Page 125 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 101

1.4.24
Casing Design [ ]

Table 1-3 Casing Design Criteria—Intermediate Casing
Drilling Criteria
Burst 1. Resist the anticipated formation breakdown pressure at the shoe
with a gas gradient to surface (0.1 psi/ft inside). Maximum burst at
surface; depth (ft) x [formation gradient - 0.1 psi/ft].
2. Casing test pressure at top of casing.
3. Casing test pressure plus mud hydrostatic to shoe less external
pressure at shoe exerted by the mud used to run casing.

Safety factor 1.1.
Collapse 1. Calculate collapse pressure at shoe when cement is displaced. Use
planned fluid heights and densities inside and out.
2. Assume losses taken at next hole TD. Expected external pressure at
the loss zone; planned mud in use drops to height supported by thief
zone pressure. Maximum collapse may be at the top of the mud col-
umn or at the shoe. Calculate external pressure exerted by mud in
hole when the casing was run.
3. If there are any plastic squeezing formations, resist 1 psi/ft outside
at the relevant depth(s). Ensure competent cement sheath.

Safety factor 1.1.

Tension 1. Weight of casing in air plus overpull allowance of 100,000 lbs.
Safety factor 1.1.

2. Axial loads due to buoyant weight plus test pressure.

Safety factor 1.75 if deviated or 1.5 if vertical.
3. Axial loads due to buoyant weight plus test pressure plus bending
stress if deviated.

Safety factor 1.5.
4. Triaxial analysis incorporating temperature correction for yield
strengths.
Safety factor 1.25.

Buckling If deep string is in vertical hole and where temperature and/or internal
pressure will significantly increase while drilling next hole section, cal-
culate revised F and F and take necessary action to prevent buckling.
s
b
Other If cementing against massive salts, cement sheath competence is as
important as the casing strength properties.
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