Page 286 - Drilling Technology in Nontechnical Language

Chapter 11 – WELL CONTROL 277

Shallow gas on oating rigs

The best place to be if a shallow gas flow occurs is watching from
a safe distance upwind! The second best place to be is on a floating rig,

drilling with returns to the seabed.

Surface hole on floating rigs is normally drilled without returns to
the rig. After cementing conductor pipe in the seabed, the next BHA is
run into the conductor using guidance from subsea video cameras. Mud
and cutting returns from the annulus exit into the sea. If a shallow gas
flow occurs, and if this presents a danger to the rig (gas appearing on the

sea surface close to the rig), the driller drops the drillstring, and the rig
moves away.

Sometimes on floating rigs, returns are taken back up to the rig. This
might be the case in deeper water, or if government regulations prohibit
cuttings being exhausted to the sea. It is possible to use a subsea diverter,

which is deployed on top of the conductor on the seabed. If the well flows,
the diverter bag preventer closes to stop the gas being channelled up to the
rig by the riser pipe, and the gas exhausts to the seabed. Shear rams should
be run as part of the diverter system so that the rig can easily disconnect
the riser from the diverter and move off location.

Special Well Control Considerations

There are, of course, some circumstances where well control becomes
quite complex. Some of these circumstances are briefly described so

as to give familiarity with the terms and a basic understanding of what
is meant.

High-angle/horizontal well killing

The example discussed previously in this chapter assumed that the
well was vertical. If the well in fact is drilled at a high angle, killing the
well while maintaining a constant bottomhole pressure is a bit different. In
this case, the driller’s method of killing the well may be more appropriate.
In this method, the well is killed in two circulations. No calculations
are required for the first circulation; normal density mud is used to

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