Page 285 - Drilling Technology in Nontechnical Language

276 Drilling Technology in Nontechnical Language Second Edition

Dynamic killing of a shallow gas flow is unlikely to be successful

except in rather narrow circumstances. To have any chance of working, a
dynamic kill must have the following:

1. Very high flow rates. The maximum pump output on most rigs

will be insufficient.
2. Small hole diameter. Once the blowout is established, large
volumes of formation solids are blown out of the well so that
the hole rapidly enlarges. This means also that an attempt to
dynamically kill the well must be made within seconds of a flow

being identified.
3. Small annular clearance. If large-diameter drill collars and
drillpipe are used, this reduces annular clearance and hence
increases the annular pressure loss.
4. Increased drilling fluid density and viscosity. Surface holes are

generally drilled with low-density fluids because the formations

drilled are weak. Using high-density, highly viscous fluid would
increase the risk of mud losses into the formation. It is even
possible to cause formations to fracture by using high-density
drilling fluid. This implies that a tank of heavy, viscous kill mud

must be kept ready to pump at maximum rate as soon as the need

is identified.
If a dynamic kill is the chosen method of killing a shallow gas blowout,
this must be planned in advance to address each of these elements if
there is to be any realistic prospect of success. Crew training is vital, as
is implementing equipment and procedures to detect and respond to an
influx as early as possible.

Ultimately if the well is not killed quickly and the blowout develops,
the chance of equipment failure is high. The extremely erosive nature of

the flow (gas plus formation solids being an effective sandblaster) means
that sooner or later, holes will appear in the diverter system, allowing the
flow to enter the rig. H S is sometimes encountered in shallow gas flows,

2
which presents an immediate and serious threat to life. It is to be hoped
that the accumulation of shallow gas is small, and after a short while, it

depletes, and the flow stops. It is also possible that chunks of rock coming
up the wellbore get stuck and form a bridge, stopping the flow.

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