Page 172 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 148

[ ] Well Programming
2.2.1

If the equipment is available offshore, consider subsea diverter
lines and valves. It is better to avoid bringing shallow gas up to the rig.
Run the casing with the diverter in place; do not nipple down until
the cement is hard. Flush cement from below the wellhead with small
diameter lines. A well can kick as cement sets and hydrostatic is lost.
Losses below the conductor shoe are common when drilling in divert
mode offshore, due to the extra hydrostatic of bringing returns above sea
level. LCM will not stop these losses. This will reduce the integrity of the
shoe and, on a jack-up, could affect spud can integrity. It is risky to drill
ahead with losses while diverter drilling; serious losses should be cured
with cement prior to drilling ahead. Refer to “Suggested procedure for
curing total losses with cement.”
Other precautions when shallow gas is likely. A small diameter
pilot hole should be drilled and opened up later. This will reduce the
flow rate if shallow gas blows out and it will make bridging off more
likely in the early stages, though it can enlarge fairly quickly. Care must
still be taken when opening hole, since primary control can still be lost
by swabbing, etc.
Logs can be run after drilling the pilot hole. If gas is seen, plug and
abandon the well rather than risk opening the hole and running cas-
ing. If a kick occurs during either of these operations, the situation is
dangerous. Alternatively a cement plug can be set from bottom to
above the gas and casing set higher, if the formation strengths allow a
BOP to be nippled up afterwards.
If the wellbore penetrates shallow gas, the drilling fluid may con-
trol the pressure. The well could kick later by swabbing, not filling the
well on trips, lost circulation, cement gelling, etc. The driller must
maintain good practices. The cement and mud programs should aim to
minimize the chances of problems. Fast ROP is usual in top hole and
if gas-bearing cuttings are produced, liberated gas from the cuttings
could reduce hydrostatic enough for the well to kick, so restrict ROP.
Look at reaction times. Large bag preventers can take over a minute
to close. Use the shortest length of large diameter hydraulic control
lines on both sides (both open and close lines). The diverter line valve
has to open the valve fully before the diverter fully closes, or a pressure
surge could break down the shoe. A low-pressure bursting disk could be
used in place of a valve. Ideally the divert sequence is started by the
driller pushing one button, causing the overboard lines to open and

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