Page 322 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 298

[ ] Well Programming
2.9.1

into the drilling program. The drill crews must be properly briefed on
the effect of some drilling practices on wellbore stability. Connection
and tripping practices in particular are within the control of the drill
crew and a lack of knowledge and/or care here can make the problems
much worse.
Surge and swab pressures—connection practices. Pumps should be
started and stopped carefully. Kick in one pump at low speed, and as cir-
culating pressure comes up, increase flow rate steadily. Similarly, do not
switch off the pumps too quickly since this reduces ECD too fast for the
formation near wellbore stresses to react to the removal of pressure.
Tripping practices. Tripping causes substantial mud pressure varia-
tions (swab and surge). This will promote hole failure—both collapse
and fracturing; the more tripping, the more risk of hole instability.
However, tripping serves other purposes such as bit changes and hole
cleaning; these will usually override borehole stability considerations.
The exception here should be wiper tripping. Routine wiper trips are
often done without justification and can lead to wellbore instability
with no improvement in hole condition. Do not wiper trip unless this
can be justified. When tripping, do not try to run or pull pipe too
quickly. Keeping rheology and gels low will minimize the effects of
flow rate changes and pipe movement.

Open hole time.
Borehole collapse increases with open hole time. This is demon-
strated routinely by comparing caliper runs taken at different intervals
after drilling. Time-dependent rock deformation is especially noticeable
in salt zones; creep inevitably results in hole closure. Other types of
rocks show some degree of time dependency. Unconfined compressive
strength may decrease by 50% as time progresses, and thus explains at
least part of the observations when drilling with a WBM. Another
important factor is the fluid invasion in low-permeability rocks (e.g.,
shale). In general, fluid invasion and the swelling associated continue
with time, gradually enlarging the affected zone and reducing stability.
Therefore, factors such as bit selection and parameters that influence
the rate of progress can have an effect on wellbore stability.

Hole orientation
Collapse. The combination of in-situ stresses and hole orientation
governs the stresses that act to destabilize the rock (collapse mode)

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