Page 333 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 309

2.9.3
Drilling Problems--Avoidance Planning [ ]

4. Drill at controlled rates to reduce annulus loading.
5. Pump slugs around which contain solids to plaster the wall. Solids
suitable for plugging off large pore spaces would be taken out by
the solids control equipment, making it difficult to add to the
whole system. With a slug this does not matter.
6. Modify the casing program to case off the problem zones as soon
as possible.

Heavily fractured cavernous formations. Losses are likely to start
as soon as the formation is penetrated. Attempting to cure the losses
with LCM is likely to be expensive and ultimately futile and the best
options will probably be to drill blind with water if the area is well
known or to drill with foam if not. Set casing as soon as possible.
Cement can work well if done properly. Refer to the
“Recommended procedure for curing total losses with cement” in
Section 3.3.2, “Lost Circulation.”
Normally pressured, deeper formations. These formations 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 anywhere in
the open hole—not necessarily at the formation just drilled in to.
Several factors will contribute to the mud loss, such as annulus loaded
with cuttings, high ECD, excessive mud density, high water loss (low
solids content to plaster the wall), excessive surge pressures, breaking
the formation during a formation integrity test (FIT), or closing in the
well after a kick.
The following techniques will reduce the incidence of losses:

1. Use low rheologies to minimize ECD.
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 the minimum AV that will effectively clean the well. If high
ROP is expected then it may be necessary to control the ROP to
avoid excessive annulus loading. Displace viscous (low inclina-
tion) or turbulent (over 45°) pills to clean the well if annulus load-
ing is suspected.

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