circulation, and chemical caused by interaction of borehole fluid with the formation.
There are four different types of borehole instabilities:
hole closure or narrowing, hole
enlargement or washouts, fracturing, and collapse.
Fig. 10.7 illustrates hole-instability problems.
Fig. 10.7—Types of hole instability problems.
Hole Closure. Hole closure is a narrowing time-dependent process of borehole instability. It
sometimes is referred to as creep
under the overburden pressure, and it generally occurs in
plastic-flowing shale and salt sections. Problems associated with hole
closure are an increase in
torque and drag, an increase
in potential pipe sticking, and an increase in the difficulty of
casings landing.
Hole Enlargement. Hole enlargements are commonly called washouts because the hole
becomes undesirably larger than intended. Hole enlargements are generally
caused by hydraulic
erosion, mechanical abrasion caused by drillstring, and inherently sloughing shale. The
problems associated with hole enlargement are an increase in cementing difficulty,
an increase
in potential hole deviation, an increase in hydraulic requirements
for effective hole cleaning,
and an increase in potential problems during logging operations.
Fracturing. Fracturing occurs when the wellbore drilling-fluid pressure exceeds the formation-
fracture pressure. The associated problems are lost circulation and possible kick occurrence.
Collapse. Borehole collapse occurs when the drilling-fluid pressure
is too low to maintain the
structural integrity of the drilled hole. The associated problems are pipe sticking and possible
loss of well.
Before drilling, the rock strength at some depth is in equilibrium with the in-situ rock stresses
(effective
overburden stress, effective horizontal confining stresses). While a hole is being
drilled, however, the balance between the rock strength and the in-situ stresses is disturbed. In
addition, foreign fluids are introduced, and an interaction process begins between the formation
Dostları ilə paylaş: 
