Page 318 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 294

[ ] Well Programming
2.9.1

lapse of the hole, bit balling, sloughing, or creep. The problem is esti-
mated to cost the industry at least $500 million per year.
Shale instability is closely connected with bulk properties of shales
such as strength and deformation, which are a function of deposition-
al environment, porosity, water content, clay content, composition,
compaction history, etc. The mud bulk properties such as continuous
phase chemical makeup and concentration, properties of any internal
phase, additives associated with the continuous phase, and system
maintenance are also important. Understanding the fundamental
physics and chemistry of the mud/shale interaction is critical.
Other factors such as in-situ stresses, pore pressure, temperature,
time in open hole, depth and length of open hole interval, and sur-
rounding geological environment (salt dome, tectonics, etc.) directly
impact drilling and completion operations.
This is an extremely complex subject in which our knowledge
is not yet complete. For more in-depth information, refer to the
many SPE/IADC technical papers and other authoritative sources
of information.
The parameters influencing wellbore stability can be divided into
five main groups: drilling fluid, rock properties, in-situ stresses,
drilling practices, and drilling mechanics.
Drilling fluid. Shales are made up of layers of flat crystals. Between
the crystals are tiny spaces (in the order of nanometers in size). Drilling
through shales exposes these pore spaces to the wellbore.
With water-based muds, filtrate may enter into these pores and
cause shale instability through two mechanisms:

Causing a local increase in the pore pressure in the near-wellbore
region (pore pressure penetration)
Chemically hydrating the shales

Assuming for a moment that the pore pressure and the wellbore
hydrostatic pressure are the same, then the formation stresses will act
to push shale into the wellbore. If the shale is weak in tension (which
it is), these stresses will be large enough to cause tensile failure and the
shale will destabilize. If, however, the mud hydrostatic exceeds the
pore pressure by a margin which equals or exceeds the stresses in the
formation, then tensile failure will not take place and the shale will be
physically stable.

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