Page 80 - Practical Well Planning and Drilling Manual

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

[ ] Well Design
1.4.6

Surface casing is always cemented to surface. This is usually with
a drillpipe stinger so that cement can be pumped until returns are seen
at surface, when the tail slurry can be displaced around the shoe.
Intermediate and production casing/liner points. Intermediate
casing may not be needed on a shallow well, but one or more may be
needed on a deeper well.
Often the intermediate casing is set in a transition zone where pore
pressure (and usually fracture gradient) is increasing. This allows you
to drill deeper for the next casing point.
Based on the pore pressures and fracture gradients prognoses, cal-
culate how far you can now drill ahead within the acceptable limits of
kick tolerance from the assumed casing point. This is the maximum
depth you can safely drill to before setting casing, assuming that the
predicted pore pressure/fracture gradient happens in practice. See
Appendix 1 for calculating kick tolerances.
Now look over your hole section summary for the interval between
the surface casing shoe and the intermediate shoe. You need to decide
if there are any factors, which make it better to set casing higher than
the maximum you can drill to.
Can you drill through all those formations in one open hole inter-
val? What hole problems might you encounter, and would it be desir-
able to set casing higher to isolate one problem interval from another?
Can one mud type be used throughout the section, modifying the prop-
erties as the well deepens if necessary? Where would be a good place
to set your casing shoe—preferably competent shale? If you drilled to
the maximum allowed by kick tolerance would this separate different
target zones, which you might want to keep together in the same hole
section? What are the directional requirements likely to be and might
they affect your casing point? Look over the well objectives as well as
lithology and problem areas to ensure that you consider everything
that is relevant.
Once you have determined the optimum casing point, repeat the
exercise for the next section so that you can define all the casing point
vertical depths to the well TD. Then review the complete directional
and casing plans so far.
Now you have decided where casings need to be set and you have
finalized the directional plan. You know the vertical and measured
depths of the shoes and the hole and casing sizes. Before looking at the
forces that the casing has to resist, it is worth summarizing what mate-
rial considerations apply. (See Fig. 1-8)

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