Page 324 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 300

[ ] Well Programming
2.9.1

rock material. This may lead to creation of cracks or cavings. The ini-
tiation of conductive cracks facilitates the invasion of mud and may
jeopardize pressure isolation measures. Cracks also lead to a reduction
in overall rock strength. Drillstring vibrations are more severe for hard
rocks since the excitation at the drill bit is larger. It is feasible that
severe hole enlargement observed in some hard rock sections may be
related to drillstring vibrations. On the other hand, severe washouts
enhance vibrations. Minimizing drillstring vibrations should reduce
borehole instability.
A short story. Time to make a connection. The driller stops the
pumps by winding the pump controls rapidly to off. ECD drops very
fast (in a few seconds). The pore pressure in the near wellbore region,
which has built up during the time since the last connection, now
exceeds the wellbore hydrostatic. Since the permeability is very low,
pressures have no time to equalize and so the shale has temporarily lost
the hydrostatic overbalance that was keeping it stable. Cracks start to
appear in the formation. After the connection, the driller winds up the
pump control to the chalk mark he has made on the control panel. The
pump kicks in rapidly. The mud in the annulus has a lot of inertia due
to its weight and has gelled up a bit while static. These two factors
resist the sudden initiation of flow therefore a rapid pressure peak is
built up. Whole mud is driven into the tiny cracks and filtrate is forced
into the pore spaces. The whole mud lubricates the crack faces and
allows deeper filtrate penetration into the formation. The shale starts
to destabilize and the problems have just begun; hole enlargement, cut-
tings beds buildup in the washed-out sections, packing off, stuck pipe,
problems logging and running casing, and bad cement jobs.
Meanwhile, the drilling supervisor loves to wiper trip. It’s the
magic bullet, the answer to his prayers. By regularly wiping the hole he
feels that those tricky shales will be caressed into submission and
things will be better than they otherwise would be. So after every 24
hours drilling he calls the driller and tells him to circulate clean and do
a 10 stand trip before continuing to drill.
Pipe movement up or down will always cause some pressure fluc-
tuation in the wellbore. It is like death and taxes, impossible to totally
avoid. The driller really likes making hole so he is keen to get this trip
over with so he can get back to turning and burning. However, he is
conscientious and so after circulating for around 90 minutes, he sees

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