Page 384 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 360

[ ] Practical Wellsite Operations
3.3.2

It is better to wait until losses are fairly severe before doing this to
ensure an easy flow path for the cement. This should not take long
once losses have started.
Losses when drilling in divert mode must be cured; it is not safe to
drill blind. If the cement does not cure the losses, consider running
casing or respudding the well.
Severe or total losses in shallow unconsolidated formations. The
principal cause of losses in these formations is extremely high perme-
ability. 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.
Several factors will contribute to the mud loss, such as annulus loaded
with cuttings, excessive mud density, insufficient mud viscosity, high
water loss (low solids content to plaster the wall), or excessive surge
pressures. One or more of the following actions may be appropriate:

1. Add solids to the mud to increase rheology and plastering charac-
teristics, such as bentonite, lime, cement, or polymer (CMC HV).
Do not add high gravity solids such as barite, which increase the
mud density.
2. Reduce mud density if possible by dilution and/or maximizing the
use of solids control equipment.
3. Increase the circulation rate. The increase in ECD will be small
compared to the reduction in annular density due to cuttings load-
ing in large shallow holes. If in deep water with a floating rig,
increase riser booster pump output if possible.
4. Drill at controlled rates to reduce annulus loading.
5. Add coarse LCM to the mud system. High strength is not necessary,
so use the cheapest bulk material available, such as sawdust or
ground nut shells.
6. Drill ahead if familiar with the area, confident that there are no well
control problems ahead, and can replace the volume lost to the
hole. Several precautions must be taken to drill blind: a float
should be in the string and you must circulate fast enough to lift
the cuttings to the loss zone (minimum AV around 50 fpm outside
the DP). Monitor carefully the torque and drags because the string
may get packed off. Drill at controlled ROP. A good supply of water
must be available, which is a problem in the desert. Do not drill
blind with a diverter in use.

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