Page 385 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 361

Drilling Problems [ ]
3.3.2

7. If drilling ahead blind is not an option, consider using a foam or
air-assisted circulating system.
8. Use of barite plugs, diesel oil bentonite plugs, or cement plugs may
be considered as methods of last resort before attempting to drill
blind or switch to foam. The cost of the rigtime lost will have to be
considered against how likely success is, and whether the losses
will simply reoccur after drilling ahead a few meters.

There are some problems that will have to be considered. Well con-
trol has already been mentioned as has packing off, but large washouts
are likely and may lead to blocks of formation falling in.
Casing should be set as soon as a competent formation is drilled far
enough to have a strong shoe. Apart from the well control considera-
tions, loss of hydrostatic may destabilize other formations in the open
hole. Cementing may be a problem if serious losses persist, in which
case special cementing techniques may have to be considered such as
adding LCM or using extended or foamed cement.
Losses in heavily fractured cavernous formations. Losses are like-
ly to start as soon as the formation is penetrated, unless fractures are
created while drilling. One option may be to drill blind with water if
the area is well known or to drill with foam if not. Use the precautions
as in the preceding topic and set casing as soon as possible.
Another option is the careful spotting of large volumes of cement.
This has been successful in the field but requires careful planning and
supervision. See “Recommended procedure for curing total losses with
cement” later within this section.
Losses in normally pressured, deeper formations. These forma-
tions may be unconsolidated, naturally fractured, become fractured by
the drilling operation, or consolidated but highly permeable with pore
sizes too large for the mud solids to plaster. The loss zone can be any-
where in the open hole, not necessarily the formation just drilled into.
Several factors can contribute to the mud loss, such as annulus loaded
with cuttings, high ECD, excessive mud density, insufficient mud vis-
cosity, high water loss (low solids content to plaster the wall), exces-
sive surge pressures, breaking the formation during an FIT, or closing
in the well after a kick.
It will be useful to identify the type of loss zone and the mecha-
nism causing the losses to start. Knowing the depth and type of loss
zone will help formulate a strategy to cure the losses.

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