
an acceptable level. Do not attempt to
short‑cut this step and continue out of
hole immediately, as the cuttings bed
will be encountered again further up
the wellbore.
•
If the tight spot persists, then some
other mechanism is responsible, most
probably a mechanical restriction such
as a ledge or key‑seat and it will be
necessary to ream the tight spot before
continuing out of hole.
If a cuttings bed is confirmed but proves
difficult to remove form the wellbore,
then it may be necessary to commence
back‑reaming out of hole, using the
procedure described in the next section.
Back‑reamingoutofhole
Occasionally it may be preferable to completely
remove the cuttings bed from the wellbore,
for example, prior to installing sand screens
or where flotation assisted casing runs are
planned. The only way to achieve this is by
back‑reaming, which allows the high annular
velocities along the length of the BHA to effectively sweep cuttings to
the top of the BHA. Above the BHA, the fluid annular velocity drops
and the cuttings settle back to the low side of the wellbore. As the
cuttings above the BHA will have already achieved an equilibrium bed
height, these additional cuttings will be added to the mobile surface
layer and be swept up‑hole as the system attempts to maintain
equilibrium bed‑height.
The following simple guidelines can greatly reduce the risks
associated with back‑reaming:
Ì
Perform a clean‑up cycle (as per previous section), the intention
being to reduce the bed height to lower the pack‑off risk and
improve back‑reaming efficiency.
Ì
Commence back‑reaming slowly.
Flow rate and rotary speed must be maintained above hole
cleaning thresholds (to prevent the equilibrium bed height
from rising as additional material is deposited above the BHA).
Restrict pulling speed initially to no more than
30 min per stand (slips to slips).
Observe the first 5 ‑ 10 stands for pack‑off indicators (erratic
stand‑pipe pressure, torque and PWD).
Ì
If trends remain steady, gradually increase the back‑reaming
speed while continuing to monitor surface parameters.
Approximately 15 min per stand is a reasonable back‑reaming
speed (i.e. 4 stands/hr, including connection time). At higher
speeds, the risk of pack‑off increases significantly.
Ì
At the first signs of pack‑off indicators, stop pulling and allow
trends to stabilise.
If trends quickly return to background levels, continue
back‑reaming but at a reduced speed. This is an indication
that the material being cleared from around the BHA is not
being carried up‑hole quickly enough and is increasing the
equilibrium bed height above the BHA.
If trends do not stabilise immediately, then remedial
measures must be taken to prevent excessive pressure
Figure 4.
Back‑reaming in enlarged hole: Significant volumes of cuttings canbe rapidlymobilised
as theBHA enters enlarged hole, increasing the pack‑off riskwhere the surge of cuttingsmeets the
next interval of gauge hole.
Figure 3.
Equilibriumbedheight ingaugeandenlargedhole: Bedheight increases inenlarged
sectionsof holeas the flowareaabove thecuttingsbedattempts toequalise (i.e. achieveaconstant
fluidannular velocitywhich is sufficient tokeep themobile surface layer of cuttings inmotion).
