Page 328 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 304

[ ] Well Programming
2.9.2

Top-hole collapse. This may occur since very shallow formations are
unconsolidated and there is little overbalance to help stabilize the wall.
If possible on an offshore well, drill with returns to the rig and use a
mud with good wall cake-forming characteristics or pump slugs
around to help plaster the hole.
Reactive formations. For hydratable shales, inhibit the mud with
KCl, Polymer, and/or glycols; or use a silicate mud system to prevent
or reduce the rate of hydration. (Refer to the notes on these mud sys-
tems in Section 2.5.5.) Keep the level of inhibition according to pro-
gram or change it if necessary. Oil mud will prevent hydration. For
brittle failure type formations, higher mud weight is the best stabiliz-
ing mechanism. Ensure that the hole is kept clean while drilling.
For shales with natural microfractures, use sized fluid loss addi-
tives to plug off the fractures since they are exposed and keep mud
densities low. Minimize swab and surge pressures.
Wiper trips can initiate or accelerate instability in shales. See
“Drilling practices” in Section 2.9.1.
Geopressured formation. In shales where the pore pressure is greater
than mud hydrostatic, slivers of shale will be pushed into the wellbore.
Use higher mud weight if possible to reduce or eliminate this and keep
the hole clean using pills if necessary.
Where the formation is exposed for a long time, hole enlargement
can occur which may lead to other problems with large chunks of for-
mation falling in, big cuttings beds forming, fish becoming impossible
to recover (if they fall over in the washout), and bad cement jobs. If the
formation cannot be controlled with mud weight, if possible, ensure
that casing can be run within a short time of drilling the formation.
Fractured and faulted. Some formations are already naturally frac-
tured before being drilled into. This can again cause chunks of rock to
enter the wellbore, causing mechanical sticking. As the damage is
already done, avoid making it worse by minimizing swab/surge pres-
sures and drillstring vibrations, drilling at low angle if possible, getting
through it quickly, and casing it off.
Junk. Ensure all drillstring components are inspected in accordance
with API RP7G and that good handling practices are used. Specify the
correct size and grade of drillpipe to withstand drilling stresses. These
will help avoid downhole failures leading to junk in the hole.

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