Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Types and Associated Problems
Principles of Borehole Instability

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and borehole fluids. The result is a potential hole-instability problem. Although a vast amount of
research has resulted in many borehole-stability simulation models, all share the same
shortcoming of uncertainty in the input data needed to run the analysis. Such data include in-
situ stresses, pore pressure, rock mechanical properties, and, in the case of shale, formation and
drilling-fluids chemistry.
Mechanical borehole failure occurs when the stresses acting on the rock exceed the compressive
or the tensile strength of the rock. Compressive failure is caused by shear stresses as a result of
low mud weight, while tensile failure is caused by normal stresses as a result of excessive mud
weight. 
The failure criteria that are used to predict hole-instability problems are the maximum-normal-
stress criterion for tensile failure and the maximum strain energy of distortion criterion for
compressive failure. In the maximum-normal-stress criterion, failure is said to occur when,
under the action of combined stresses, one of the acting principal stresses reaches the failure
value of the rock tensile strength. In the maximum of energy of distortion criterion, failure is
said to occur when, under the action of combined stresses, the energy of distortion reaches the
same energy of failure of the rock under pure tension.
More than 75% of drilled formations worldwide are shale formations. The drilling cost attributed
to shale-instability problems is reported to be in excess of one-half billion U.S dollars per year.
The cause of shale instability is two-fold: mechanical (stress change vs. shale strength
environment) and chemical (shale/fluid interaction—capillary pressure, osmotic pressure,
pressure diffusion, borehole-fluid invasion into shale).