Page 170 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 146

[ ] Well Programming
2.2.1

high probability of equipment failure. The risk of shallow gas should
be assumed to be present in pre-BOP drilling, even for the last well on
a multislot platform.
In top hole, the conductor shoe strength is too low to close in on
a kick. If closed in, the flow could fracture formation from the shoe to
the surface. A jack-up or platform could lose support. If the well is
brought under control after such a blowout to surface, the well would
probably have to be abandoned. A well can be drilled quite deep on a
diverter system; any gas accumulation encountered before BOPs are
nippled up is classified as “shallow gas.”
A shallow gas kick may occur by drilling into an overpressured
gas-bearing zone, by drilling into a producing well, or into an artifi-
cially charged zone due to a poorly cemented annulus on a nearby well.
It may be high or low volume, it may or may not deplete quickly, and
it may or may not bridge itself off before it depletes. It could also be
normally pressured and kicks due to swabbing, losses, not filling the
hole, gas from cuttings reducing hydrostatic, or other mechanisms that
reduce hydrostatic below normal.
Advance detection. There are techniques that will aid in pre-spud
detection of shallow gas. They include:

Shallow seismic surveys (gives indications of gas accumulations
but is no guarantee either way)
Pilot hole drilling using a soil-sampling vessel. This allows opti-
mizing of the conductor setting depth before spudding a well off-
shore and can help determine likely spud can penetration.
Offset well reports
Platform piling reports

Assess the likelihood of shallow gas, then decisions can be made
regarding rig selection, drilling techniques, and contingency planning.
Drilling riserless from a floating rig is the best situation offshore if
shallow gas is hit. The rig can be quickly moved off location, and the
sea will inhibit flow and fill the hole once it has depleted or bridged off.
Jack-ups normally cannot move off quickly enough and could capsize
if the seabed fractures.
Planning and training are essential preparation to handle shallow gas.
Riserless drilling. When drilling from a floating rig, the riser can-

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