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1.4.5
Casing Design [ ]
This formula can be simplified to:
o − pv
f + p
=
1 − v
where
ρf= Fracture gradient, psi/ft
ρo= Overburden gradient, psi/ft (generally assume 1 psi/ft)
ρp= Pore pressure gradient, psi/ft (may be measured or estimated)
v = Poissons ratio
1.4.5. Casing Shoe Depth Determination: General Points
Defining the casing setting depths has to take several different fac-
tors into account. In a directional well, casing points and directional
planning are intimately entwined and may take several iterations to
achieve a good overall design.
The first step is to decide which formations would give a compe-
tent shoe (i.e., one that will hold a reasonable wellbore pressure assum-
ing that the cement job is good). Refer to the geological information,
lithology column, pore pressure and frac gradient prognosis, hole sec-
tion summaries of the offset wells, and any other available data. You are
normally looking for competent shales or unfractured limestones that
are impermeable and have a reasonable fracture gradient.
Of particular significance is the existence of a pressure transition
zone. If casing is set just above a transition zone then pore pressure in
the next section will increase shortly after drilling out and the kick tol-
erance will reduce as mud density is increased. It may reduce enough
to prevent drilling the following section to the planned depth. On the
other hand, if a transition zone is penetrated too deeply a kick may
result in the worst circumstances since there will be a lot of open hole
under the previous casing shoe. (See Fig. 1-7)
Ideally, casing should be set deeply enough in the transition zone
to give a sufficient kick tolerance for the next hole section, while main-
taining enough kick tolerance in the current hole section.
Where a drop in pore pressure is expected, casing could be set just
above it to give the best shoe strength prior to drilling in to the weak-
er zone (see Fig. 1-7)
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