Page 171 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 147

2.2.1
Well Control [ ]

not be connected until surface casing has been set. A jack-up will often
drive conductor pipe extended to surface and set a diverter on top. If
drilled and cemented rather than driven, it will be drilled riserless since
there is nothing to attach a diverter to. If shallow gas blows out during
riserless drilling, the well will flow to the sea until the gas accumula-
tion is exhausted or until the well bridges itself off.
When drilling from a floating rig, riserless drilling is preferred
to using a surface diverter. The slip joint is especially vulnerable to
leaks and damage under diverted flow conditions, jeopardizing the
rig and personnel.
A small diameter pilot hole will reduce the gas flow and will be
more likely to bridge off due to produced formation. If the gas flow rate
causes instability or fire concerns, a floater can move off location. The
deeper the water, the more that the gas will disperse as it rises and any
current will take the gas bubbles away from the rig. It is unlikely that
rig stability will be seriously compromised except if there is a very
strong flow in shallow water.
Santa Fe in Kalimantan has used a technique for diverterless
drilling of a surface pilot hole for a jack-up rig. The rig is lightly pinned
in place on the seabed and the towboats left connected. With zero air
gap a small diameter bit is run in and the pilot hole drilled. If shallow
gas is encountered, the string is dropped, the legs lifted, and the rig
pulled off location by the towboats. This is only applicable where the
bottom is fairly hard and gives little spud can penetration. See refer-
ences in Section 2.2.5, “References for Well Control—Shallow Gas.”
Note that shallow gas can occur at any depth. Therefore, it is pos-
sible that shallow gas can be met while conductor driving.
Diverter drilling. If the shoe strength is insufficient to close in a
kick, flow must be diverted away from the rig—preferably downwind.
Two lines are needed to prevent diverting upwind, whichever way the
wind is blowing. A windsock should be visible to the driller. The
diverter lines must be large diameter (at least 12 inches ID), straight,
with no restrictions, and securely attached to the rig. Large pieces of
formation may be produced with the gas. Erosion from produced sand
can cut steel lines on bends. The line valve mechanisms must open
fully and quickly, or bursting disks can be used. The line has to be kept
clear of cuttings and debris.

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