Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Mechanical Instability. As stated previously, mechanical rock instability can occur because the
in-situ stress state of equilibrium has been disturbed after drilling. The mud in use with a certain
density may not bring the altered stresses to the original state; therefore, shale may become
mechanically unstable. 
Chemical Instability. Chemical-induced shale instability is caused by the drilling-fluid/shale
interaction, which alters shale mechanical strength as well as the shale pore pressure in the
vicinity of the borehole walls. The mechanisms that contribute to this problem include capillary
pressure, osmotic pressure, pressure diffusion in the vicinity of the borehole walls, and
borehole-fluid invasion into the shale when drilling overbalanced. 
Capillary Pressure. During drilling, the mud in the borehole contacts the native pore fluid in
the shale through the pore-throat interface. This results in the development of capillary
pressure, p
cap
, which is expressed as 
....................(10.8) 
where σ is the interfacial tension, ϴ is the contact angle between the two fluids, and r is the pore-
throat radius. To prevent borehole fluids from entering the shale and stabilizing it, an increase
in capillary pressure is required, which can be achieved with oil-based or other organic low-
Mechanical Rock-Failure Mechanisms
Shale Instability

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polar mud systems. 
Osmotic Pressure. When the energy level or activity in shale pore fluid, a
s
, is different from the
activity in drilling mud, a
m
, water movement can occur in either direction across a
semipermeable membrane as a result of the development of osmotic pressure, p
os
, or chemical
potential, μ
c
. To prevent or reduce water movement across this semipermeable membrane that
has certain efficiency, E
m
, the activities need to be equalized or, at least, their differentials
minimized. If a
m
is lower than a
s
, it is suggested to increase E
m
and vice versa. The mud activity
can be reduced by adding electrolytes that can be brought about through the use of mud systems
such as seawater, saturated-salt/polymer, KCl/NaCl/polymer, and lime/gypsum.