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198 Part III Underbalanced Drilling Systems

the dominant fluid. Thus, the point defining the capillary parachor is
used to represent the critical point (point C) on the capillary pressure
curve.
Owing to the parabolic nature of the capillary pressure curve, it is
usually impossible to use a UBD pressure differential that is high enough
to completely balance the capillary pressure at the initial water saturation.
This is especially true for low permeability reservoirs, where the capillary
pressure is normally very high at the initial water saturation. Fortunately,
such a high-UBD pressure differential is not required because the capil-
lary pressure drops rapidly with a small amount of water imbibition that
should not cause significant formation damage. The UBD pressure differ-
ential should be just high enough to balance the critical capillary pressure
defined at the capillary parachor. It is possible to keep the formation
damage minimal using the critical pressure differential that is equal to the
critical capillary pressure.

9.3.2 Collapse Pressure Analysis

A number of methods are available in the literature for wellbore collapse
analyses. These methods include those used by Bradley (1979) and
Aadnoy and Chenevert (1987). McLean and Addis (1990a) presented a
review of these methods. The major difference among these methods is
that they use different rock failure criteria such as Mohr-Coulomb, von
Mises, Outer Drucker-Prager, Middle Drucker-Prager, and Inner Middle
Drucker-Prager. McLean and Addis (1990b) presented their studies on
the effect of rock strength criteria on mud weight recommendations.
While the Mohr-Coulomb criterion suggests that an inclined borehole
collapses first at the lateral sides of the hole, other criteria imply that
an inclined borehole collapses first at the top side of the hole. More
evidence from horizontal well drilling supports the Mohr-Coulomb
criterion.
Borehole collapse analysis involves comparing failure stress to failure
strength. The following data are required to complete a borehole collapse
analysis:
• In situ formation stresses (both magnitude and direction)
• Pore pressure
• Rock elastic properties (elastic modulus and Poisson’s ratio)
• Rock failure parameters (shear strength, tensile strength, and friction
angle)