Page 231 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 207

2.5.4
Drilling Fluids Program [ ]

as a lignosulfonate added to it which has defocculated it and ultimate-
ly allowed it to disperse. Fully dispersed, a clay may be at its most
expanded condition and it will not be able to impart any more viscos-
ity than it already has to the mud. This can give some temporary sta-
bility to the mud in respect of its rheology, but the resultant mud may
be capable of causing further dispersion of formation clays. As such, it
would be considered to have very little inhibition to clay dispersion.
Some degree of inhibition can be provided to a dispersed mud. The
addition of some form of calcium (from lime or gypsum for example)
to a fluid whose makeup clays has been deflocculated and dispersed
will cause drilled clays to remain in the aggregated condition or, at the
least, limit their dispersion. Lime or gypsum muds are examples of
such inhibited dispersed systems. Nondispersed variants of lime or
gypsum muds now exist but the amount of clay in them is minimized.
A characteristic difference between a dispersed and a nondispersed
mud can be seen in their typical rheologies. A dispersed system will
usually have a low yield point value and a high value for plastic vis-
cosity. The plastic viscosities of dispersed muds will be higher than
those of nondispersed muds. Usually improved drilling performance,
better hole conditions, and less formation impairment can be achieved
if a nondispersed mud can be used (as compared with a dispersed sys-
tem). In the early days, in many cases, only dispersed systems could be
used. It was the improvement in efficiency of rig solids control equip-
ment that allowed nondispersed systems to be successful. Following
are some dispersed mud systems.
Spud mud. Often in top hole drilling, a fluid based upon bentonite
in water is used to provide viscosity and filter cake. This fluid is often
described as “spud mud.” Such mud would be described as dispersed
mud when the bentonite was in a dispersed condition. It would
become flocculated mud when it was mixed in with seawater or had
lime (calcium hydroxide) added to it. The flocculation of a prehydrat-
ed bentonite (dispersed in fresh water) with lime, in the case of spud
mud, is done on purpose to obtain additional viscosity and gel
strengths. The gel structure of flocculated mud is invariably stronger
than in the dispersed state.
Caustic lignosulfonate mud. This is a very simple form of dis-
persed mud. It will cope with varying quantities of clay present in the
mud. However, in using a caustic lignosulfonate mud those varying
quantities of clay will tend to increase. That is because this type of mud

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