Page 319 - Drilling Technology in Nontechnical Language

310 Drilling Technology in Nontechnical Language Second Edition

if the severity of the situation warrants it and if that might bypass the loss
zone. Aerated mud techniques as described in chapter 7 might also be
applicable, but these must be planned well in advance.

Losses in heavily fractured or cavernous formations

Some formations contain very large fractures (typical of limestone
reservoirs) or large caverns. When the bit penetrates one of these spaces,
total losses are likely to start immediately—a good sign that this type of
formation has been penetrated.
One option may be to drill blind with water if the area is well known,
or to drill with foamed mud if not. Casing is set as soon as possible after
drilling right through the loss zone (drilling far enough to give a reasonable
column of cement above the casing shoe).
Another option is the careful spotting of large volumes of cement as
described later. If done correctly, this will work to plug off fractures or
caverns around the wellbore. Drilling further after this cement cure can
open up more fractures, which could lead to one or more repeats until the
fractured zone has been completely drilled through.

Diesel oil bentonite (DOB) plug

DOB plugs are also known as gunk plugs. They work by holding
bentonite clay in suspension in diesel oil until the plug is in place downhole,
and then arranging for water to hydrate the bentonite. The bentonite
hydrates rapidly, becoming extremely viscous. They can be successful

in shutting off flow in an underground blowout, especially if the flow
is water.
A DOB plug will not maintain strength indefinitely. Cement should

be spotted to give a permanent seal once the plug has worked. The main
potential problem with the DOB plug is that it will set up inside the
drillstring if it contacts any water. A good diesel spacer ahead and behind
are essential to prevent this.
To place the plug, 150 lb of bentonite are mixed for each barrel of
diesel. Normally, a DOB plug will be between 30 bbl and 150 bbl in size.
There must be no water present in the mixing pit or in the pump lines.

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