Page 407 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 383

Casing [ ]
3.4.1

drillpipe. During jetting, seawater or spud mud is pumped down the
drillstring with returns coming up inside the conductor.
Setting the conductor by jetting can be done quickly since it takes
only one trip. To be successful the seabed has to be soft enough to jet.
This may be determined by offset records and site surveys. It may be
necessary to wait on slack water to ensure that the conductor is verti-
cal before jetting begins.
Drilling and cementing the conductor. In some cases offshore, a
hole is drilled with returns to the seabed and the conductor pipe is run
and cemented to seabed. The bit will spud directly into the seabed
when no supporting structure is present on the seabed (e.g., a tem-
plate or temporary guide base). It can be tricky to get started; any
weight on bit and the drillstring will buckle slightly, causing the bit to
walk away across the seabed. Any current may also cause the well to
be spudded off vertical. Start with very low parameters but a reason-
able flow rate, which will erode the top part of the seabed. Once estab-
lished then WOB can be increased. Often the hole will be drilled as a
smaller diameter pilot, opened up to full size. A large hole requires
very high flow rates to keep clean. In addition, if any flow were
encountered, a smaller hole will produce less—until it washes out and
becomes a large hole!
You may have problems re-entering the hole if no guiding structure
is available. An ROV can be used to monitor the position of the bit rel-
ative to the hole. Once the conductor is run, it will be cemented via an
inner string until cement returns are seen at the seabed, then heavier
tail slurry would normally be pumped.
Driving the conductor. Check the conductor driving records for
offset wells in the area. This will give you an idea of how much pene-
tration you can expect. It may vary for your new well depending on the
pipe size, drive shoe configuration, hammer size/energy setting, and
water depth.
Soil-boring surveys can be done. These can be used to analyze the
likely depth of freefall and conductor penetration. If using a jack-up
rig, these surveys can also be used to estimate spud-can penetration for
the specific rig to be used. Often the drilling contractor’s insurers will
demand that a soil-boring survey be done with a spud-can penetration
analysis of the results. Soil borings can be done in advance of rig arrival

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