Page 91 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 67

Casing Design [ ]
1.4.10

job), then it is easier to use Archimedes’ principle by calculating
the weight of casing + contents and subtracting the weight of the
displaced fluids.
Inside the casing there is cement tail slurry in the shoetrack and
mud above the float collar. Outside the casing there is (from bot-
tom to top) tail slurry, lead slurry, spacer, and mud. If the tail slur-
ry is 500 ft measured depth interval and weighs 15.8 ppg outside
9 /8 in casing then the weight of displaced slurry is (500 ft x 15.8 x
5
3.78 [gals/ft displacement]) or 29,862 lbs. Make the same calcula-
tion for all of the fluids outside the casing, total these figures up
and this is the total buoyant force acting upwards on the bottom of
the casing.
In a vertical well, the total force up acts on the bottom end of the
casing. The weight of the fluids inside the casing act downwards on
the cement plug at the bottom of the casing. The difference between
these two forces puts the bottom of the casing in compression.
The neutral point for tension can be calculated by working out the
length of casing in air that equals this net force up and subtracting
that length from the total length. This will give you approximately
the depth of the neutral point, above which will be in tension and
below which will be in compression. This will not be the case in a
deviated well since the buoyancy force acts perpendicular to the
surface of the immersed object, causing a net buoyant effect on the
sides of an inclined pipe as well as at the end. This is explained in
point #4. If different weights of casing are used, this will also
change the neutral point, as explained in point #3.
Buoyancy force calculated as described above is useful to deter-
mine the buoyant weight of a closed-end object that has different
fluid densities inside and outside; e.g., casing after bumping the
cement plug. In a casing string where one weight of casing is used
for the whole string, axial stress can be calculated at any depth by
subtracting the weight in air of the pipe below the depth of inter-
est from the buoyancy force.

3. Calculate hydrostatic pressures and multiply by cross-sectional
areas (see Fig. 1-12).

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