Page 332 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 308

[ ] Well Programming
2.9.3

3. Maximize annular velocity. Use the largest liners at maximum SPM
and consider an extra skid-mounted pump for top hole. Circulate
viscous sweeps around each connection. If equipment problems
restrict the flow rate, stop drilling and circulate until resolved. High
flow rates in large, shallow hole will not add much ECD. It is more
beneficial to clean the hole effectively than to restrict the flow rate.
If restricting the flow rate does help, slow down the ROP as well to
avoid loading up the hole.
4. Maximize AV with large drill collars and possibly large drillpipe.
5. Drill with controlled ROP to minimize annulus loading. In practice,
as long as sufficient AV is maintained this should not be necessary.
6. Use bentonite or polymers to increase mud viscosity.

The easiest way to cure these losses is probably with large volumes
of cement. Refer to the “Recommended Procedure for curing total loss-
es with cement” in Section 3.5.2, “Slurry Mixing Options.” Prevention
is better than cure.

Shallow unconsolidated formations—severe or total losses. The
principal cause of losses in these formations is very high permeability.
The mud does not make an effective mud cake to seal the loss zone.
Losses are likely to start as soon as the formation is penetrated. An
annulus loaded with cuttings, excessive mud density, insufficient mud
viscosity, high water loss (low solids content to plaster the wall), or
excessive surge pressures will all contribute to the losses. The follow-
ing precautions can be taken:

1. Use high rheologies by adding flocculating agents such as lime or
cement. Thickening the mud will give greater resistance to flow
into the formation, but in shallow large diameter holes will not
make a significant difference to the ECD. This will also help keep
the hole clean.
2. Drill with the minimum safe density to control formation pressures
with a trip margin. This will have to be balanced against any for-
mations that may destabilize with insufficient hydrostatic and cave
into the hole. Some caving can probably be tolerated if this allows
the losses to be avoided.
3. Use a high circulation rate to clean the well. If possible, increase
riser booster pump output when in deep water with a floating rig.

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