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Pilot holes in deepwater.
In deep water drilling operations, it is often practiced to drill a pilot hole to assess
potential geo-hazards near the planned well location without the use of a marine riser. A
small pilot hole, typically 9 7/8 – 12 ¼”, is drilled to a depth which is below the planned
setting depth of the surface casing. Pump rate is held constant while drilling the pilot
hole and pump and annular pressure measurements are utilised as the primary indicator
of well flow. If changes in pump pressures are observed while drilling the pilot hole,
drilling is ceased and the ROV camera or drilling measurement tools will be used to
determine If well flow is detected. If flow is observed, dynamic well killing operations are
then immediately initiated by pumping kill mud at a pre-determined modelled kill rate with
the rig pumps until the well is killed.
If no well flow occurs after reaching total depth of the pilot hole, the hole is displaced with
mud before pulling out with the drilling assembly. Once it has been determined that the
well can be safely drilled without the riser, the well is drilled riser less to the setting depth
of the surface casing string(s).
The two basic reasons for sacrificial riserless pilot holes in deepwater operations are.
1. It is difficult to bring mud returns to the surface as water depth increases, due to
the fact that the formation fracture gradient decreases with increasing water
depth. The formation below the “normal” setting depth is thus unable to support the
hydrostatic pressure of the mud column in the riser.
2. Drilling a small pilot hole and having seawater hydrostatic pressure available for
back pressure may enhance well killing ability (although there are differing opinions
on this).
If a gas or shallow water flow does occur while drilling in deep water, flow is deflected
down current and will safely surface away from the rig. I.e. To reduce risk of fire or
explosion occurring on the rig. |
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发表于 2009-5-4 14:58
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