Page 103 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 79

Casing Design [ ]
1.4.13

Shoe at 15,000 ft TVD, plugs at (15,000 - [80 Cos 25]) =
14,928 ft TVD. Internal pressure is therefore (14,928 x 0.65)
+ (72 x 0.82) = 9762 psi, acting down on an area of 59.19 in 2
= 577,789 lbs.

Shoe at 15,000 ft, vertical interval of tail slurry is 500 Cos 25 =
453 ft, vertical interval of lead slurry is 3500 Cos 25 = 3172 ft,
therefore, top of cement (TOC ) is 11,375 ft TVD. Weight of
displaced fluids = (11,375 x 0.65) + (3172 x 0.686) + (453 x
0.82) = 9941 psi, acting on an area of 72.76 in 2 = 723,321 lbs.

Net axial hydraulic force at the shoe = 145,532 lbs.
So total tension due to buoyant casing weight at kickoff point
(KOP) = 658,000 - 145,532 = 512,468 lbs.
Add tension due to pressure test (as previously calculated),
tension at KOP = 690,468 lbs.

This is over a cross-sectional area of 0.7854(D - d ) = 13.57 in so
2
2
2
the stress at this point (load ÷ area) is 50,881 psi. Tensile stress due to
bending in the outer fibers of the casing is 5248 psi. Therefore, the total
stress at the outside of the curve is the sum of the two, or 56,130 psi.
The minimum yield stress of 47 ppf N80 casing is 80,000 psi so the
safety factor at this point is 1.43 (too low). With a safety factor of 1.5,
the minimum yield stress should be 84,195 psi so 47 ppf C90 run over
the deviated section would suffice, or dispensation could be requested
to accept the lower safety factor. In practice, the tension in the steel
along the outside of the curve would be transmitted to the adjacent cas-
ing, taking some distance to dissipate the load. Running 100 ft of 47
ppf C90 above and below the dogleg section will suffice.
Axial forces in casing due to friction in a deviated wellbore (tangent sec-
tion). In order to calculate the effect of friction, we need to calculate the actual
force exerted between the wellbore wall and the casing.

Refer to Section 1.4.10, Methods of Applying Buoyancy Effects,
which describes how to calculate the effect of hydrostatic (buoy-
ancy) forces acting on an inclined pipe so that the force against the
wellbore wall can be found.

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