Page 109 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 85

1.4.16
Casing Design [ ]

the actual coefficients have been derived from data obtained while
drilling on the same or an offset well.
Actual m will depend on hole rugosity, lithology, wallcake thick-
ness and lubricity, mud lubricity, and relative sizes of casing and hole.
To calculate the torque:

Refer to Section 1.4.10, “Methods of Applying Buoyancy Effects,”
which describes how to calculate the effect of hydrostatic (buoy-
ancy) forces acting laterally on an inclined pipe.
Refer to Section 1.4.13, “Calculating Axial Loads,” which describes
how to calculate the effect of increasing hole angle on sidewall
forces in air.
Subtract the buoyancy forces from the sidewall forces to give the
actual force pressing the pipe against the wellbore wall.
In general,

Force due to friction between surfaces = coefficient of friction
(m) x area x force pressing the surfaces together.

However, in calculating frictional forces between casing and wall,
length is used rather than area and m is generally assumed to be 0.35
in open hole and 0.20 inside another casing.
Therefore, to calculate the frictional force acting against pipe rota-
tion, calculate the actual force pressing the pipe against the wellbore
wall F , multiply by m and then by the length L (in feet) under con-
s
sideration; divide by the pipe radius r (in feet).

L××µ Fs
Torque, ft lbs =
r

1.4.16. Triaxial Stress Analysis

Triaxial analysis of your casing design may be done in normal
cases and should be done in critical cases. What triaxial analysis aims

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