Page 96 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 72

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1.4.12 Well Design

collapse loadings need to be calculated together and the net burst or
collapse obtained for the worst case while drilling, producing, stimula-
tion, or other operations to which it may be subjected.
Burst loads may occur from combinations of:

Hydrostatic head. For a fluid of single density, the burst load equals
the vertical depth multiplied by the fluid gradient. If casing is full
of mud of 0.5 psi/ft gradient, the burst pressure from hydrostatic at
8000 ft TVD will be 8000 x 0.5 = 4000 psi. Where multiple fluids
are involved (e.g., in cementing), the hydrostatic pressure of each
is calculated separately and added up.
Applied surface pressure. This may occur from pressure testing,
from a kick while drilling, from a leaking production string, from
support pressure while fracturing, from injection pressure for
gaslift, or chemical injection in the production casing. This surface
pressure is added to the hydrostatic head to give the total internal
pressure at any particular depth. For instance, with 500 psi surface
pressure applied to the previous example, the internal pressure at
8000 ft TVD is 4500 psi.

Collapse loads may occur from:

Hydrostatic head of mud or cement outside the casing. Calculated
for burst as previously discussed in this section.
Flow from lower zones migrating upwards outside the casing,
exerting extra pressure in addition to hydrostatic head of annulus
fluids. For this reason annular pressures are monitored with a
gauge on the wellhead side outlet. Calculated for surface pressure,
as previously discussed in this section.
Drawdown pressure while producing in the production casing or
liner.
Mobile formations such as massive salts. It is normally assumed that
mobile salts can impose overburden pressure (1 psi/ft) on the cas-
ing. This may require extra heavy wall pipe across a massive salt.

Biaxial effects account for the change in burst and collapse resis-
tance due to tension or compression of the casing. A pipe under ten-
sion will have increased burst and reduced collapse resistance; con-

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