Page 78 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 54

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1.4.6 Well Design

away—because they may give useful data. Now get together the lithol-
ogy and pore pressure/fracture gradient prognoses for your well, with
the hole section summaries, well proposal, and site survey (including
shallow gas indicators, if any).

1.4.6. Individual Casing Points

The individual casing points will now be discussed. Criteria for
physical properties of each of these casings will be summarized later.
Conductor. Start off at the top of the well by defining where your
conductor pipe should be set. It protects topsoil from washing out and
allows returns to a flowline. It may have to support future wellhead,
casing, and completion loads. A conductor will allow a diverter to be
set for drilling ahead.
The conductor depth will be determined by one of several factors:

1. On an offshore bottom-supported rig, it should be set at least deep
enough to withstand the extra hydrostatic head imposed by bring-
ing returns up to the flowline. This can be calculated using the for-
mulae in Appendix 5.
2. If driving the conductor (whether on land or offshore), it should be
driven to refusal. If the refusal depth is insufficient to allow returns
to the flowline, you will have to secure the conductor and drill out
of it and then recommence driving.
3. Offshore, you may drill into the seabed (perhaps with a template or
guide base on a floater) and cement the conductor in place. The set-
ting depth does not need to be greater than what is required to pre-
vent washing out at the seabed, if the next section will be drilled for
surface casing from a floater with returns to seabed. If a riser is to
be nippled up or if the conductor extends to the flowline then the
shoe depth should be calculated as per factor 1.
4. On a floating rig where the seabed is soft enough, the conductor
may be jetted in if the surface hole can then be drilled with returns
to the seabed. For jetting in, the intention is to set the pipe deep
enough only to prevent washing at the seabed, and the following
section will be drilled for surface casing riserless with returns to
seabed. If a guideline system is used, the guidebase is made up on
top of the conductor and run with it; otherwise a guide funnel is
made up on top for a guidelineless wellhead system. The length of
conductor is such that you can jet in to leave the guidebase about
5 ft above the mudline.

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