Page 100 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 76

[ ] Well Design
1.4.13

If you are not calculating further tensile forces then compare the
total tension to lesser of [API minimum yield or connection tensile
strength] ÷ safety factor (use SF of 1.50 for a vertical well, 1.75 for a
directional well).
For instance, intermediate casing string of 9 /8 in casing, assuming
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first choice is 47 lbs/ft N80 from TD at 15,000 ft measured depth; 80 ft
shoe track. Test pressure will be 3000 psi. Cement is 500 ft of 0.82
psi/ft (15.8 ppg) neat “G” cement from the shoe and 3500 ft of 0.70
psi/ft (13.2 ppg) cement above that. Mud in use is 0.65 psi/ft (12.5
ppg). Connection is buttress. Well is vertical.
Weight in air is 705,000 lbs, plus 100,000 lbs overpull allowance
equals 805,000 lbs. Minimum yield strength is 1,086,000/1.1 (safety
factor for overpull) = 987,000 lbs so the tension due to its own weight
plus overpull is within limits. Note also that for a regular buttress con-
nection, pipe body strength is less than connection strength. If the
connection were a special clearance buttress, then the connection
would be weaker than the body strength and that would be the limit-
ing factor—in this case, 983,000 lbs, which is still adequate for this
example.
Weight of casing contents; casing capacity is 3.073 gals/ft. Weight
of mud therefore is 14,920 x 3.073 x 12.5 = 573,100 lbs. Weight of
cement in the shoetrack = 3900 lbs. Weight of steel = 705,000 lbs. Total
downward force = 1,282,000 lbs.
Casing displacement is 3.778 gals/ft. Weight of displaced fluids =
(500 x 3.778 x 15.8) + (3500 x 3.778 x 13.2) + (11000 x 3.778 x 12.5)
= 724,000 lbs total upward buoyant force. So the net tension due to
buoyant weight at the top is 558,000 lbs.
The force exerted by a 3000 psi test on an ID of 8.681 is 0.7865 x
8.681 x 3000 = 178,000 lbs.
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So after cementing the casing and bumping the plug, the tension at
the top joint will be 736,000 lbs. Applying a safety factor of 1.50 (safe-
ty factor for a vertical well) to the minimum body yield strength of
1,086,000 lbs gives an allowable tension of 724,000 lbs—not enough
in this case.
Without further calculations to reduce the safety factor or dispen-
sation to allow a lower safety factor, a mixed casing string would have
to be run. P110 grade 47 ppf run at the top would give an allowable
tension of 995,333 lbs at a safety factor of 1.50. If a triaxial analysis
were to be done, it is possible that a mixed 53.5 or 47 ppf N80 string

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