BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
Competent Formations
There are two extremes of mechanical hole
instability, referred to as compressive failure
and formation breakdown (Figure 7 - 1).
Compressive failure occurs when the drilling
fluid density is too low – this results in hole
closure (tight hole) or hole collapse. In
contrast to this, formation breakdown occurs
if the drilling fluid density is too high.
Drilling fluid pressure may induce a fracture
or open a natural fracture system, leading to
massive mud losses.
In general, hole sections containing
shales/mudstones will collapse if given
insufficient support, and sands/carbonates
will lead to mud losses and/or differential
sticking if drilled with too high an
overbalance.
Figure 7 - 1 Forms of Mechanical Instability
To drill a hole section with little or no instability problems requires the maximum drilling fluid
density tolerated by the
sand/carbonates to exceed the
minimum drilling fluid density
required to support the mudstones.
These upper and lower bounds to
the drilling fluid density define the
“mud weight window” (see Figure 7
- 2). The wider the window the
easier the well is to drill.
Conversely, the narrower the
window the more difficult it is to
contain the drilling fluid density
within the stable region, and hence
the risk of wellbore instability is
much greater.
Figure 7 - 2 Defining “Mud Weight Window”
In certain highly tectonically stressed regions (e.g. foothills of the Casanare region in
Colombia) the collapse gradient in the shales can exceed the fracture gradient in the sands,
even for nominally vertical wells. In such cases there is no drilling window and it is impossible
to select a drilling fluid density to simultaneously avoid both losses and collapse. Hence, one or
both forms of instability must be tolerated to some extent.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-3